Fatigue Loading and Design Methodology for High-Mast Lighting Towers (2012)

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B-1 A p p e n d i x B Proposed Specification and Commentary

B-2 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-i SECTION 11: FATIGUE DESIGN TABLE OF CONTENTS 11 11.1—SCOPE .............................................................................................................................................................................. 11-1 11.2—DEFINITIONS ................................................................................................................................................................. 11-1 11.3—NOTATION ...................................................................................................................................................................... 11-1 11.4—APPLICABLE STRUCTURE TYPES............................................................................................................................ 11-3 11.5—DESIGN CRITERIA ........................................................................................................................................................ 11-3 11.6—FATIGUE IMPORTANCE FACTORS .......................................................................................................................... 11-5 11.7—FATIGUE DESIGN LOADS ........................................................................................................................................... 11-8 11.7.1—Sign and Traffic Signal Structures ......................................................................................................................... 11-8 11.7.1.1—Galloping ..................................................................................................................................................... 11-9 11.7.1.2—Vortex Shedding ........................................................................................................................................ 11-11 11.7.1.3 11.7.3—Natural Wind Gust.......................................................................................................................... 11-13 11.7.1.4 11.7.4—Truck-Induced Gust ........................................................................................................................ 11-14 11.7.2—High-mast Lighting Towers ................................................................................................................................. 11-15 11.8—DEFLECTION ............................................................................................................................................................... 11-17 11.9—FATIGUE RESISTANCE ............................................................................................................................................. 11-18 11.10—REFERENCES ............................................................................................................................................................. 11-28

proposed Specification and Commentary B-3 11-1 SECTION 11: FATIGUE DESIGN 11.1—SCOPE C11.1 This Section contains provisions for the fatigue design of cantilevered and noncantilevered steel and aluminum structural supports for highway signs, luminaires, and traffic signals. This Section focuses on fatigue, which is defined herein as the damage that may result in fracture after a sufficient number of stress fluctuations. It is based on NCHRP Report 412, Fatigue Resistant Design of Cantilevered Signal, Sign and Light Supports (Kaczinski et al., 1998), NCHRP Report 469, Fatigue-Resistant Design of Cantilever Signal, Sign, and Light Supports (Dexter and Ricker, 2002), and NCHRP Report 494, Structural Supports for Highway Signs, Luminaires, and Traffic Signals (Fouad et al., 2003), NCHRP Project 10-74, Development of Fatigue Loading and Design Methodology for High-Mast Light Poles (Connor et al, 2012). 11.2—DEFINITIONS Constant-Amplitude Fatigue Limit (CAFL)—Nominal stress range below which a particular fatigue detail can withstand an infinite number of repetitions without fatigue failure. Fatigue—Damage resulting in fracture caused by stress fluctuations. In-Plane Bending—Bending in-plane for the main member (column). At the connection of an arm or arm’s built-up box to a vertical column, the in-plane bending stress range in the column is a result of galloping or truck-induced gust loads on the arm and/or arm’s attachments. Limit State Wind Load Effect—A specifically defined load criteria. Load-Bearing Attachment—Attachment to main member where there is a transverse load range in the attachment itself in addition to any primary stress range in the main member. Nonload-Bearing Attachment—Attachment to main member where the only significant stress range is the primary stress in the main member. Out-of-Plane Bending—Bending out-of-plane for the main member (column). At the connection of an arm or arm’s built-up box to a vertical column, the out-of-plane bending stress range in the column is a result of natural wind-gust loads on the arm and the arm’s attachments. Pressure Range—Pressure due to a limit state wind load effect that produces a stress range. Stress Range—The algebraic difference between extreme stresses used in fatigue design. Yearly Mean Wind Velocity—Long-term average of the wind speed for a given area. HMLT – Acronym for high-mast lighting tower 11.3—NOTATION b = flat-to-flat width of a multisided section (m, ft) Cd = appropriate drag coefficient from Section 3, “Loads,” for given attachment or member d = diameter of a circular section (m, ft) D = inside diameter of exposed end of female section for slip-joint splice (mm, in.) E = modulus of elasticity (MPa, ksi) fn = first natural frequency of the structure (cps)

B-4 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-2 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS fn1 = first modal frequency (cps) (F)n = fatigue strength (CAFL) (MPa, ksi) g = acceleration of gravity (9810 mm/s2, 386 in./s2) H = effective weld throat (mm, in.) I = moment of inertia (mm4, in.4) Iavg = average moment of inertia for a tapered pole (mm4, in.4) Itop = moment of inertia at top of tapered pole (mm4, in.4) Ibottom = moment of inertia at bottom of tapered pole (mm4, in.4) IF = fatigue importance factors applied to limit state wind load effects to adjust for the desired level of structural reliability L = length of the pole (Article 11.7.2) (mm, in.) L = slip-splice overlap length (example 1 of Figure 11-111-2) (mm, in.) L = length of reinforcement at handhole (example 13 of Figure 11-111-2) (mm, in.) L = length of longitudinal attachment (examples 12, 14, and 15 of Figure 11-111-2) (mm, in.) PCW = combined wind pressure range for fatigue design of HMLTs (Pa, psf) PFLS = fatigue-limit-state wind pressure range for fatigue design of HMLTs (Pa, psf) PG = galloping-induced vertical shear pressure range (Pa, psf) PNW = natural wind gust pressure range (Pa, psf) PTG = truck-induced gust pressure range (Pa, psf) PVS = vortex shedding-induced pressure range (Pa, psf) r = radius of chord or column (mm, in.) R = transition radius of longitudinal attachment (mm, in.) Sn = Strouhal number SR = nominal stress range of the main member or branching member (MPa, ksi) t = thickness (mm, in.) tb = wall thickness of branching member (mm, in.) tc = wall thickness of main member (column) (mm, in.) tp = plate thickness of attachment (mm, in.) Vc = critical wind velocity for vortex shedding (m/s, mph) Vmean = yearly mean wind velocity for a given area (m/s, mph) VT = truck speed for truck-induced wind gusts (m/s, mph) W = weight of the luminaire (N, k) w = weight of the pole per unit length (N/mm, k/in.) = damping ratio = angle of transition taper of longitudinal attachment (example 14 of Figure 11-111-2) ( ) = ovalizing parameter for bending in the main member (note b of Table 11-211-4) F = constant amplitude fatigue limit stress range (MPa, ksi) = indication of stress range in member

proposed Specification and Commentary B-5 SECTION 11: FATIGUE DESIGN 11-3 11.4—APPLICABLE STRUCTURE TYPES C11.4 Design for fatigue shall be required for the following type structures: a. overhead cantilevered sign structures, b. overhead cantilevered traffic signal structures, c. high-level, high-mast lighting structures, d. overhead noncantilevered sign structures, and e. overhead noncantilevered traffic signal structures. NCHRP Report 412 is the basis for the fatigue design provisions for cantilevered structures. NCHRP Report 494 is the basis for the fatigue design provisions for noncantilevered support structures. The fatigue design procedures outlined in this Section may be applicable to steel and aluminum structures in general. However, only specific types of structures are identified for fatigue design in this Article. Common lighting poles and roadside signs are not included because they are smaller structures and normally have not exhibited fatigue problems. An exception would be square lighting poles, as they have exhibited poor fatigue performance. Square cross-sections have been much more prone to fatigue problems than round cross-sections. Caution should be exercised regarding the use of square lighting poles even when a fatigue design is performed. The provisions of this Section are not applicable for the design of span-wire (strain) poles. 11.5—DESIGN CRITERIA C11.5 Cantilevered and noncantilevered support structures shall be designed for fatigue to resist each of the applicable equivalent static wind load effects specified in Article 11.7, and modified by the appropriate fatigue importance factors given in Article 11.6. Stresses due to these loads on all components, mechanical fasteners, and weld details shall be limited to satisfy the requirements of their respective detail categories within the constant-amplitude fatigue limits (CAFL) provided in Article 11.9. Accurate load spectra and life prediction techniques for defining fatigue loadings are generally not available. The assessment of stress fluctuations and the corresponding number of cycles for all wind-induced events (lifetime loading histogram) is practically impossible. With this uncertainty, the design of sign, luminaire, and traffic signal supports for a finite fatigue life becomes impractical. Therefore, an infinite life fatigue design approach is recommended and is considered sound practice. Fatigue stress limits are based on the CAFL. The CAFL values provided in Table 11-311-5 are approximately the same as those given in Table 10.3.1A of the Standard Specifications for Highway Bridges6.6.1.2.5-3 of the LRFD Bridge Design Specifications. An infinite life fatigue approach was developed in an experimental study that considered several critical welded details (Fisher et al. 1993). The infinite life fatigue approach can be used when the number of wind load cycles expected during the lifetime of the structures is greater than the number of cycles at the CAFL. This is particularly the case for structural supports where the wind load cycles in 25 years or greater lifetimes are expected to exceed 100 million cycles, whereas typical weld details reach the CAFL at 10 to 20 million cycles.

B-6 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-4 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS Fatigue-critical details are designed with nominal stress ranges that are below the appropriate CAFL. To assist designers, typical support structure details based on AASHTO and American Welding Society (AWS) fatigue design categories are provided in Table 11-211-4 and Figure 11-111-2. The above-referenced details were produced based on a review of state departments of transportation standard drawings and manufacturers’ literature. This list should not be considered a complete set of all possible connection details, but rather it is intended to remove the uncertainty associated with applying the provisions of the Standard Specifications for Highway Bridges LRFD Bridge Design Specifications to the fatigue design of support structures. Choice of details improves the fatigue resistance of these structures, and it can eliminate or reduce increases in member size required for less fatigue- resistant details. The notes for Table 11-211-4 specify the use of Stress Category K2. This stress category corresponds to the category for cyclic punching shear stress in tubular members specified by the AWS Structural Welding Code D1.1—Steel. Fatigue design for the column’s wall under this condition may require sizes of the built-up box connection or column wall thicknesses that are excessive for practical use. For this occurrence, an adequate fatigue-resistant connection other than the built-up box shown in Figure 11-111-2 should be considered. Fatigue testing has shown the advantage of ring stiffeners that completely encircle a pole relative to a built-up box connection. For built-up box connections, it is recommended that the width of the box be the same as the diameter of the column (i.e., the sides of the box are tangent to the sides of the column). Regarding full-penetration groove-welded tube-to- transverse plate connections, NCHRP Report 412 did not fully investigate the effects from the possible use of additional reinforcing fillet welds. Additional research and testing of these types of detail configurations are needed to support future updates of this Section. Stress categories in Table 11-211-4 for weld terminations at the end of longitudinal stiffeners were based, in part, on assigned categories for attachments in the AASHTO Bridge Specifications. Fatigue testing of many fillet-welded tube-to-longitudinal stiffener connections indicates that the angle of intersection, the transitional radius to the pole wall, the length of the stiffener, and the ratio of the stiffener thickness to pole wall thickness, for example, all have effects on the fatigue life of the detail. Some tube-to- stiffener connections have a potential to develop high stress concentrations in the tube wall in the vicinity of the weld termination at the end of longitudinal stiffeners. Testing on poles having wall thickness less than 6 mm (0.25 in.) indicates that longitudinal stiffeners yielded little or no improvement of the fatigue performance of the connection (Koenigs et al., 2003). Until further research can give reliable estimates of the effects of stiffeners, all welds terminating at the end of longitudinal stiffeners shall be classified as Stress Category E .

proposed Specification and Commentary B-7 SECTION 11: FATIGUE DESIGN 11-5 Equal leg welds in socket connections have been shown by fatigue testing to have a fatigue strength less than Stress Category E . The fatigue strength of a socket-welded connection can be improved by using an unequal leg fillet weld. 11.6—FATIGUE IMPORTANCE FACTORS C11.6 A fatigue importance factor, IF, that accounts for the risk of hazard to traffic and damage to property shall be applied to the limit state wind-load effects specified in Article 11.7. Fatigue importance factors for traffic signal, sign, and luminaire support structures exposed to the four wind load effects are presented in Table 11-1. The importance categories given in Table 11-2 shall be used for high-mast lighting towers. Fatigue importance factors are introduced into the Specifications to adjust the level of structural reliability of cantilevered and noncantilevered support structures. Fatigue importance factors should be determined by the Owner. For combined structures, where traffic signals and luminaires are joined, the use of the more conservative fatigue importance factor is recommended. The importance categories and fatigue importance factors found in Table 11-1 (rounded to the nearest 0.05) are results from NCHRP Reports 469 and 494. Three categories of support structures are presented in Table 11-1. Structures classified as Category I present a high hazard in the event of failure and should be designed to resist rarely occurring wind loading and vibration phenomena. It is recommended that all structures without effective mitigation devices on roadways with a speed limit in excess of 60 km/hr (35 mph) and average daily traffic (ADT) exceeding 10 000 or average daily truck traffic (ADTT) exceeding 1000 should be classified as Category I structures. ADT and ADTT are for one direction regardless of the number of lanes. Structures without mitigation devices may be classified as Category I if any of the following apply: 1. Cantilevered sign structures with a span in excess of 16 m (50 ft) or high-mast towers in excess of 30 m (100 ft), 2. Large sign structures, both cantilevered and noncantilevered, including variable message signs, and 3. Structures located in an area that is known to have wind conditions that are conducive to vibration. Structures should be classified as Category III if they are located on roads with speed limits of 60 km/hr (35 mph) or less. Structures that are located such that a failure will not affect traffic may be classified as Category III. All structures not explicitly meeting the Category I or Category III criteria should be classified as Category II. Maintenance and inspection programs should be considered integral to the selection of the fatigue importance category. There are many factors that affect the selection of the fatigue category and engineering judgment is required. The fatigue importance factors for HMLTs found in Table 11-2 are based on the research conducted as part of NCHRP 10-74. The importance factors for HMLTs have been separated and simplified from those in Table 11-1. Since HMLTs are generally only used on high ADTT roadways, whether a pole can or cannot fall in the path of traffic is selected as the critical parameter..

B-8 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-6 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS

proposed Specification and Commentary B-9 SECTION 11: FATIGUE DESIGN 11-7 Table 11–1—Fatigue Importance Factors, IF Fatigue Category Fatigue Importance Factor, IF Galloping Vortex Shedding Natural Wind Gusts Truck-Induced Gusts Ca nt ile ve re d I Sign Traffic Signal Lighting 1.0 1.0 x x* x* 1.0 1.0 1.0 1.0 1.0 1.0 x II Sign Traffic Signal Lighting 0.70 0.65 x x* x* 0.65 0.85 0.80 0.75 0.90 0.85 x III Sign Traffic Signal Lighting 0.40 0.30 x x* x* 0.30 0.70 0.55 0.50 0.80 0.70 x N o n ca n til ev er ed I Sign Traffic Signal x x x* x* 1.0 1.0 1.0 1.0 II Sign Traffic Signal x x x* x* 0.85 0.80 0.90 0.85 III Sign Traffic Signal x x x* x* 0.70 0.55 0.80 0.70 Notes: x Structure is not susceptible to this type of loading. * Overhead cantilevered and noncantilevered sign and traffic signal components are susceptible to vortex shedding prior to placement of the signs and traffic signal heads, i.e., during construction. Table 11-2—Fatigue Importance Categories for HMLTs Hazard Level Importance Category High (distance to roadway height of HMLT) I Low (distance to roadway > height of HMLT) II

B-10 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-8 S TA ND AR D S PECIFICA TIONS FOR S TRUCTUR AL S UPPORTS FOR H IGHWA Y S IGN S , L UM IN AI RE S , A ND T RA FFIC S IG NA LS 11.7—FATIGUE DESIGN LOADS C11.7 To avoid large-amplitude vibrations and to preclude th e developm ent of fatigue cracks in various connection details and at other critical locations, cantilevered an d noncantilevered s upport structures shall be designed to resi s t each of the following applicable limit state equivalent static wind loads acting separately. These loads shall be used to calculate nominal stress ranges near fatigue-sensitive connection details described in Article 11.5 and deflections for service limits described in Article 11.8. In lieu of using the equivalent static pressures provide d in th is Specification, a dynamic analysis of the structure ma y b e perform ed using appropriate dynamic load functions derived from reliable data. Fatigue loading provisions for high-mast lighting towers (HMLTs) are differentiated fro m t hose associated w ith othe r traffic structures. HMLTs shall be designed for the loading given in Article 11.7.2. Cantilevered and noncantilevered support structures are exposed to several wind pheno me na that can produce cyclic loads. Vibrations associated with these cyclic forces ca n beco me significant. NCHRP Report 412 identified galloping, vortex shedding, natural wind gusts, and truck-induced gusts as wind-loading mechanisms that can induce large-a mp l itude vibrations and/or fatigue damage in cantilevered traffic signal, sign, and light support structures. NCHRP Report 494 identified natural wind gusts and truck-induced gusts as wind-loading mechanisms that can induce large-a mp l itude vibrations and/or fatigue damage in noncantilevered traffic signal and sign support structures. The am plitude of vibratio n and resulting stress ranges are increased by the low levels o f s tiffness and damping possessed by many of these structures. In so me cases, the vibration is only a serviceability probl e m b ecause mo torists cannot clearly see the ma st ar m attachments or are concerned about passing under the structures. In other cases, where deflections may or ma y no t b e considered excessive, th e ma gnitudes of stress ranges i nduced in these structures have resulted in the develop me n t of fatigue cracks at various connection details including the anchor bolts. The provisions for fatigue lo ading of HMLTs is base d on the research conducted as part of NCHRP Project 10-74, which developed a loadi ng spectru m inclusive of all a pplicable load effects due to natural wind. The wind-loading phenom ena specified in this sectio n possess the greatest potential for creating large-am plitude vibrations in cantilevered support structures. In particular, galloping and vortex shedding are aeroelastic instabilities that typically in duce vibrations at the natural frequency of the structure (i.e., resonance). These conditions can l ead to fatigue failures in a relatively short period of time. Design pressures for f ourfatigue wind-loading mechanisms are presented as an equivalent static win d p ressure range, or a shear stress range in the case o f galloping. These pressure (or shear stress) ranges should be applied as prescribed by static analysis to determine stress ranges near fatigue-sensitive details. In lieu of designing fo r galloping or vortex-shedding limit state fatigue wind lo a d effects, m itigation devices may be used as approved by the Owner. Mitigation devices are discussed in NCHRP Reports 412 and 469.412, 469, and 10-74. 11.7.1—Sign and Traffic Signal Structures C11.7.1 Equivalent static wind loads for the fatigue design o f sign and traffic signal structures shall be determined fro m Articles 11.7.1.1 through 11.7.1.4 as applicable. The structures included in this s ection are defined in Article 11.5 and the associated commentary .

proposed Specification and Commentary B-11 SECTION 11: FATIGUE DESIGN 11-9 11.7.1.1—Galloping C11.7.1.1 Overhead cantilevered sign and traffic signal support structures shall be designed for galloping-induced cyclic loads by applying an equivalent static shear pressure vertically to the surface area, as viewed in normal elevation of all sign panels and/or traffic signal heads and backplates rigidly mounted to the cantilevered horizontal support. The vertical shear pressure range shall be equal to the following: 1000G FP I (Pa) (11-1) 21G FP I (psf) In lieu of designing to resist periodic galloping forces, cantilevered sign and traffic signal structures may be erected with effective vibration mitigation devices. Vibration mitigation devices should be approved by the Owner, and they should be based on historical or research verification of its vibration damping characteristics. Alternatively, for traffic signal structures, the Owner may choose to install approved vibration mitigation devices if structures exhibit a galloping problem. The mitigation devices should be installed as quickly as possible after the galloping problem appears. The Owner may choose to exclude galloping loads for the fatigue design of overhead cantilevered sign support structures with quadri-chord (i.e., four-chord) horizontal trusses. Galloping, or Den Hartog instability, results in large- amplitude, resonant oscillations in a plane normal to the direction of wind flow. It is usually limited to structures with nonsymmetrical cross-sections, such as sign and traffic signal structures with attachments to the horizontal cantilevered arm. Structures without attachments to the cantilevered horizontal arm support are not susceptible to galloping- induced wind load effects. The results of wind tunnel (Kaczinski et al., 1998) and water tank (McDonald et al., 1995) testing, as well as the oscillations observed on cantilevered support structures in the field, are consistent with the characteristics of the galloping phenomena. These characteristics include the sudden onset of large-amplitude, across-wind vibrations that increase with increases in wind velocity. Galloping is typically not caused by wind applied to the support structure, but rather applied to the attachments to the horizontal cantilevered arm, such as signs and traffic signals. The geometry and orientation of these attachments, as well as the wind direction, directly influence the susceptibility of cantilevered support structures to galloping. Traffic signals are more susceptible to galloping when configured with a backplate. In particular, traffic signal attachments configured with or without a backplate are more susceptible to galloping when subject to flow from the rear. Galloping of sign attachments is independent of aspect ratio and is more prevalent with wind flows from the front of the structure.

B-12 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-10 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS By conducting wind tunnel tests and analytical calibrations to field data and wind tunnel test results, an equivalent static vertical shear of 1000 Pa (21 psf) was determined for the galloping phenomenon. This vertical shear range should be applied to the entire frontal area of each of the sign and traffic signal attachments in a static analysis to determine stress ranges at critical connection details. For example, if a 2.5 × 3.0 m (8 × 10 ft) sign panel is mounted to a horizontal mast arm, a static force of 7500 × IF, N (1680 × IF, lb) should be applied vertically at the area centroid of the sign panel. A study (Florea et al, 2007) has shown that the equivalent static force that an attachment experiences depends on the location along the arm where it is attached. Equivalent static pressures or vertical shear ranges applied to the frontal area of each sign or traffic signal attachment are greater towards the tip of the mast arm. The specification does not consider the effect of the attachment location when calculating the galloping force. Further testing is necessary to verify this and to suggest location- specific ranges. A pole with multiple horizontal cantilevered arms may be designed for galloping loads applied separately to each individual arm, and need not consider galloping simultaneously occurring on multiple arms. Overhead cantilevered sign support structures with quadri-chord horizontal trusses do not appear to be susceptible to galloping because of their inherent stiffness. Two possible means exist to mitigate galloping-induced oscillations in cantilevered support structures. The dynamic properties of the structure or the aerodynamic properties of the attachments can be adequately altered to mitigate galloping. The installation of a device providing positive aerodynamic damping can be used to alter the structure’s response from the aerodynamic effects on the attachments. A method of providing positive aerodynamic damping to a traffic signal structure involves installing a sign blank mounted horizontally and directly above the traffic signal attachment closest to the tip of the mast arm. This method has been shown to be effective in mitigating galloping- induced vibrations on traffic signal support structures with horizontally mounted traffic signal attachments (McDonald et al., 1995). For vertically mounted traffic signal attachments, a sign blank mounted horizontally near the tip of the mast arm has mitigated large-amplitude galloping vibrations occurring in traffic signal support structures. This sign blank is placed adjacent to a traffic signal attachment, and a separation exists between the sign blank and the top of the mast arm. In both cases, the sign blanks are required to provide a sufficient surface area for mitigation to occur. However, the installation of sign blanks may influence the design of structures for truck-induced wind gusts by increasing the projected area on a horizontal plane. NCHRP Reports 412 and 469 provide additional discussion on this possible mitigation device and on galloping susceptibility and mitigation.

proposed Specification and Commentary B-13 SECTION 11: FATIGUE DESIGN 11-11 11.7.1.2—Vortex Shedding C11.7.1.2 Cantilevered lighting structures shall be designed to resist vortex shedding-induced loads for critical wind velocities less than approximately 20 m/s (45 mph). The critical wind velocity, Vc (m/s, mph), at which vortex shedding lock-in can occur may be calculated as follows: For circular sections: n c n f d V S (m/s) (11-2) 0.68 nc n f d V S (mph) For multisided sections: n c n f b V S (m/s) (11-3) 0.68 nc n f b V S (mph) where fn is a natural frequency of the structure (cps); d and b are the diameter and flat-to-flat width of the horizontal mast arm or pole shaft for circular and multisided sections (m, ft), respectively; and Sn is the Strouhal number. The Strouhal number shall be taken as 0.18 for circular sections, 0.15 for multisided sections, and 0.11 for square or rectangular sections. For a tapered pole, d and b are the average diameter and width. The equivalent static pressure range to be used for the design of vortex shedding-induced loads shall be: 2 0.613 2 c d F vs V C I P (Pa) (11-4) 2 0.00256 2 c d F vs V C I P (psf) where Vc is expressed in m/s (mph); Cd is the drag coefficient as specified in Section 3, “Loads,” which is based on the critical wind velocity Vc; and is the damping ratio, which may be estimated as 0.005. The equivalent static pressure range Pvs shall be applied transversely to poles (i.e., horizontal direction) and horizontal mast arms (i.e., vertical direction). In lieu of designing to resist periodic vortex-shedding forces, effective vibration mitigation devices may be used. The shedding of vortices on alternate sides of a member may result in oscillations in a plane normal to the direction of wind flow. Typical natural frequencies and member dimensions preclude the possibility of most cantilevered sign and traffic signal support structures from being susceptible to vortex shedding-induced vibrations. NCHRP Report 469 shows that poles with tapers exceeding 0.0117 m/m (0.14 in./ft) can also experience vortex shedding in lighting structures. Observations and studies indicate that tapered poles can experience vortex shedding in second or third mode vibrations and that those vibrations can lead to fatigue problems. Procedures to consider higher mode vortex shedding on tapered poles are demonstrated in NCHRP Report 469. Structural elements exposed to steady, uniform wind flows shed vortices in the wake behind the element in a pattern commonly referred to as a von Karmen vortex street. When the frequency of vortex shedding approaches one of the natural frequencies of the structure, usually the first mode (or higher modes as demonstrated in NCHRP Report 469), significant amplitudes of vibration can be caused by a condition termed lock-in. The critical velocity at which lock- in occurs is defined by the Strouhal relationship: n c n f d V S (C11-1) For the first mode of vibration, a lower bound wind speed can be established for traffic signal and sign structures. Although vortices are shed at low wind velocities for wind speeds less than 5 m/s (16 fps, 11 mph), the vortices do not impart sufficient energy to excite most structures. Typical natural frequencies and member diameters for sign and traffic signal support structures result in critical wind velocities well below the 5 m/s (16 fps, 11 mph) threshold for the occurrence of vortex shedding. Because of extremely low levels of damping, vortex shedding may significantly excite resonant vibration. At wind speeds greater than about 20 m/s (65 fps, 45 mph), enough natural turbulence is generated to disturb the formation of vortices. Because Vc is relatively low, the largest values of Cd for the support may be conservatively used. Horizontal arms may be susceptible to vortex shedding before sign and signal heads are attached, i.e., during construction. Although possible, tests (Kaczinski et al., 1998; McDonald et al., 1995) have indicated that the occurrence of vortex shedding from attachments to cantilevered sign and traffic signal support structures is not critical. These attachments are more susceptible to galloping-induced vibrations.

B-14 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-12 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS Calculation of the first modal frequency for simple pole structures (i.e., without mast arms) can be computed using: 1 4 1.75 n EIgf wL (C11-2) (without luminaire mass) 1 3 4 1.732 2 0.236n EIgf WL wL (C11-3) (with luminaire mass) where W is the weight of the luminaire (N, k), w is the weight of the pole per unit length (N/mm, k/in.), g is the acceleration of gravity (9810 mm/s2, 386 in./s2), L is the length of the pole (mm, in.), and I is the moment of inertia of the pole (mm4, in.4). For tapered poles, Iavg is substituted for I, where: 2 top bottom avg I I I (C11-4) Itop is the moment of inertia at the tip of the pole and Ibottom is the moment of inertia at the bottom of the pole. The first modal frequency for poles with mast arms, however, is best accomplished by a finite element based modal analysis. The mass of the luminaire/mast arm attachments shall be included in the analysis to determine the first mode of vibration transverse to the wind direction. Poles that may not have the attachments installed immediately shall be designed for this worst-case condition. Because the natural frequency of a structure without an attached mass is typically higher than those with an attachment, the resulting critical wind speed and vortex shedding pressure range are also higher for this situation.

proposed Specification and Commentary B-15 SECTION 11: FATIGUE DESIGN 11-13 11.7.1.3 11.7.3—Natural Wind Gust C11.7.1.3 11.7.3 Cantilevered and noncantilevered overhead sign and overhead traffic signal and high level lighting supports shall be designed to resist an equivalent static natural wind gust pressure range of: 250NW FdCP I (Pa) (11-5) 5.2NW FdCP I (psf) where Cd is the appropriate drag coefficient based on the yearly mean wind velocity of 5 m/s (11.2 mph) specified in Section 3, “Loads,” for the considered element to which the pressure range is to be applied. If Eq. C11-5 is used in place of Eq. 11-5, Cd may be based on the location-specific yearly mean wind velocity Vmean. The natural wind gust pressure range shall be applied in the horizontal direction to the exposed area of all support structure members, signs, traffic signals, and/or miscellaneous attachments. Designs for natural wind gusts shall consider the application of wind gusts for any direction of wind. The design natural wind gust pressure range is based on a yearly mean wind speed of 5 m/s (11.2 mph). For locations with more detailed wind records, particularly sites with higher wind speeds, the natural wind gust pressure may be modified at the discretion of the Owner. Because of the inherent variability in the velocity and direction, natural wind gusts are the most basic wind phenomena that may induce vibrations in wind-loaded structures. The equivalent static natural wind gust pressure range specified for design was developed with data obtained from an analytical study of the response of cantilevered support structures subject to random gust loads (Kaczinski et al., 1998). Because Vmean is relatively low, the largest values of Cd for the support may be conservatively used. This parametric study was based on the 0.01 percent exceedance for a yearly mean wind velocity of 5 m/s (11.2 mph), which is a reasonable upper bound of yearly mean wind velocities for most locations in the country. There are locations, however, where the yearly mean wind velocity is larger than 5 m/s (11.2 mph). For installation sites with more detailed information regarding yearly mean wind speeds (particularly sites with higher wind speeds), the following equivalent static natural wind gust pressure range may shall be used for design: 2 250 5 / mean NW d F V P C I m s (Pa) (C11-5) 2 5.2 11.2 mean NW d F V P C I mph (psf) The largest natural wind gust loading for an arm or pole with a single arm is from a wind gust direction perpendicular to the arm. For a pole with multiple arms, such as two perpendicular arms, the critical direction for the natural wind gust is usually not normal to either arm. The design natural wind gust pressure range shall be applied to the exposed surface areas seen in an elevation view orientated perpendicular to the assumed wind gust direction.

B-16 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-14 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS 11.7.1.4 11.7.4—Truck-Induced Gust C11.7.1.4 11.7.4 Cantilevered and noncantilevered overhead sign and traffic signal support structures shall be designed to resist an equivalent static truck gust pressure range of 900TG d FP C I (Pa) (11-6) 18.8TG d FP C I (psf) where Cd is the drag coefficient based on the truck speed of 30 m/s (65 mph) from Section 3, “Loads,” for the considered element to which the pressure range is to be applied. If Eq. C11-6 is used in place of Eq. 11-6, Cd should be based on the considered truck speed VT. The pressure range shall be applied in the vertical direction to the horizontal support as well as the area of all signs, attachments, walkways, and/or lighting fixtures projected on a horizontal plane. This pressure range shall be applied along any 3.7-m (12-ft) length to create the maximum stress range, excluding any portion of the structure not located directly above a traffic lane. The equivalent static truck pressure range may be reduced for locations where vehicle speeds are less than 30 m/s (65 mph). The magnitude of applied pressure range may be varied depending on the height of the horizontal support and the attachments above the traffic lane. Full pressure shall be applied for heights up to and including 6 m (20 ft), and then the pressure may be linearly reduced for heights above 6 m (20 ft) to a value of zero at 10 m (33 ft). The truck-induced gust loading shall be excluded unless required by the Owner for the fatigue design of overhead traffic signal support structures. The passage of trucks beneath support structures may induce gust loads on the attachments mounted to the horizontal support of these structures. Although loads are applied in both horizontal and vertical directions, horizontal support vibrations caused by forces in the vertical direction are most critical. Therefore, truck gust pressures are applied only to the exposed horizontal surface of the attachment and horizontal support. A pole with multiple horizontal cantilever arms may be designed for truck gust loads applied separately to each individual arm and need not consider truck gust loads applied simultaneously to multiple arms. Recent vibration problems on sign structures with large projected areas in the horizontal plane, such as variable message sign (VMS) enclosures, have focused attention on vertical gust pressures created by the passage of trucks beneath the sign. The design pressure calculated from Eq. 11-6 is based on a truck speed of 30 m/s (65 mph). For structures installed at locations where the posted speed limit is much less than 30 m/s (65 mph), the design pressure may be recalculated based on this lower truck speed. The following equation may be used: 2 900 30 T TG d F V P C I m s (Pa) (C11-6) 2 18.8 65 T TG d F V P C I mph (psf) where VT is the truck speed in m/s (mph). The given truck-induced gust loading shall be excluded unless required by the Owner for the fatigue design of overhead traffic signal structures. Many traffic signal structures are installed on roadways with negligible truck traffic. In addition, the typical response of traffic signal structures from truck-induced gusts is significantly overestimated by the design pressures prescribed in this article (NCHRP Report 469). This has been confirmed in a recent study (Albert et al, 2007) involving full-scale field tests where strains were monitored on cantilevered traffic signal structures. Over 400 truck events were recorded covering a variety of truck types and vehicle speeds; only 18 trucks produced even a detectable effect on the cantilevered traffic signal structures and the strains were very small relative to those associated with the design pressures in this Article.

proposed Specification and Commentary B-17 SECTION 11: FATIGUE DESIGN 11-15 11.7.2—High-mast Lighting Towers C11.7.2 High-mast lighting towers shall be designed for fatigue to resist the combined wind effect, an equivalent static pressure range of CW FLS dP P C (11-7) where PFLS is the fatigue-limit-state static pressure range presented in Table 11-3. For the structural element considered, Cd is the appropriate drag coefficient specified in Section 3, “Loads,” and shall be based on the yearly mean wind velocity, Vmean. The combined wind effect pressure range shall be applied in the horizontal direction to the exposed area of all high-mast lighting tower components. Designs for combined wind shall consider the application of wind from any direction. The yearly mean wind velocity used in determining PFLS shall be as given in Figure 11-1. Designers are cautioned of the effects of topography when considering location-specific mean wind velocity in their design. These effects can cause considerable variation in wind speed. For locations with more detailed wind records, the yearly mean wind velocity may be modified at the discretion of the Owner. NCHRP Project 10-74 is the basis for fatigue loads identified in this section. Prior to 2012, these AASHTO specifications made no distinction between high-mast lighting towers and other signal, sign, or luminaire support structures. Failures of HMLTs resulting from wind-induced fatigue led to field testing, laboratory wind tunnel testing, and analytical studies to determine appropriate load models for the fatigue design of high-mast lighting towers. The fatigue-limit-state static pressure range values listed in Table 11-3 account for fatigue importance factors and variation in mean wind speed. The combined wind pressure range includes the cumulative fatigue damage effects of vortex shedding. Figure 11-1 serves as a broad guide for determining regional mean wind speed. Local conditions are known to vary and may not necessarily be represented by the map. NCHRP Report 412 and NCHRP 10-74 found the design method to be conservative in most cases; however, designers are encouraged to check local wind records and/or consider topographical effects in choosing a yearly mean wind speed for design if the local wind conditions are suspected to be more severe than suggested by Figure 11-1. Table 11-3–Fatigue-limit-state Pressure Range for HMLT Design, PFLS Yearly Mean Wind Velocity, Vmean Importance Category I II Vmean 9 mph 310 Pa (6.5 psf) 280 Pa (5.8 psf) 9 mph < Vmean 11 mph 310 Pa (6.5 psf) 310 Pa (6.5 psf) Vmean > 11 mph 340 Pa (7.2 psf) 340 Pa (7.2 psf)

B-18 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-16 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS Figure 11-1—Yearly Mean Wind Velocity, mph No separate load is specified to account for vortex shedding since it is incorporated in the combined wind pressure range for HMLTs, PCW used for fatigue design in Article 11.7.2. Where serviceability and maintenance requirements due to vortex shedding induced vibrations are an issue, devices such as strakes, shrouds, mechanical dampers, etc. may be used to mitigate the effect. High-mast lighting towers can be highly susceptible to vibrations induced by vortex shedding, leading to the rapid accumulation of damaging stress cycles (depending on the fatigue detail category selected) that lead to fatigue failure. Prior to 2012, HMLTs were included in Section 11.7.1.2. NCHRP Project 10-74 studied the response of these structures in the field and determined that the previous edition did not properly quantify vortex shedding. Rather than separate the effect of vortex shedding from all other wind phenomena, a loading spectrum was developed to encompass all typical wind load effects. The fatigue-limit- state static wind pressures listed in Table 11-3 represent this combined wind load effect. Maintenance and serviceability issues resulting from vortex shedding may have a detrimental effect of the performance of HMLTs. Issues with anchor bolts loosening and rattling of the luminaire have been known to occur. Where fatigue-prone details exist, which may shorten the life of HMLTs due to a lower fatigue resistance than initially considered, or in cases where an HMLT initially designed for a finite lifetime may wish to be extended, mitigation devices have proved reliable in reducing the number of damaging stress cycles. Information pertaining to the performance and sizing of strakes and shrouds on HMLTs is presented in NCHRP Report 10-74 and Reduction of Wind-Induced Vibrations in High-mast Light Poles (Ahearn and Puckett).

proposed Specification and Commentary B-19 S ECTION 11: F ATIGUE D ESIGN 11-17 11.8—DEFLECTION C11.8 Galloping and truck-gust-induced vertical deflections o f cantilevered single-ar m sign supports and traffic signal arms and noncantilevered supports should not be excessive so as to result in a serviceability problem , because mo torist s cannot clearly see the attachments or are concerned abou t passing under the structures. Because of the low le vels of stiffness and da mp i ng inherent in cantilevered single ma st arm sign and traffic signal support structures, even structures that are adequately designe d to resist fatigue damage may experience excessive verti cal deflections at the free end of the horizontal ma st arm . The p ri ma ry objective of th is provision is to mi ni m ize the numb e r of motorist complaints. N CHRP Re port 412 recommends that the total deflection at the free end of single-ar m sign supports and all t raffic signal ar ms be limited to 200 mm (8 in.) vertically, when the equivalent static design wind effect from galloping and truck-induced gusts are applied to the structure. NCHRP Report 494 recommends applying the 200-mm (8-in.) vertical limit to noncantilevered support structures. Double- member or t russ-type cantilevered horizontal sign supports were not required to have vertical deflections checke d b ecause of their inherent s tiffness. There are no provisions for a displacement limitation in the horizontal direction.

B-20 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-18 S TA ND AR D S PECIFICA TIONS FOR S TRUCTUR AL S UPPORTS FOR H IGHWA Y S IGN S , L UM IN AI RE S , A ND T RA FFIC S IG NA LS 11.9—FATIGUE RESISTANCE C11.9 The allowable CAFLs are provided in Table 11-3 Table 11-5 . A summary of the ty pical fatigue-sensitive connectio n details are presented in Table 11-2 Table 11- 4 a nd illustrat e d in Figure 11-1 11-2 . Wi nd loads of Article 11.7 shall be considered in com puting the fatigue stress range. Unless noted in Table 11-2 Table 11-4 , the me mb e r cross-section adjacent to the weld toe shall be used to com pute the nominal stress range. The CAFLs were established b ased on fatigue testing and the resistances were computed based on elastic section analysis, i.e., no mi nal values in the cross-section. Therefore, it is assumed that these resistances include effects of residual stresses due to fabrication, ou t-of-plane distortions, etc. A t this time, only stress range due to wind is used; therefore, dead load effects may be neglected. Residual stresses and anchor bolt pretension are generally not considered in the com putations. Table 11-2 Table 11-4 —Fatigue Details of Cantilevered and Noncantilevered Support Structures Construction Detail Stress Category Application Example Plain Members 1. With rolled or cleaned surfaces. Flame-cut edges with ANSI/AASHTO/AWS D5.1 (Article 3.2.2) smoothness of 1000 µ-in. or less. A — — 2. Slip-joint splice where L is greater than or equal to 1.5 diameters. B High-level lighting poles. 1 Mechanically Fastened Connections 3. Net section of fully tightened, high-strength (ASTM A 325, A 490) bolted connections. B Bolted joints. 2 4. Net section of other mechanically fastened connections: a. Steel: b. Aluminum: D E — 3 5. Anchor bolts or other fasteners in tension; stress range based on the tensile stress area. Misalignments of less than 1:40 with firm contact existing between anchor bolt nuts, washers, and bas e plate. D Anchor bolts. Bolted mast-arm-to- column connections. 8 , 16 6. Connection of members or attachment of miscellaneous signs, traffic signals, etc. with clamps or U-bolts. D — — Holes and Cutouts 7. Net section of holes and cutouts. D Wire outlet holes. Drainage holes. Unreinforced handholes. 5 Continued on next page Groove Welded Connections 8. Tubes with continuous full- or partial- penetration groove welds parallel to the direction of the applied stress. B Longitudinal seam welds. 6 9. Full-penetration groove-welded splices with welds ground to provide a smooth transition between members (with or without backing ring removed). D Column or mast arm butt-splices. 4 10. Full-penetration groove-welded splices with weld reinforcement not removed (with or without backing ring removed). E Column or mast arm butt-splices. 4 11. Full-penetration groove-welded tube-to- transverse plate connections with the backing ring attached to the plate with a full- penetration weld, or with a continuous fillet weld around interior face of backing ring. The thickness of the backing ring shall not exceed 10 mm (0.375 in.) when a fillet weld attachment to plate is used. Full-penetration groove-welded tube-to-transverse plate connections welded from both sides with backgouging (without backing ring). E Column-to-base-plate connections. Mast-arm-to-flange-plate connections. 5 12. Full-penetration groove-welded tube-to- E Column-to-base-plate 5

proposed Specification and Commentary B-21 S ECTION 11: F ATIGUE D ESIGN 11-19 Construction Detail Stress Category Application Example transverse plate connections with the backing ring not attached to the plate with a continuous full-penetration weld, or with a continuous interior fillet weld. connections . Mast-arm-to-flange-plate connections . Fillet-Welded Connections 13. Fillet-welded lap splices. E Column or mast arm lap splices. 3 14. Members with axial and bending loads with fillet-welded end connections without notches perpendicular to the applied stress. Welds distributed around the axis of the member so as to balance weld stresses. E Angle-to-gusset connections with welds terminated short of plate edge. Slotted tube-to-gusset connections with coped holes . e 2, 6 15. Members with axial and bending loads with fillet-welded end connections with notches perpendicular to the applied stress. Welds distributed around the axis of the member so as to balance weld stresses. E Angle-to-gusset connections . Slotted tube-to-gusset connections without coped holes. 2, 6 16. Fillet-welded tube-to-transverse plate connections. j E Column-to-base-plate or mast-arm-to-flange-plate socket connections. 7, 8, 16 17. Fillet-welded connections with one-sided welds normal to the direction of the applied stress. E Built-up box mast-arm- to-column connections. 8, 16 18. Fillet-welded mast-arm-to-column pass- through connections. E f Mast-arm-to-column pass-through connections . 9 Continued on next page 19. Fillet-welded T-, Y-, and K-tube-to-tube, angle-to-tube, or plate-to-tube connections. a, b Chord-to-vertical or chord-to-diagonal truss connections.a Mast-arm directly welded to column. b Built-up box connection.b 8, 10, 11 25. Fillet-welded ring-stiffened box-to-tube connection. g Ring-stiffened built-up box connections. 16 Attachments 20. Longitudinal attachments with partial- or full- penetration groove welds, or fillet welds, in which the main member is subjected to longitudinal loading: L < 51 mm (2 in.): 51 mm (2 in.) L 12t and 102 mm (4 in.): L > 12t or 102 mm (4 in.) when t 25 mm (1 in.): C D E Reinforcement at handholes. 13 21. Longitudinal attachments with partial- or full- penetration groove welds, or fillet welds in which the main member is subjected to longitudinal loading. E Weld terminations at ends of longitudinal stiffeners. h, i 12, 14 22. Detail 22 has been intentionally removed. 23. Transverse load-bearing fillet-welded attachments where t 13 mm (0.5 in.) and the main member is subjected to minimal axial and/or flexural loads. (When t > 13 mm C Longitudinal stiffeners welded to base plates. 12, 14

B-22 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-20 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS Construction Detail Stress Category Application Example [0.5 in.], see note d.) 24. Transverse load-bearing longitudinal attachments with partial- or full-penetration groove welds or fillet welds, in which the nontubular main member is subjected to longitudinal loading and the weld termination embodies a transition radius that is ground smooth: 51 mm(2in.) 51 mm(2in.) R R D E c Gusset-plate-to-chord attachments. 15 Notes: a Stress Category ET with respect to stress in branching member provided that r/t 24 for the chord member. When r/t > 24, then the fatigue strength equals: 0.7 24ET n n F F r t where: ET n F is the CAFL for Category ET. Stress Category E with respect to stress in chord. b Stress Category ET with respect to stress in branching member. Stress Category K2 with respect to stress in main member (column) provided that: r/tc 24 for the main member. When r/tc > 24, then the fatigue strength equals: 0.7 2 24 n c KF F n r t where: 2KF n is the CAFL for Category K2. The nominal stress range in the main member equals (SR) main member = (SR) branching member (tb/tc) where tb is the wall thickness of the branching member, tc is the wall thickness of the main member (column), and is the ovalizing parameter for the main member equal to 0.67 for in-plane bending and equal to 1.5 for out-of-plane bending in the main member. (SR) branching member is the calculated nominal stress range in the branching member induced by fatigue design loads. (See commentary of Article 11.5.) The main member shall also be designed for Stress Category E using the elastic section of the main member and moment just below the connection of the branching member. c First check with respect to the longitudinal stress range in the main member per the requirements for longitudinal attachments. The attachment must then be separately checked with respect to the transverse stress range in the attachment per the requirements for transverse load-bearing longitudinal attachments. d When t > 13 mm (0.5 in.), the fatigue strength shall be the lesser of Category C or the following:

proposed Specification and Commentary B-23 S ECTION 11: F ATIGUE D ESIGN 11-21 1 6 0.094 1.23 MPa c p n p H t F F t 1 6 0.0055 0.72 ksi c p n p H t F F t where c n F is the CAFL for Category C , H is the effective weld throat (mm, in.), and t p is the attachment plate thickness (mm, in.). e The diameter of coped holes shall be the greater of 25 mm (1 in.), twice the gusset plate thickness, or twice the tube thickne ss. f In addition to checking the branching member (mast arm), the main member (column) shall be designed for Stress Category E usin g the elastic section of the main member and moment just below the connection of the branching member (mast arm). g Stress Category E with respect to stress in branching me mber (ring-stiffened built-up box connection). The main member shall be designed for Stress Category E using the elastic section of the main member and moment just below the connection of the branchi ng me mber. h Only longitudinal stiffeners with lengths greater than 102 mm (4 in.) are applicable for Detail 21. On column-to-base-plate or mast - arm-to-flange plate socket connections having a wall thickness greater than 6 mm (0.25 in.) that have exhibited satisfactory fi eld performance, the use of stiffeners having a transition radius or taper with the weld termination ground smooth ma y be designed at a higher stress category with the approval of the Owner. Under this exception, the Owner shall establish the stress category to w hich the detail shall be designed. See commentary for Article 11.5. i Nondestructive weld inspection should be used in the vicinity of the weld termination of longitudinal stiffeners. Grinding of weld terminations to a smooth transition with the tube face is not allowed in areas with fillet welds or partial-penetration welds c onnecting the stiffener to the tube. Full-penetration welds shall be used in areas where grinding ma y occur. See commentary for Article 1 1.5. j Fillet welds for socket connections (Detail 16) shall be unequal leg welds, with the long leg of the fillet weld along the col umn or ma st arm. The termination of the longer weld leg should contact the shaft’s surface at approximately a 30º angle.

B-24 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-22 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS Table 11-3 Table 11-5—Constant-Amplitude Fatigue Limits Detail Category Steel Aluminum MPa ksi MPa ksi A 165 24 70 10.2 B 110 16 41 6.0 B 83 12 32 4.6 C 69 10 28 4.0 D 48 7 17 2.5 E 31 4.5 13 1.9 E 18 2.6 7 1.0 ET 8 1.2 3 0.44 K2 7 1.0 2.7 0.38

proposed Specification and Com m entary B-25 SECTION 11: FATIGUE DESIGN 11-23 Continued on next page Figure 11-1 11-2—Illustrative Examples

B-26 Fatigue Loading and design M ethodology for High-M ast Lighting Tow ers 11-24 Figure 11-111-2—Illustr ative Examples— Continued STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS, LUMINAIRES, AND TRAFFIC SIGNALS Continued on next page

proposed Specification and Com m entary B-27 SECTION11:FATIGUE DESIGN Figure 11-111-2—Illustrative Examples—Continued Co 11-25 ntinued on next page

B-28 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-26 F i gure 11-1 11-2 — L > STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS,LUMINAIRES, AND TRAFFIC SIGNALS Illustrative Examples—Continued 102 mm (4 in ) Longitudinal Attachment Stiffener thickness = t Example 12 ∆σ Full-Penetration, Partial-Penetration or Fillet Weld (Detail 21) Fillet Weld (Detail 23) Note: Tube-to-transverse plate connections (Details 11, 12, and 16) checked using combined moment of inertia of tube and stiffeners Continued on next page

proposed Specification and Commentary B-29 SECTION 11: FATIGUE DESIGN Figure 11-111-2—Illustrative Examples—Continued L > Smoo 102 mm (4 in) th Transition to T Stiffener th ube Longitudina Exam ∆σ α ickness = t Full-P or Fil R Fi (D l Attachment ple 14 enetration, Parti let Weld (Detail 2 llet Weld etail 23) Note: Tube-to-tr (Details 1 combined and stiffen al-Penetration 1) ansverse plate co 1, 12, and 16) che moment of inerti ers nnections cked using a of tube 11-27

B-30 Fatigue Loading and design Methodology for High-Mast Lighting Towers 11-28 STANDARD SPECIFICATIONS FOR STRUCTURAL SUPPORTS FOR HIGHWAY SIGNS,LUMINAIRES, AND TRAFFIC SIGNALS 11.10—REFERENCES AASHTO. 2002. AASHTO Standard Specifications for Highway Bridges, 17th Edition, HB-17. American Association of State Highway and Transportation Officials, Washington, DC. Ahearn, E.B., and J.A. Puckett. 2010. “Reduction of Wind-Induced Vibrations in High-mast Light Poles,” Report No. FHWA-WY-10/02F, University of Wyoming, Laramie, WY. Albert M. N., L. Manuel, K. H. Frank, and S. L. Wood. 2007. Field Testing of Cantilevered Traffic Signal Structures under Truck-Induced Gust Loads, Report No. FHWA/TX-07/4586-2. Center for Transportation Research, Texas Department of Transportation, Austin, Texas. Amir, G., and A. Whittaker. 2000. “Fatigue-Life Evaluation of Steel Post Structures II: Experimentation,” Journal of Structural Engineering. American Society of Civil Engineers, New York, NY. Vol. 126, No. 3, Vol. 2 (March 2000), pp. 331–340. Connor, R.J., S.H. Collicott, A.M. DeSchepper, R.J. Sherman, and J.A. Ocampo. “Development of Fatigue Loading and Design Methodology for High-Mast Lighting Towers,” NCHRP Project 10-74. Purdue University, West Lafayette, IN (2011). Cook, R. A., D. Bloomquist, A. M. Agosta, and K. F. Taylor. 1996. Wind Load Data for Variable Message Signs, Report No. FL/DOT/RMC/0728-9488. University of Florida, Gainesville, FL. Report prepared for Florida Department of Transportation. Creamer, B. M., K. G. Frank, and R. E. Klingner. 1979. Fatigue Loading of Cantilever Sign Structures from Truck Wind Gusts, Report No. FHWA/TX-79/10+209-1F. Center for Highway Research, Texas State Department of Highways and Public Transportation, Austin, TX. Dexter, R. J., and K. W. Johns. 1998. Fatigue-Related Wind Loads on Highway Support Structures: Advanced Technology for Large Structural Systems, Report No. 98-03. Lehigh University, Bethlehem, PA. Dexter, R., and M. Ricker. 2002. Fatigue-Resistant Design of Cantilever Signal, Sign, and Light Supports, NCHRP Report 469. Transportation Research Board, National Research Council, Washington DC. Fisher, J. W., A. Nussbaumer, P. B. Keating, and B. T. Yen. 1993. Resistance of Welded Details Under Variable Amplitude Long-Life Fatigue Loading, NCHRP Report 354. Transportation Research Board, National Research Council, Washington, DC. Florea M. J., L. Manuel, K. H. Frank, and S. L. Wood. 2007. Field Tests and Analytical Studies of the Dynamic Behavior and the Onset of Galloping in Traffic Signal Structures, Report No. FHWA/TX-07/4586-1. Center for Transportation Research, Texas Department of Transportation, Austin, Texas. Fouad, F., et al. 2003. Structural Supports for Highway Signs, Luminaries, and Traffic Signals, NCHRP Report 494. Transportation Research Board, National Research Council, Washington, DC. Kaczinski, M. R., R. J. Dexter, and J. P. Van Dien. 1998. Fatigue Resistant Design of Cantilevered Signal, Sign and Light Supports, NCHRP Report 412. Transportation Research Board, National Research Council, Washington, DC. Koenigs, M. T., T. A. Botros, D. Freytag, and K. H. Frank. 2003. Fatigue Strength of Signal Mast Arm Connections, Report No. FHWA/TX-04/4178-2. Center for Transportation Research, Texas Department of Transportation, Austin, TX. McDonald, J. R., et al. 1995. Wind Load Effects on Signals, Luminaires and Traffic Signal Structures, Report No. 1303-1F. Wind Engineering Research Center, Texas Tech University, Lubbock, TX. NOAA. 2010. “Comparative Climatic Data for the United States Through 2010,” National Climatic Data Center, Asheville, NC.

Abbreviations and acronyms used without definitions in TRB publications: AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S.DOT United States Department of Transportation