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GROUND MOTION MP STRUCTURAL RESPONSE DETERMINATION OF SHOCK SPECTRA The dynamics of ground shocks and a i r shocks associated with nuclear explosions i n the atmosphere were well understood because of e a r l i e r studies with respect to Atlas, Titan, and Minuteman weapon systems. The f a m i l i a r quadra-log plottmgs of f r e e - f i e l d shock response spectra were available f o r various design overpressures. The "ps i " overpressure was not considered to be a simple dynamic load factor. Instead, equipment suppliers were assumed capable of making the necessary dynamic response analysis of t h e i r structures and equipments. The attenuation factors available f o r some of the higher overpressures were not necessarily applicable to ABM environments. This anticipated problem area was resolved by the following suggestion: Determine by computer-aided analyses the f r e e - f i e l d ground motion and pressures i n the outrunning regions of air-induced ground shock. Obtain results i n the form of shock spectra suitable f o r design of ABM i n s t a l l - ations and equipment. No important problems concerning the strength of equip- ment under shock were envisioned. However, i t was recognized that the normally available commercial-type equipment might have to be upgraded. A possible dividend would be that r e s i l i e n t mounting of large structures, or entire f l o o r areas, to mitigate ground shock would prove to be unnecessary. -36-
On the other hand, i n view of the precise power require- ments i t was considered that malfunction would occur i n sensitive high-speed relay or computer-type apparatus. This second a n t i c i - pated problem area was resolved by the following: Study the possible v u l n e r a b i l i t y of the kinds of equipment to be used i n the ABM power system subject to the shock response spectra deter- mined f o r the low overpressure levels under consideration. Consider the use of available shock-test machines and techniques f o r eval- uating the performance of special items of equipment under consideration. A separate and d i s t i n c t i n s t a l l a t i o n f o r f i e l d test purposes was not recommended. However, i t was considered h e l p f u l to s t a r t building the f i r s t prototype unit i n order to learn by experience what the remaining problem areas might be. This approach was preferred to prolonged concentration on detailed design specifications i n anticipation of simultaneous f u l l deploy- ment at numerous locations. I n view of the low overpressures i t was recommended that high-speed blast valves be avoided. Also, Navy-type ai r - b l a s t closures were ruled out because of multiple-shock considerations. The merits of a closed cycle f o r the a i r versus attenuation with hardened intakes led to an investigation of the l a t t e r problem as follows: Study by means of an experimental program the various blast attenuating devices applicable to the ABM system. This should include: -37-
(a) shock-tube tests of individual devices, (b) tests by means of scale models exposed to blast waves, and (c) steady-flow tests of scaled intake and exhaust systems. Using available shock-tube f a c i l i t i e s i t was recommended that tests be made of the cooling tower fan, a section of the finned heat exchanger, and models of the cooling tower. There was concern about the need to produce shock waves with positive phase of r e l a t i v e l y long time duration. Regarding reinforced concrete structures, i t was not considered r e l i a b l e to model the dynamic response of the prototype. Preliminary shock-tube tests were used as the basis f o r obtaining satisfactory mathematical models of the performance of a diesel engine and gas turbine subject to overpressure transients simulating an a i r blast shock. Successful computer simulations led to disposition of this second problem area as follows: Develop a special ABM shock-tube f a c i l i t y to produce the shock-front pressure waves and accompanying overpressure at the prime mover i n l e t and exhaust systems. Two tubes were necessary to provide the inlet-exhaust timing of the a i r blast e f f e c t s , thereby ensuring that precise power could be main- tained under appropriate conditions. The t o t a l amount of dust associated with nuclear detonation a i r blast led to the opinion that the maximum rate of dust intake could be much greater than o r i g i n a l l y estimated. This t h i r d problem was evaluated as follows: -38-
