Calibration of AASHTO LRFD Concrete Bridge Design Specifications for Serviceability (2014) / Chapter Skim
Currently Skimming:
Pages 154-214
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 155... ...
Table of Contents A.1 Approach ................................................................................................................. A-4 A.2 Questionnaire of Bridge Owners............................................................................
|
From page 156... ...
List of Tables Table A-1 Existing Service Limit States in AASHTO LRFD ................................................... A-21 Table A-2 Recommended Minimum Depth of Concrete Structures in 1989 AASHTO ...........
|
From page 157... ...
A. A.1 Approach As part of Phase I of the project an assessment of the current state of the art related to service limit states was conducted as follows: • A survey of Bridge owners was conducted and is presented in Section A2.
|
From page 158... ...
Twenty nine responses were received. These responses came from: Alabama Department of Transportation Alaska Department of Transportation Arizona Department of Transportation Arkansas Department of Transportation California Department of Transportation Florida Department of Transportation Hawaii Department of Transportation Kansas Department of Transportation Kentucky Department of Transportation Louisiana Department of Transportation Michigan Department of Transportation Minnesota Department of Transportation Mississippi Department of Transportation Missouri Department of Transportation Montana Department of Transportation New Mexico Department of Transportation New York Department of Transportation North Carolina Department of Transportation Oklahoma Department of Transportation Pennsylvania Department of Transportation South Carolina Department of Transportation South Dakota Department of Transportation Texas Department of Transportation Virginia Department of Transportation Washington Department of Transportation Wisconsin Department of Transportation Wyoming Department of Transportation Kansas Turnpike Authority Ontario Ministry of Transport Most responses included answering all questions.
|
From page 159... ...
The modifications to the HL 93 loading included: California: The application of the tandem load for negative moment near, and reaction of, interior supports is modified. 100% of two tandems spaced 26 to 40 ft.
|
From page 160... ...
New York: The New York Permit Vehicle is an eleven-axle vehicle. The total weight of the vehicle is 220 kips and the distance between the front and rear axles is 51 ft.
|
From page 161... ...
Hawaii: No tensile stresses allowed in precompressed tensile zone after all losses have occurred except when computing load capacity ratings at the operating level and for Legal and permit loads Kansas: Allowed tensile stress in concrete is 0.0948√f′c (ksi) for inventory rating and 0.19√f′c (ksi)
|
From page 162... ...
__7__ Current requirements with modifications (Please specify revisions below) __2__ Pre-2005 Interim requirements (the Z equation)
|
From page 163... ...
If Yes, please specify: Among the six responses indicating checking bridges under overloads for the service limit state, only one response indicated which limit states the bridges are checked for: In Kansas, bridges are checked for fatigue and crack control under overloads.
|
From page 164... ...
__1__ Immediately after construction __5__ Within 1 year of construction __3__ 2 to 5 years after construction _____ More than 5 years after construction _13__ Not sure What type of cracks and in bridges of what age group (e.g. bridges constructed before 1970's)
|
From page 165... ...
Other types of reported cracking included: Arkansas: Vertical cracks over piers of simple spans made continuous for bridges from between 1973 and 2005. Florida: Insignificant longitudinal cracks in upper/lower flanges predominately over bearing areas, Hairline diagonal cracks in the web, running from the top of the web near beam end diagonally 3 to 4 feet in length.
|
From page 166... ...
0% 23 Responses 1%-20% 1 Response 20% to 40% 1 Response 40% to 60% 60% to 80% 80% to 99% 2 Response 100% Did decks designed using the empirical design method perform as good as traditionally designed decks? Yes 4 No 1 If No, please explain in what way: More cracking than conventional decks was cited as the reason for the negative answer above.
|
From page 167... ...
Generally, it appears that the 12" maximum reinforcement spacing allowed by most jurisdictions for a wide range of concrete components is enforced for decks designed using the empirical method. It also appears that tighter reinforcement spacing is sometimes used in an attempt to minimize deck cracking problems.
|
From page 168... ...
Is the deck cracking type and extent in decks designed using the empirical design method different from cracking in decks designed using the traditional method? Yes 1 No 3 Not Applicable 23 (Empirical deck design not used)
|
From page 169... ...
Range of deck life span from 25 years to full bridge life was given. No general trend could be deduced.
|
From page 170... ...
clear whether culverts are counted in determining reinforced concrete bridges causing them to appear the dominant type.. It appears that the type of construction is mostly a function of past practice and the sections produced by local/regional precasters.
|
From page 171... ...
Generally, it appears that problems with bearings on concrete bridges are not wide spread. Except for the steel sliding bearings cited above, when bearing problems arise, they do not seem to cause significant damage to the girders.
|
From page 172... ...
18. What is the average service life span of the concrete substructures under your agency's jurisdiction ____ Years One respondent cited a life span of 40 years for substructures in salt water.
|
From page 173... ...
A.2.3 Lessons Learned from the Questionnaire The responses to the questionnaire indicated that most bridge owners apply the service limit states included in AASHTO LRFD with no, or with few, revisions. The additional limit states used by bridge owners appeared to be related either to owner-specified vehicles, or to address a specific issue that does not seem to be shared by other bridge owner as evident by the lack of use of these additional limit states by other owners.
|
From page 174... ...
Table A-1 Existing Service Limit States in AASHTO LRFD AASHTO LRFD Article Basic Provision 2.5.2.6.2 Criteria for Deflection In the absence of other criteria, the following deflection limits may be considered for steel, aluminum, and/or concrete construction: Vehicular load, general - Span/800, Vehicular and/or pedestrian loads - Span/1000, Vehicular load on cantilever arms - Span/300, and Vehicular and/or pedestrian loads on cantilever arms - Span/375. 3.4.1 and 3.6.1.4 Fatigue Fatigue truck and load factors in Table 3.4.1-1.
|
From page 175... ...
AASHTO LRFD Article Basic Provision 5.5.3.4 Welded or Mechanical Splices of Reinforcement "For welded or mechanical connections that are subject to repetitive loads, the range of stress, f f, shall not exceed the nominal fatigue resistance given in Table 1." 5.6.3.6 Crack Control Reinforcement "The ratio of reinforcement area to gross concrete area shall not be less than 0.003 in each direction." 5.7.3.4 Control of Cracking by Distribution of Reinforcement "The spacing s of mild steel reinforcement in the layer closest to the tension face shall satisfy the following: 700 2e c s ss s d f γ β ≤ − " "If the effective depth, de, of nonprestressed or partially prestressed concrete members exceeds 3.0 ft., longitudinal skin reinforcement shall be uniformly distributed along both side faces of the component for a distance de/2 nearest the flexural tension reinforcement. The area of skin reinforcement Ask in in.2/ft.
|
From page 176... ...
AASHTO LRFD Article Basic Provision 5.9.3 Stress Limitations for Prestressing Tendons "The tendon stress due to prestress or at the service limit state shall not exceed the values: • Specified in Table 1, or • Recommended by the manufacturer of the tendons or anchorages." 5.9.4.1.1 Compression Stresses "The compressive stress limit for pretensioned and posttensioned concrete components, including segmentally constructed bridges, shall be 0.60 f′ci (ksi) ." 5.9.4.1.2 Tension Stresses "The limits in Table 1 shall apply for tensile stresses." 5.9.4.2.1 Compression Stresses "Compression shall be investigated using the Service Limit State Load Combination I specified in Table 3.4.1-1.
|
From page 177... ...
AASHTO LRFD Article Basic Provision 5.10.8 Shrinkage and Temperature Reinforcement "For bars or welded wire fabric, the area of reinforcement per foot, on each face and in each direction, shall satisfy: ( ) 1.30 2s y bh A b h f ≥ + 0.11 0.60 s A≤ ≤ " 5.10.10.1 Splitting Resistance "The splitting resistance of pretensioned anchorage zones provided by reinforcement in the ends of pretensioned beams shall be taken as: r s sP f A= with the stress in steel not to exceed 20 ksi 5.14.2.3.3 Construction Load Combinations at the Service Limit State "Flexural tension and principal tension stresses shall be determined at service limit states as specified in Table 1, for which the following notes apply: • Note 1: equipment not working, • Note 2: normal erection, and • Note 3: moving equipment.
|
From page 178... ...
deemed uncalibrateable or "deemed to satisfy". The literature search yielded information on the following limit states and the information is summarized in the following sections:: • live load deflection of structures, • fatigue of rebar and prestressing strands, • cracking of reinforced concrete components, • tensile stresses of prestressed concrete components, • compressive stresses of prestressed concrete components, As stated in Chapter 1, these limit states and the associated load and resistance factors for SLS are based on apparent successful past practice and have not been subject to a reliability-based calibration.
|
From page 179... ...
• Damping characteristics of bridge and vehicle The use of depth-to-span ratios began in the early 1900's with the American Railway Engineering and Maintenance of Way Association (AREMA) specification (at that time AREA)
|
From page 180... ...
equally divided between those that concluded that girder flexibility affects deck cracking and those that concluded that girder flexibility does not affect deck cracking. As indicated by some of the studies, concrete material factors may be more important to reduce the formation of earlyage deck cracks.
|
From page 181... ...
dynamics but also have adjustments for apparent behavior in the field. Modern refined computational methods make the determination of frequencies and mode shapes relatively straightforward.
|
From page 182... ...
Finite-life Fatigue This limit state is not used for concrete structures and, therefore, is not discussed in this report. Limited information related to revisions to the finite-life fatigue (Fatigue II limit state)
|
From page 183... ...
For this study, the research used to define these S-N curves, Ople and Hulsbos (1966) , was re-evaluated to estimate the fatigue resistance to about 108 cycles (100 million)
|
From page 184... ...
Prestressing Tendons Fully prestressed components satisfying the tensile stress limits specified in AASHTO LRFD Table 5.9.4.2.2-1 at the Service III limit-state load combination are exempt from fatigue considerations. (The Service III limit-state load combination and its calibration is discussed in other sections of this report)
|
From page 185... ...
NCHRP Research Results Digest 197 (1994) found that there is substantial uncertainty in the fatigue performance of different types of welds and connectors much as structural-steel details.
|
From page 186... ...
However, for calibration purposes, these equations were evaluated with regard to accuracy and applicability. The results from various equations were compared and validated using data collected from available literature.
|
From page 187... ...
A = average effective concrete area around reinforcing bar, having same centroid as reinforcement, in² sf = steel stress calculated by elastic crack section theory, ksi bw = maximum crack width, 0.001 in. β = ratio of distances to neutral axis from extreme tension fiber and from centroid of reinforcement Broms (1965)
|
From page 188... ...
Based on the physical model, the equation to calculate the maximum crack width for uncoated reinforcement was developed as shown below (Frosch, 1999)
|
From page 189... ...
s = maximum spacing of reinforcement, in. sα = reinforcement factor cγ = reinforcement coating factor: 1.0 for uncoated reinforcement; 0.5 for epoxy-coated reinforcement, unless test data can justify a higher value Frosch (2001)
|
From page 190... ...
In order to prevent excessive cracks throughout the depth of the section, maximum spacing of the reinforcement should be determined. According to Frosch (2002)
|
From page 191... ...
permitted to take sf as not more than 60 percent of the specified yield strength yf . s = maximum spacing of reinforcement, in.
|
From page 193... ...
but not greater than 40,000 12 sf , where cc is the least distance from surface of reinforcement or prestressing steel to the tension face. If there is only one bar or wire nearest to the extreme tension face, s used in Equation (A-17)
|
From page 194... ...
corrosive environments.
|
From page 195... ...
cuσ , tuσ = ultimate strengths of concrete under compression-tension biaxial state of stress Equation (A-23) is obtained by combining Equation (A-21)
|
From page 196... ...
allowance for losses, where pkf is characteristic tensile strength of prestressing steel. The exact meaning of "characteristic" tensile strength is not defined in EN1992-2 and is interpreted herein as the specified strength.
|
From page 197... ...
6 cif ′ for members with non-prestressed reinforcement provided to resist the tensile force in concrete; computed on the basis of an uncracked section. The 1963 Building Code Requirements for Reinforced Concrete (ACI Committee 318, 1963)
|
From page 198... ...
Table A-7 Tensile Stress Limits in Prestressed Concrete at SLS after Losses, Fully Prestressed Components (AASHTO LRFD, 2012, Table 5.9.4.2.2-1) Bridge Type Location Stress Limit Other Than Segmentally Constructed Bridges Tension in the precompressed Tensile Zone Bridges, Assuming Uncracked Sections For components with bonded prestressing tendons or reinforcement that are subjected to not worse than moderate corrosion condition.
|
From page 199... ...
Fatigue of Reinforcement of Fully Prestressed Components (AASHTO LRFD, Article 5.5.3.1) For fully prestressed components designed to have extreme fiber tensile stress due to Service III Limit State within the tensile stress limit specified in the AASHTO LRFD Table 5.9.4.2.2-1, the fatigue limit-state load factors, the girder distribution factors, and dynamic load allowance cause fatigue limit-state stress to be considerably less than the corresponding value determined from SLS III.
|
From page 200... ...
A.3.2 Eurocode The Eurocode contains the following sections to which reference is made in some other sections of this report. EN 1990 (Eurocode 0)
|
From page 201... ...
– The durability, or – The functioning of the structure. In the context of serviceability, the Eurocode considers the term "appearance" to be concerned with such criteria as high deflection and extensive cracking, rather than aesthetics (EN 1990, 2002)
|
From page 202... ...
Table A-9 Eurocode Consequence Classes (EN 1990, 2002, adapted from Table (B1)
|
From page 203... ...
Table A-11 Target Probabilities of Failure (pF) and Reliability Indices (βT)
|
From page 204... ...
Table A-13 SLS Combinations SLS Load Combinations Type Description Type Acceptance of Infringement Example Reversible those limit states that will not be exceeded when the actions which caused the infringement are removed frequent specified duration and frequency of infringements are accepted the crack-width limit state of a prestressed concrete beam with bonded tendons characterized by a 0.2 mm crack width quasi-permanent specified longterm infringement is accepted the crack-width limit state for a reinforced concrete or prestressed concrete beam with unbonded tendons characterized by a 0.3 mm crack width Irreversible those limit states that remain permanently exceeded even when the actions which caused the infringement are removed characteristic (5% probability of exceedance) no infringement accepted the crack-width limit state characterized by a 0.5 mm crack width, because such a wide crack cannot completely close once the loads that caused it are removed A.3.2.3 Serviceability Design Basic Approach A.3.2.3.1 Basic Equation The basic equation in the Eurocode (EN 1990, 2002)
|
From page 205... ...
Cd = the limiting design value of the relevant serviceability criterion. Ed = the design value of the effects of actions specified in the serviceability criterion, determined on the basis of the relevant combination.
|
From page 206... ...
Effect of Action (E) Effect of actions (or action effect)
|
From page 207... ...
Table A-14 Recommended Values of Ψ Factors for Highway Bridges in the Eurocode (EN 1990, 2002, adapted from Table A2.1) Action Symbol Ψ0 Ψ1 Ψ2 Traffic Loads (EN 1991-2, 2003 , Table 4.4)
|
From page 208... ...
• 0,80 for gr1a (LM1)
|
From page 209... ...
HL93 truck, alone, i.e., without the Unified Distribution Load (UDL)
|
From page 210... ...
reviewed to determine the usefulness of the information. Any methods that could potentially be used in creating new SLSs were noted and investigated further.
|
From page 211... ...
ACI Committee 318.
|
From page 212... ...
Choi, Y., and B
|
From page 213... ...
Helgason, T., J Hanson, N
|
From page 214... ...
Standard Specifications for Highway Bridges, 15th ed.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.