Chapter 6. Seismic Code Requirements for Building Structures 2007 CBC & 2006 IBC Chapter 16 ASCE 7-05 Chapter 12. Topics to be covered

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1 esmc Desgn for Professonal Lcense (007 CBC/ ACE 7-05) Dr. hahn A. Mansour, PE Chapter 6 esmc Code Requrements for Buldng tructures 007 CBC & 006 IBC Chapter 16 ACE 7-05 Chapter 1 Topcs to be covered Bass for Desgn tructural ystems (Buldng Frames, hear Walls, & Dual) Actual and Desgn esmc Force electon of Lateral Force Procedure (ELF, Dynamc, and Alternatve mplfed Methods ) esmc Desgn Parameters (Road Map wth 19 Parameters) Base hear Calculatons (V) Dstrbuton of Base hear (F ) Redundancy Factor (ρ) Combnatons of tructural ystems Drft and Buldng eparatons P-Delta ( P ) Effects 15 ample Problems wth Detaled olutons 38 upplemental Practce Problems wth Detaled olutons Chapter 6- esmc Code Requrements for Buldng tructures 163

2 esmc Desgn for Professonal Lcense (007 CBC/ ACE 7-05) Dr. hahn A. Mansour, PE Chapter 6- esmc Code Requrements for Buldng tructures 007 CBC & 006 IBC Chapter 16/ ACE 7-05 Chapter CBC/ ACE 7-05 TRUCTURAL DEIGN BAI Purpose: The specfed earthquake loads are based upon post-elastc energy dsspaton n the structure, and because of ths fact, the requrements for desgn, detalng, and constructon shall be satsfed even for structures and members for whch load combnatons that do not contan earthquake loads ndcate larger demands than combnatons that nclude earthquake loads. The purpose of the earthquake provsons s prmarly to safeguard aganst major structural falures and loss of lfe, not to lmt damage or mantan functon. tructures desgned n conformance wth the sesmc desgn provsons prescrbed by the current Code should be able to: 1. Resst mnor ground moton wthout damage.. Resst moderate ground moton wthout structural damage but wth some nonstructural damage. 3. Resst major ground moton wthout collapse but wth possble structural and nonstructural damage. 4. The sesmc provsons n the ACE 7-05 wll consder the potental geologcal and sesmc hazards n esmc Desgn Categores C through F from: lope nstablty Lquefacton Dfferental settlement urface dsplacement due to faultng or lateral spreadng. How does the buldng Code attempt to accomplsh ths objectve when resstng earthquake ground moton? 1. Desgn the structure for forces less than those correspondng to elastc response generated by the desgn earthquake.. Rely on ductlty and detalng to prevent collapse 3. Allow the energy mparted by the earthquake to be absorbed by the structure wthout destroyng t. 4. It s assumed that the nput energy s absorbed (dsspated) upon post-elastc energy dsspaton n the structure. It should be noted that the amount or level of structural damage depends of the followng prmary parameters: TABLE 6-1 PARAMETER AFFECTING TRUCTURAL DAMAGE Earthquake Parameters te Parameters tructural Parameters Magntude Duraton Frequency Length of fault ol characterstcs Dstance to fault(s) Natural perod of the ste and ts relaton to structural perod (resonance) Natural perod of the buldng Buldng confguraton (regular vs. rregular) Type of lateral-force resstng system (LFR) and detalng (MRF, braced frames, shear walls) Constructon materal (steel, concrete, wood, masonry) Qualty control of constructon 164 Chapter 6- esmc Code Requrements for Buldng tructures

3 esmc Desgn for Professonal Lcense (007 CBC/ ACE 7-05) Dr. hahn A. Mansour, PE 6- TRUCTURAL YTEM In Chapter 3 of ths manual, loads were classfed as vertcal (gravty) and lateral loads (horzontal). The classfcaton of the structural systems n the current Code s based on whch porton of the system s responsble of resstng (carryng) the vertcal load and whch porton s responsble of carryng the lateral load. The followng abbrevatons wll be used n relaton to classfcaton of structural systems: MF IMF MMWF OMF EBF CBF = pecal moment frame = Intermedate moment frame = Masonry moment wall frame = Ordnary moment frame = Eccentrcally braced frame = Concentrcally braced frame 11. of ACE 7 gves the defntons of the dfferent structural systems as follows: 1- Buldng Frame ystem: A structural system wth an essentally complete space frame provdng support for vertcal loads. esmc force resstance s provded by shear walls or braced frames. - Dual ystem: A structural system wth an essentally complete space frame provdng support for vertcal loads. esmc force resstance s provded by moment resstng frames and shear walls or braced frames as prescrbed n ecton hear Wall-Frame Interactve ystem: A structural system that uses combnatons of ordnary renforced concrete shear walls and ordnary renforced concrete moment frames desgned to resst lateral forces n proporton to ther rgdtes consderng nteracton between shear walls and frames on all levels. 4- pace Frame ystem: A 3-D structural system composed of nterconnected members, other than bearng walls, that s capable of supportng vertcal loads and, where desgned for such an applcaton, s capable of provdng resstance to sesmc forces. Also, the dfferent types of frames are defned as follows: 1- Braced Frame: An essentally vertcal truss, or ts equvalent, of the concentrc or eccentrc type that s provded n a buldng frame system or dual system to resst sesmc forces. - Concentrcally Braced Frame (CBF): A braced frame n whch the members are subjected prmarly to aal forces. CBFs are categorzed as ordnary concentrcally braced frames (OCBF) or specal concentrcally braced frames (CBF). 3- Eccentrcally Braced Frame (EBF): A dagonally braced frame n whch at least one end of each brace frames nto a beam a short dstance from a beam-column or from another dagonal brace. 4- Moment Frame: A frame n whch members and jonts resst lateral forces by fleure as well as along the as of the members. Moment frames are categorzed as ntermedate moment frames (IMF), ordnary moment frames (OMF), and specal moment frames (MF). Table 1.-1 of ACE 7 lsts the desgn coeffcents and factors for EIGHT categores [A through H] total of about 83 systems. Chapter 6- esmc Code Requrements for Buldng tructures 165

4 166 esmc Desgn for Professonal Lcense (007 CBC/ ACE 7-05) Dr. hahn A. Mansour, PE In general, structural systems can be categorzed as follows n regard to resstng lateral forces: TABLE 6-3 LATERAL-FORCE-REITING- YTEM (LRF) hear Wall ystem Frame ystem Braced Concentrc (CBF) Eccentrc (EBF) Unbraced (UBF) Dual ystem A system where the shear wall s desgned to ressts lateral forces parallel to the plane of the wall Also known as vertcal daphragm or structural wall system A vertcal truss system to resst lateral forces The members are subjected prmarly to aal forces. A vertcal truss system to resst lateral forces A system conssts of beams and columns to carry gravty (vertcal) loads The lateral forces are ressted ether by shear walls or by the members and jonts (fleure acton) The total sesmc force resstance s to be provded by the combnaton of the moment frames and the shear walls or braced frames n proporton to ther rgdtes. For a dual system, the moment frames shall be capable of resstng at least 5 percent of the desgn sesmc forces. Chapter 6- esmc Code Requrements for Buldng tructures

5 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE A- Bearng Wall ystem: A structural system wthout a complete vertcal load-carryng space frame. Bearng walls or bracng systems provde support for all or most gravty loads. Resstance to lateral load s provded by shear walls or braced frames. The followng table shows the dfference between the shear walls and braced frames TABLE 6-4 BEARING WALL YTEM A-Bearng Wall ystems (1-15 n Table 1.-1) Carry vertcal (gravty) loads Resst horzontal (lateral ) loads Eamples: concrete and masonry shear walls, lght-framed walls wth shear panels. Fgure 6- Bearng Wall ystem The followng table shows the heght lmtatons of some of the bearng wall systems lsted n Table for the IX DEIGN CATEGORIE (DC): TABLE 6-5 BEARING WALL YTEM (ystems 1 through 15 n Table 1.-1) A-Bearng Wall ystems Heght lmtaton per DC, ft A * or B C D d E d F e 1. pecal renforced concrete shear walls NL NL Ordnary renforced concrete shear walls NL NL NP NP NP 7. pecal renforced masonry shear walls NL NL Intermedate renforced masonry shear walls NL NL NP NP NP 9. Ordnary renforced masonry shear walls NL 160 NP NP NP 13. Lght frame walls wth wood structural panels NL NL NL = No lmt NP = Not permtted d ee ecton for a descrpton of buldng systems lmted to buldngs wth a heght of 40 ft (73. m) or less. e ee ecton for buldng systems lmted to buldngs wth a heght of 160 ft (48.8 m) or less. * DC A s not lsted n Table 1.-1 because n ths DC the D < 0.167g and D1 < 0.067g whch ndcates that the structures n ths DC are the least vulnerable to earthquake forces compared to other DC. Therefore, NL for DC B also means NL for DC A Chapter 6- esmc Code Requrements for Buldng tructures 167

6 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE 6-3 HEIGHT LIMIT Table 1.-1 gves the heght lmts for the structural systems lsted and n the IX esmc Desgn Categores (DC). Note that NL means no lmt and NP means not permtted Increased Buldng Heght Lmt for teel Braced Frames and pecal Renforced Concrete hear Walls. The heght lmts n Table 1.-1 are permtted to be ncreased from 160 ft (50 m) to 40 ft (75 m) for structures assgned to esmc Desgn Categores D or E and from 100 ft (30 m) to 160 ft (50 m) for structures assgned to esmc Desgn Category F that have steel braced frames or specal renforced concrete cast-n-place shear walls and that meet both of the followng requrements: 1- The structure shall not have an etreme torsonal rregularty as defned n Table 1.-1 (horzontal structural rregularty Type 1b). - The braced frames or shear walls n any one plane shall resst no more than 60 percent of the total sesmc forces n each drecton, neglectng accdental torsonal effects CBC/ ACE 7-05 CLAIFICATION OF TRUCTURE 1.3. Irregular and Regular Classfcaton. tructures shall be classfed as regular or rregular based upon the crtera n ths secton. uch classfcaton shall be based on horzontal and vertcal confguratons Horzontal Irregularty. tructures havng one or more of the rregularty types lsted n Table shall be desgnated as havng horzontal structural rregularty. uch structures assgned to the sesmc desgn categores lsted n Table shall comply wth the requrements n the sectons referenced n that table. TABLE 6-1 REGULAR AND IRREGULAR TRUCTURE Regular tructures Irregular tructures Regular structures have no sgnfcant physcal dscontnutes n plan or vertcal confguraton or n ther lateral-forceresstng systems such as the rregular features. Irregular structures have sgnfcant physcal dscontnutes n confguraton or n ther lateralforce-resstng systems. Irregular features. There are EVEN vertcal and IX horzontal Irregulartes. Vertcal rregulartes: Eamne the sde vew of the structure Plan (Horzontal ) rregulartes: Eamne the plan vew of the structure 168 Chapter 6- esmc Code Requrements for Buldng tructures

7 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE TABLE HORIZONTAL TRUCTURAL IRREGULARITIE 1a. 1b Irregularty Type and Descrpton Torsonal Irregularty s defned to est where the mamum story drft, computed ncludng accdental torson, at one end of the structure transverse to an as s more than 1. tmes the average of the story drfts at the two ends of the structure. Torsonal rregularty requrements n the reference sectons apply only to structures n whch the daphragms are rgd or semrgd. Etreme Torsonal Irregularty s defned to est where the mamum story drft, computed ncludng accdental torson, at one end of the structure transverse to an as s more than 1.4 tmes the average of the story drfts at the two ends of the structure. Etreme torsonal rregularty requrements n the reference sectons apply only to structures n whch the daphragms are rgd or semrgd. Reentrant Corner Irregularty s defned to est where both plan projectons of the structure beyond a reentrant corner are greater than 15% of the plan dmenson of the structure n the gven drecton. Daphragm Dscontnuty Irregularty s defned to est where there are daphragms wth abrupt dscontnutes or varatons n stffness, ncludng those havng cutout or open areas greater than 50% of the gross enclosed daphragm area, or changes n effectve daphragm stffness of more than 50% from one story to the net. Out-of-Plane Offsets Irregularty s defned to est where there are dscontnutes n a lateral force-resstance path, such as out-of-plane offsets of the vertcal elements. Nonparallel ystems-irregularty s defned to est where the vertcal lateral force-resstng elements are not parallel to or symmetrc about the major orthogonal aes of the sesmc force resstng system. Chapter 6- esmc Code Requrements for Buldng tructures 169

8 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE TABLE 6-13 HORIZONTAL TRUCTURAL IRREGULARITIE # Type Defnton Illustraton The mamum story drft at one Torsonal end s more than 1. tmes the 1a Irregularty average of the story drfts of the two ends of the structure 1 1. ( ) 0.6 ( 1 ) 1b Etreme Torsonal Irregularty The mamum story drft at one end s more than 1.4 tmes the average of the story drfts of the two ends of the structure ( 1 ) Reentrant Corner Irregularty Where both projectons of the structure beyond a re-entrant corner are greater than 15 % of the plan dmenson of the structure n the gven drecton. projecton a > 0.15, and projecton b > 0.15 y 3 Daphragm Dscontnuty Irregularty Area of openng ab > 0.5 of the area XY, OR Changes n effectve daphragm stffness > 50% from one story to the net. 4 Out-ofplane Offsets Irregularty Dscontnutes n lateral-forceresstng path, such as out-of-plane offsets of vertcal elements. 170 Nonparallel ystems 5 Irregularty Vertcal lateral-force-resstng elements are not parallel to OR symmetrc about the major orthogonal aes of the lateral-forceresstng system. Chapter 6- esmc Code Requrements for Buldng tructures

9 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE 6-6 ROAD MAP FOR EQUIVALENT LATERAL FORCE (ELF) PROCEDURE 1- te classfcaton characterstcs: A, B, C, D, E, & F Tables & Mamum consdered earthquake spectral response acceleratons 1 & s Three sources: 1) usgs.gov )NEHRP 3)Maps-007CBC, 006 IBC& ACE te Coeffcents F a & F v Tables (1) & () 4- Adjusted mamum consdered earthquake spectral response acceleratons M = Fa s (Equaton 16-37) = F v 1 (Equaton 16-38) M1 5- Desg n spectral response acceleraton parameters D = /3 M (Equaton 16-39) =/3 (Equaton 16-40) D1 M1 6- Perod of the structure T Equatons: 1.8-7, 1.8-8, 1.8-9, wth the lmtaton 7- Occupancy mportance factor: I, Table ACE esmc desgn category (DC): Tables (1) & () 9- Lateral-force-resstng systems (LFR): 11. & Table 1.-1, ACE Response modfcaton coeffcent, R, Table 1.-1, ACE esmc dead load: W, 1.7-, ACE esmc Response Coeffcent, C s, 1.8 ACE esmc base shear: V 1.8 ACE Vertcal dstrbuton of base shear: F ACE Horzontal and vertcal components of E: E h & E v 1.4 ACE Redundancy factor: ρ, ACE Overstrength factor: 0, Table 1.-1, ACE Deflecton control: C I, Table 1.-1 & d 19- Combnatons of systems: Table 1.-1, & e Chapter 6- esmc Code Requrements for Buldng tructures 171

10 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE Mamum s Value n Determnaton of C s. For regular structures fve stores or less n heght and havng a perod, T, of 0.5 s or less, C s s permtted to be calculated usng a value of 1.5 for s 13- esmc Base hear, V 1.8 ACE esmc Base hear. The sesmc base shear, V, n a gven drecton shall be determned n accordance wth the followng equaton: V = Cs W (1.8-1) where: C s = the sesmc response coeffcent determned n accordance wth ecton W = the effectve sesmc weght per ecton TABLE 6-4 TEP OF THE EQUIVALENT LATERAL FORCE (ELF) PROCEDURE tep 1: Calculate the structural perod T usng: ) Appromate Method: Ta Ct hn ) T a = 0.1 N ) Ta hn Cw v) Raylegh Method (Note: Always check the lmt f applcable T C u T a) n n T w g f 1 1 D1 tep : Calculate the perod T T (T s not part of any base shear equatons, ts used for D comparson only) D1 tep 3: Calculate the perod T 0 T 0 0. tep 4: If T T 0, the acceleraton s gven as D T T0 a D and the base shear wll be as a W follows: V R I tep 5: If T 0 T T, Eq. (1.8-) governs and there s no need to check the mnmum values of (Eq ) & (Eq ). tep 6: If T T T L, Eq. (1.8-3) governs and the mnmum values of (Eq ) & (Eq ) should be checked. tep 7: If T T L, Eq.(1.8-4) governs 17 Chapter 6- esmc Code Requrements for Buldng tructures

11 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE Fgure 6-15 Equvalent Lateral Force Procedure (ELF) Chapter 6- esmc Code Requrements for Buldng tructures 173

12 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE 14- Vertcal Dstrbuton of Base hear, F ACE Vertcal Dstrbuton of esmc Forces. The lateral sesmc force (F ) (kp or kn) nduced at any level shall be determned from the followng equatons: and F = C v V (1.8-11) C v w n 1 h k w h k (1.8 1) Where: C v = vertcal dstrbuton factor, V = total desgn lateral force or shear at the base of the structure (kp or kn) w and w = the porton of the total effectve sesmc weght of the structure (W) located or assgned to Level or h and h = the heght (ft or m) from the base to Level or k = an eponent related to the structure perod as follows: for structures havng a perod of 0.5 s or less, k = 1 for structures havng a perod of.5s or more, k = for structures havng a perod between 0.5 and.5 s, k shall be or shall be determned by lnear nterpolaton between 1 and NOTE: The justfcaton of the eponent k > 1 for buldngs havng a perod grater than 0.5 seconds can be eplaned as follows: Tall buldngs ( > 4 stores and hgher dependng on the structure type and story heght) wll have hgher modes of vbraton under the sesmc force effect. To account for the hgher mode effects, a parabolc mode shape s assumed where larger forces wll be assgned to upper levels of the structure. TABLE 6-5 VALUE OF THE EXPONENT k T 0.5sec 0.5secT.5sec.5sec.0 k 1.0 OR Lnear Interpolaton between 1 & *.0 Vertcal Dstrbuton Factor (C v ) C w v n 1 h w h *Lnear Interpolaton: If T = 0.8 seconds. The value of the eponent k based on lnear nterpolaton s: T 0.5 k1 ( T 0.5) k C w v n 1 h k w h k C w v n 1 h w h k k Chapter 6- esmc Code Requrements for Buldng tructures

13 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE mplfed (Alternatve) Lateral Force Procedure At a Glance Occupancy Category I & II (ee Table ) Mamum Heght Three tores (ee Table ) ol Type (te Class) A to D (NO E & F Type) tructural ystems 1- Bearng Wall -Buldng Frame (ee Table ) D = /3 F a s ( ) F a s permtted to be taken as 1.0 for rock stes & 1.4 for sol stes s 1.5 ( ) ρ = 1 Ω o =.5 ( ) Torsonal moment due to M t,.e., NO M ta Drft = 1% of buldng heght unless computed to be less The advantages of the smplfed method could be summarzed as follows: esmc Desgn Category (DC) B to E NO F Only Table (based on D and ) 1.5 a. NO need to calculate the perod of the structure. b. NO need to calculate the drft and t wll be taken as 1% of the buldng heght unless computed to be less per ,.e., the smplfed method s used for buldngs for whch the drft s not controllng factor n desgn. c. Redundancy factor ρ wll be taken 1.0,.e. NO need to consder t. d. mplfed dstrbuton (rectangular) of the base shear. Fgure 6-7 mplfed (Alternatve) esmc Lateral Force Procedure Chapter 6- esmc Code Requrements for Buldng tructures 175

14 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE esmc Base hear, V, Alternate mplfed Desgn Procedure ( ) Determne n accordance wth where 1.5 Calculate D = /3 F a s F = 1.0 for rock stes a = 1.4 for sol stes, or = value determned n accordance wth Determne the Response Modfcaton Coeffcent R from Table for the approprate structural system based on esmc Desgn Category (DC) Determne effectve weght W n accordance wth Determne base shear V by Eq F D V W R F = 1.0 for one story buldngs = 1.1 for two-story buldngs = 1. for three-story buldngs Fgure 6-8 Flowchart for mplfed Determnaton of esmc Base hear 176 Chapter 6- esmc Code Requrements for Buldng tructures

15 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE 6-1 AMPLE PROBLEM ample Problem 6.1: tructural Perod Appromate Method Gven: Three-story steel specal concentrcally braced frame (CBF) offce buldng wth a penthouse on the roof as shown s located n central Calforna. The buldng wth sol type C and DC C. The story heght s 1 feet. D & D1 are 0.4g and 0.6g respectvely, The effectve sesmc dead load W for levels 1& s 10 kps each and for the roof ncludng the penthouse s 90 kps. Fnd: The structure perod T s most nearly: oluton: (A) 0.9 s (B) 0.30 s (C) 0.44 s (D) 0.49 s Whch method should be used to calculate T? ) Appromate method Eq. (1.8-7): YE, enough nformaton are gven ( h n, type of structure to get C t & ) ) Appromate method Eq. (1.8-8): NO, only for concrete and steel moment resstng frames not eceedng 1 stores and the story heght s at least 10 ft T a = 0.1 N (1.8-8) ) Raylegh Method Eq. (15.4-6): NO, not enough nformaton (forces & dsplacements) T n 1 w g n 1 f (15.4 6) T a C h t n ( 1.8 7) For CBF, C t = 0.00 & =0.75 (for all other structures, Table 1.8-) 4 T a 0.00(36) 0. 9 ec 3 Tme aver Usng Table 6-19 page 196 CBF & h n = 35ʹ (the closest to 36ʹ) 0.88 ec Answer B: wrong answer, 0.1 N = 0.30 s Answer C: wrong answer f C t = s chosen Answer D: wrong answer f C t = 0.08 s chosen Answer: (A) Note: When you pck the value of C t, the type of the structure should match the descrpton gven n the Code. Chapter 6- esmc Code Requrements for Buldng tructures 177

16 esmc Desgn for Professonal Lcense (007 CBC/ACE 7-05) Dr. hahn A. Mansour, PE Gven: ame as ample problem 6.1 ample Problem 6.: Base hear (V) Calculaton Fnd: tatc desgn base shear, V s most nearly: (A) 51.3 kps (B) kps (C) 7.54 kps (D) 3.10 kps oluton: Whch method should be used to calculate V?. ELF ( 1.8): YE, number of stores, type of structure, sol type, occupancy category.. mplfed ( 1.14): MAYBE, f the requrements of secton are met.. Dynamc: NO, not enough nformaton (acceleraton or response spectra). teps usng ELF Method: 1) Calculate the structure perod T D1 ) Calculate the perod T (T s gong to be used for comparson only) D 3) If T T, Eq. (1.8-) governs and no need to check the mnmum values of V (Eqs & 1.8-6) I = 1.0, 007 CBC Table & Table (Occupancy Category II) R = 6.0, Table 1.-1( Buldng Frame ystems- pecal teel CBF) 0.6g T 0.6 ec T 0. 9 ec (from prob. 6.1), Eq. (1.8-) governs 0.4g Also, T 0.9 s T0 0.0Ts s D 0.4 C 0.07 (1.8-) R 6 1 I V = C s W = 0.07 ( ) = 0.07 (330) = 3.10 kps (Ma.) (1.8-1) Note : Eq. (1.8-3) yelds a value greater than Eq. (1.8-) whch s the mamum specfed by the Code. D C for T TL R 1494 (1.8-3) 0.9(6 1) T I V = C s W = ( ) = (330) = kps (1.8-1) Answer: (D) Note: Buldngs up to 4 stores (low rse structures = short perod structures) most lkely wll be controlled by the Eq (mamum) 178 Chapter 6- esmc Code Requrements for Buldng tructures