Design Example 1 Special Moment Frame

Transcription

1 Design Exmple 1 pecil Moment Frme OVERVIEW tructurl steel specil moment frmes (MF) re typiclly comprised of wide-flnge bems, columns, nd bem-column connections. Connections re proportioned nd detiled to resist internl forces (flexurl, xil, nd sher) tht result from imposed displcement s result of wind or erthquke ground shking. Inelsticity nd energy dissiption re chieved through loclized yielding of the bem element outside of the bem-column connection. pecil proportioning nd detiling of this connection is essentil to chieving the desired inelstic behvior. The nticipted seismic behvior of the MF system is long-period, high-displcement motion, with well distributed inelstic demnd shred by ll prticipting bem-column connections. ystem yielding mechnisms re generlly limited to frme bems with the intent to invoke yielding t the bse of frme columns. In mny cses, engineers my model MF system with pin-bsed columns s significnt stiffness is required to yield the bse of lrge wide-flnge members. If yielding t the bse of the frme is desired to occur within the column section, the column might be extended below grde nd tied into bsement wll or ground-level bem, which is dded to crete bem-column connection. Economies of construction usully limit the size of bem nd column elements bsed on imposed displcement/drift limits. Design regultions for steel MF re promulgted in series of stndrds: ACE/EI 7, ANI/AIC 341, ANI/AIC 358, nd ANI/AIC 360. AIC 358 provides specific regultions relted to prequlifiction of certin MF connection types tht obvite project-specific testing required by AIC 341. This design exmple follows the provisions of AIC 358 for the RB connection type for the steel MF seismic-forceresisting system. The six-story steel office structure depicted in the figure bove hs lterl-force-resisting system comprising structurl steel specil moment frmes. The typicl floor frming pln is shown in Figure 1 1. A typicl frme elevtion is depicted in Figure 1 2. This design exmple utilizes simplifying ssumptions 2012 IBC EAOC tructurl/eismic Design Mnul, Vol. 4 1

2 Design Exmple 1 pecil Moment Frme for ese of clcultion or computtionl efficiency. Becuse by sizes vry, the exmple frmes cn be designed with different prticipting bys in ech direction, which will result in different sizes of bems nd columns for ech frme depending on loction. This exmple explores the design of single frme nd single connection of tht frme. Assumptions relted to bse-of-column rottionl restrint (ssumed fixed), pplied forces (tken from the bse exmple ssumptions), nd pplied wind force (not considered) re ll incorported into the exmple in silent considertion. Bem nd column element sizes were determined using liner elstic computer model of the building. These element sizes were determined through itertion such tht code-required drift limits, element chrcteristics, nd strength requirements were met. While this exmple is ccurte nd pproprite for the design of steel MF structures, different methodologies for nlysis, connection design, nd inelstic behvior cn be utilized, including the use of proprietry MF connection design. This exmple does not explore every possible option, nor is it intended to be integrted with other exmples in this document (i.e. Bse Plte Design, Pssive Energy Dissiption). OUTLINE 1. Building Geometry nd Lods 2. Clcultion of the Design Bse her nd Lod Combintions 3. Verticl nd Horizontl Distribution of Lod 4. MF Frme 5. Element nd RB Connection Design 6. Detiling of RB Connection 1. Building Geometry nd Lods 1.1 GIVEN INFORMATION Per Appendix A Office occupncy on ll floors Locted in n Frncisco, CA, t the ltitude nd longitude given ite Clss D 120 feet 150 feet in pln with typicl floor frming shown in Figure 1 1 Frme bem nd column sizes for lines 1 nd 5 (Figure 1 2) Bem nd column sizes will vry from those on lines A nd F ix-stories s shown in Figure 1 2 tructurl mterils Wide-flnge shpes ATM A992 (F y = 50 ksi) Ptes ATM A572, Grde 50 Weld electrodes E70X-XX IBC EAOC tructurl/eismic Design Mnul, Vol. 4

3 Design Exmple 1 pecil Moment Frme A B C D E F 30' 0'' = 150' 0'' Figure 1 1. Typicl fl oor frming pln A B C D E F ROOF TOP OF PARAPET 6th FLR 5th FLR 4th FLR 3rd FLR 2nd FLR 1st FLR Figure 1 2. Frme elevtion line 1 (line 2 in bckground) 2012 IBC EAOC tructurl/eismic Design Mnul, Vol. 4 3

4 Design Exmple 1 pecil Moment Frme 1.2 FLOOR WEIGHT For development of seismic forces per Appendix A: Tble 1 1. Development of seismic forces per Appendix A Level Typicl floor Roof Assembly Unit W t (psf) Are (ft 2 ) Weight (kips) Floor 78 15, Ext Wll Roof 36 15, Ext Wll/Prpet Floor W t (kips) W = 5(1320 kips) kips = 7256 kips 2. Clcultion of the Design Bse her nd Lod Combintions ACE CLAIFY THE TRUCTURAL YTEM AND DETERMINE PECTRAL ACCELERATION Per ACE 7 Tble for specil steel moment frme: R = 8.0 Ω o = 3 C d = DEIGN PECTRAL ACCELERATION The spectrl ccelertions to be used in design re derived in Appendix A: D = 1.00g D1 = 0.60g 2.3 DEIGN REPONE PECTRUM Determine the pproximte fundmentl building period, T, using ection : C t = nd x = 0.8 T T C h x = = 086 sec (see discussion below) Eq n t 08 T = 0.86 sec T o D1 = 02 = = 012 sec D IBC EAOC tructurl/eismic Design Mnul, Vol. 4

5 Design Exmple 1 pecil Moment Frme T T T D T = For T < T o D1 060 = = = 100 D D1 060 = = T T o Eq sec For T > T s. Eq The long-period eqution for does not pply here becuse the long-period trnsition occurs t 12 sec (from ACE 7 Figure 22 12). Design pectrl Accelertion, (g) T o =0.12sec = T T =0.60sec =0.60/T D =1.0g Period (ec) MF Building Period T =0.86sec, =0.70g T mx =1.20sec, =0.50g Figure 1 3. Design Response pectrum for the exmple building Figure 1 3 depicts the design spectrl ccelertion determined from T, which is greter thn T, so the design spectrl ccelertion is 0.70g. ACE 7 ection indictes tht the fundmentl period of the structure cn be estblished using the structurl properties nd deformtionl chrcteristics of the resisting elements in properly substntited nlysis, which might llow liner elstic modl nlysis to suffice. ection , however, limits the period tht cn be used to clculte spectrl ccelertion to vlue of T mx = C u T, where C u is fctor found in Tble In this cse T mx = = 1.20 sec. For preliminry design, the pproximte period, T, will be used to design the MF. As MF designs re hevily dependent on meeting drift requirements, the initil vlue (usully found to be much lower thn the period found through mthemticl modeling) will suffice for the first design itertion IBC EAOC tructurl/eismic Design Mnul, Vol. 4 5