9.2 Seismic Loads Using ASCE Standard 7-93

Transcription

1 CHAPER 9: Wnd and Sesmc Loads on Buldngs 9.2 Sesmc Loads Usng ASCE Standard 7-93 Descrpton A major porton of the Unted States s beleved to be subject to sesmc actvty suffcent to cause sgnfcant structural damage although many of these areas do not have any sesmc desgn code requrements. Chapter 9 of Amercan Socety of Cvl Engneers Standard 7-93 has comprehensve requrements for sesmc desgn applcable throughout the Unted States. he sesmc requrements of ASCE Standard 7-93 are based on the 99 edton of NEHRP Recommended Provsons for the Development of Sesmc Regulatons for New Buldngs. hs applcaton computes the sesmc forces, shears, and overturnng moments at each level of a buldng, followng the Equvalent Lateral Force Procedure of Secton 9.4 of ASCE Standard he buldng must meet the lmtatons for use of ths procedure gven n secton of ASCE Standard 7-93 he requred nput ncludes the effectve peak acceleraton coeffcent, the effectve peak velocty-related acceleraton coeffcent, ste coeffcent, response modfcaton coeffcent, calculated perod of vbraton (optonal), numercal coeffcent used n determnng the approxmate perod of vbraton, story heghts, and the sesmc dead loads of each story. A summary of nput and computed varables s shown on pages 7 and 8. Reference: ASCE Standard 7-93 "Mnmum Desgn Loads for Buldngs and Other Structures."

2 Input Notaton Input Varables Effectve peak acceleraton coeffcent determned from Map 9- of ASCE Standard 7-93: A a 0.5 Effectve peak velocty-related acceleraton coeffcent determned from Map 9-2 of ASCE Standard 7-93: A v 0. Ste coeffcent determned from able 9.3- of ASCE Standard 7-93: S.0 Response modfcaton coeffcent determned from able of ASCE Standard 7-93: R 4.5 he user must verfy that the structural system lmtatons and heght lmts of able are not exceeded for the sesmc performance category.

3 Fundamental perod of vbraton of the structure n the drecton under consderaton establshed usng the structural propertes and deformatonal characterstcs of the resstng elements n a properly substantated analyss, ASCE 7-93, Enter = 0 sec f the fundamental perod of vbraton has not been calculated: 0 sec Factor C determned from ASCE 7-93, secton , wth unts added: C Story heghts: s ([ ]) Sesmc dead load at each story: w [ ] Computed Varables n h w W F V M Mf numercal subscrpt desgnatng the top level range varable subscrpt desgnatng levels through n heght of each level above ground level sesmc dead load at each story total sesmc dead load lateral sesmc force appled at each level lateral shear force at each story overturnng moment reducton factor overturnng moment n each story foundaton overturnng moment ORIGIN set equal to to agree wth customary usage:

4 Calculatons Number of stores: n length (s) n = 2 Heght of each story above ground: n x n h x ( x ) s x h = [ ] ft otal sesmc dead load: W w W = kp otal buldng heght: h = 49.5 ft n Approxmate fundamental perod from ASCE 7-93, Eq. (9.4-4): 3 4 a h =.496 n C Functons of Av for lnear nterpolaton of values of Ca n ASCE 7-93, able 9.4. f A v 2. 4 A v f2 A v.8 2 A v f3 A v.6 A v Coeffcent for upper lmt on calculated fundamental perod of vbraton of the structure from ASCE 7-93, able 9.4.: C a A v 0.,.7, A v 0.5, f A v, A v 0.2, f2 A v, 0.4,.2, f3 A v C a =.66 Maxmum permssble calculated fundamental perod for use n ASCE 7-93, Equaton (9.4.2)): C a a = A v Sesmc coeffcent, calculated from ASCE 7-93 Eq. (9.4-2) f the perod of vbraton has been calculated, or from Eq. (9.4-3) f the the perod of vbraton has not been calculated: C s =0, 2.5 A a.2 A, v S = R R < C a a,, C a a

5 Sesmc base shear, ASCE 7-93, Eq. 9.4-: V C = s W Perod of vbraton for use n determnng the value of exponent k for use n ASCE 7-93, Eq. (9.4-6): ' =0, a, < C a a,, C a a =.496 he value of ' s equal to a f s not calculated,, f s less than Caa, or Caa f s greater than Caa. Exponent k, related to the buldng perod as shown n ASCE 7-93, 9.4.3: ' 0.5 k ' < 0.5,, = ' 2.5 sec, 2, he value of k s lnearly nterpolated between and 2 for values of ' between 0.5 and 2.5 seconds. Vertcal dstrbuton factor Cv at levels through n, ASCE 7-93, Eq. (9.4-6), wth the formula modfed to gve the dstrbuton factor at all levels: C v h w SIUntsOf (h) k n h w = SIUntsOf (h) k C v = [ ] Lateral sesmc force nduced at any level, ASCE 7-93, Eq. (9.4-5), wth formula modfed to gve the forces at all levels: F Cv V F = [ ] kp

6 Sesmc desgn story shear n each story, ASCE 7-93, Eq. (9.4-7), wth the formula modfed to gve the shear at all levels: V x ( x) F V = [ ] kp Overturnng moment reducton factor, (ASCE 7-93, 9.4.5): τ ( n 9,.0, ( ( n 9),.0 ( n 9 ) 0.02, 0.8)) τ = [ ] Overturnng moment n each story, ASCE 7-93, Eq. (9.4-9), wth formula modfed to gve overturnng moment at all levels: M x τ x ( x) F h h x M = [ ] kp ft Foundaton overturnng moment, ASCE 7-93, 9.4.5: M f 0.75 F h M f = 80403

7 Summary Enter structural system descrpton "Ordnary moment frame of steel" (ASCE 7-93, able ) Effectve peak acceleraton coeffcent (ASCE 7-93, Map 9-): Effectve peak velocty-related acceleraton coeffcent (ASCE 7-93, Map 9-2): A a = 0.5 A v = 0. Ste coeffcent (ASCE 7-93, able 9.3-): S = Calculated perod of vbraton, = 0 sec f not calculated: = 0 Foundaton overturnng moment, ASCE 7-93, 9.4.5: M f = kp ft Response modfcaton coeffcent (ASCE 7-93, able 9.3.2): R = 4.5 Factor C (ASCE 7-93, ): C = Level numbers above ground, story heghts s, heght h of each level above ground, sesmc dead load w per story, force F at each level, story shear V at each level, overturnng reducton factor at each level, and overturnng moment M at each level: s = h = w =

8 F = kp = V kp = τ M = kp ft