DESIGN OF TENSION MEMBERS

Transcription

1 CHAPTER Srcral Seel Design LRFD Mehod DESIGN OF TENSION MEMBERS Third Ediion A. J. Clark School of Engineering Deparmen of Civil and Environmenal Engineering Par II Srcral Seel Design and Analysis 4 FALL 00 By Dr. Ibrahim. Assakkaf ENCE Inrodcion o Srcral Design Deparmen of Civil and Environmenal Engineering Universiy of Maryland, College Park CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 1 This chaper deals wih he design of ension members for exernal loads. In general, he design of ension members shold have he following properies: 1. Compacness. Dimensions ha fi ino he srcre wih reasonable relaion o oher member dimensions. 3. Minimizaion of shear blocks and lag. 1

2 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. AISC LRFD Specificaions The design srengh φ P n is he lesser of a) φ F y A g (1) b) φ F A e () c) The block shear srengh, φ R n (3) CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 3 AISC LRFD Specificaions The firs expression (Eq. 1) is saisfied if he minimm gross area is a leas eqal o he following: P min Ag (4) φ F The second expression (Eq. ) is saisfied if he minimm vale of A e is a leas min A e y P φ F (5)

3 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 4 AISC LRFD Specificaions And since A e U A n for boled members, he minimm vale of A n is given by Ae P min An min (6) U φ F U Then he minimm A g for he second expression (Eq. ) ms be a leas eqal he minimm vale of A n pls he esimaed hole area: P min Ag + esimaed hole areas (7) φ F U CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 5 AISC LRFD Specificaions The hird expression (Eq. 3) can be evalaed once a rial shape has been seleced, and he oher parameers relaed o he block shear srengh are known. The designer can sbsie ino Eqs. 4 and 7, aking he larger vale of A g so obained for an iniial size esimae. 3

4 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 6 AISC LRFD Specificaions The designer also has o check he slenderness raio ha i wold no exceed a vale of 300, ha is or L r 300 min r L 300 (8) CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 7 AISC LRFD Specificaions If no load involved oher han he dead and live loads, hen he designer ms check he following load facor expressions and ake he larger: P P (9) 1.4D 1.D + 1.6L (10) 4

5 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 8 Example 1 Selec a 30-f-long W1 secion of A99 seel o sppor a ensile service dead load P D 130 k and a ensile service load P L 110 k. As shown in he figre, he member is o have wo lines of bols in each flange for 7/8-in bols (a leas hree in a line 4 in on cener). CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 9 Example 1 (con d) Considering he load facor expressions of Eqs. 9 and 10: ( ) P 1.4D k Conrols P 1.D + 1.6L 1.( 130) + 1.6( 110) 33 k Comping he minimm A g reqired sing Eqs. 4 and 7: min P 33 A 7.38 in g φ F y ( ) Assme U 0.9 and assme he flange hickness is in from he manal for W1. 5

6 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 10 Table 1 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 11 Example 1 (con d) min A g P φ F U ( 65)( 0.9) Preferable min r: + esimaed hole areas ( 0.380) 9.09 in 30 1 min r L 1. in

7 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 1 φ P Example 1 (con d) Try W1 35, ha has he following properies (P. 1-0 and 1-1, Manal): A g 10.3 in, d 1.5 in, b f 6.56 in f 0.50 in, and r y 1.54 in Checking: F A n φ g ( 50)( 10.3) k 33 k 0.9 > OK x For half W1 35 or ha is a WT : x 1.30 in (noe i is y in P.1-49 of Manal) CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 13 Example 1 (con d) Checking (con d) L ( 4) 8 in U < An φ P φ F A 0.75 L r y y n x L e ( 0.50) < ( 65) [ 0.84( 8.) ] Therefore, USE W1 35 Secion 8. in k > 33 k OK OK 7

8 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 14 Bil-p Tension Members The LRFD Specificaion provides a definie se of rles describing how he differen pars of bil-p ension members are o be conneced ogeher: 1. When a ension member is bil p from elemen in coninos conac wih each oher, sch as a plae and a shape, or wo plaes, he longidinal spacing of connecors beween hose elemens ms no exceed 4 imes he hickness of he hinner plae, or 1 in if he member is o be pained. CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 15 Bil-p Tension Members. Shold he member consiss of npained weahering seel elemens in coninos conac and be sbjec o amospheric corrosion, he maximm permissible connecor spacings are 14 imes he hickness of he hinner plae, or 7 in. 3. Shold a ension member be bil p from wo or more shapes separaed by inermien file, he shapes ms be conneced o each oher a inervals sch ha he slenderness raio of he individal shapes beween faseners does no exceed

9 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 16 Bil-p Tension Members 4. The disance from he cener of any bols o he neares edge of he conneced par nder consideraion may no be larger han 1 imes he hickness of he conneced par, or 6 in. NOTE: refer o Page 105 of he exbook and LRFD Specificaion D for more deails abo he he design of connecing plaes. CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 17 Bil-p Tension Members Example Two C1 30s, as shown in he figre, have been seleced o sppor a dead ensile working load of 10 k and a 40-k live load ensile working load. The member is 30 f long consiss of A36 seel, and has one line of a leas hree 7/8-in bols in each channel flange 3 in on cener. Using he LRFD Specificaion, deermine wheher he member is saisfacory and design he necessary ie plaes. Assme ceners of bol holes are 1.75 in from he backs of he channels. 9

10 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 18 Bil-p Tension Members Example (con d) c.g. of C 5.36 P P - C1 30s ( A 8.8 in each) 1 lengh of ie plae P widh of P he plae CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 19 Bil-p Tension Members Example (con d) Using C1 30s, he following properies from he LRFS Manal can be obained: A g 8.81 in each, f in, I x 16 in 4 each I y 5.1 in 4 each, y axis from back of C, and r y 0.76 in. Load o be resised: ( ) P 1.4D k P 1.D + 1.6L ( ) + 1.6( 40) 58 k Conrols 10

11 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 0 Bil-p Tension Members Example (con d) Design Srenghs: φ F A 0.9 ( 36) [ ( 8.81) ] k > 58 k 7 1 An ( 0.501) 15.6 in 8 8 U 0.85 from Table (Table 3-, Tex) φ P φ F A U 0.75 y n g n ( 58)( 15.6)( 0.85) k > 58 k OK OK CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 1 Table 1 11

12 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. Bil-p Tension Members Example (con d) Slenderness Raio: I I r r x x y y 4 ( ) 34 in ( 5.1) + ( 8.81)( 5.36) ( 8.81) 511 ( 8.81) 4.9 in 5.38 in 511in since rx 4.9 in < ry 5.38 in, rx conrols Lx < 300 r 4.9 OK x 4 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 3 Bil-p Tension Members Example (con d) Design of ie plaes: 3 Disance beween lines of bols in 4 Min. lengh of ie plaes ( 8.5) 5.67 in (say 6 in) 3 Min. hickness of Min. widh of Max. preferablespacing of Max. preferable 1 3 ie plaes ( 8.5) 0.17 (say in) ie pales in (say 1 in) ie plaes: Leas r of C 0.76 in USE 3/ f L L L 8.6 in f r

13 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 4 Rods and Bars When rods and bars are sed as ension members, hey may be simply welded a heir ends, or hey may be hreaded and held in place wih ns. The LRFD nominal ensile design sress for hreaded rods is given in heir able J3. and eqals φ0.75 F, and is o be applied o he gross area of he rod A D comped wih he major hread diameer. CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 5 Rods and Bars The area reqired for a pariclar ensile load can hen be calclaed from he following expression: P A D wih φ 0.75 (11) φ0.75f 13

14 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 6 Rods and Bars Example 3 Using A36 seel and LRFD Specificaion, selec a sandard rod of A36 seel o sppor a ensile working dead load of 10 k and a ensile working live load of 0 k. Eq.11: ( ) P 1.4D P φ0.75f 14 k P 1.D + 1.6L A D ( ) + 1.6( 0) USE1 -in - diameer rod ( A 8 D k ( 0.75)( 58) 1.49 in 1.35 in ) Conrols CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 7 Design for Faige Loads The AISC has provisions for faige design as olined by he following procedre: 1. The design sress range deermined in accordance wih he AISC reqiremens is only applicable for he following siaions: a. Srcres for which he seel has adeqae corrosion proecion for he condiions expeced in ha localiy. b. Srcres for emperares do no exceed F. 14

15 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 8 Design for Faige Loads. The provisions of he Specificaion apply o sresses which are calclaed wih nfacored loads and he maximm permied sress de o hese loads is 0.66 F y. Formlas are given in Appendix K.3 of he Specificaion for comping he design sress range. CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 9 Design for Faige Loads The sress range, for mos cases, can be calclaed from F SR C f N F TH (1) Where F SR design sress range, ksi C f consan from Table A-K3.1 in LRFD Appendix N nmber of sress flcaions in design life F TH hreshold faige sress range from Table A-K3.1 in LRFD Appendix, Ksi. 15

16 CHAPTER 4. DESIGN OF TENSION MEMBERS Slide No. 30 Design for Faige Loads Example 4 A ension member is o consis of a W1 secion (F y 50 ksi) wih fille-welded end connecions. The service dead load is 40 k, while i is esimaed ha he service live load will vary from a compression of 0 k o a ension of 90 k fify imes per day for an esimaed design life of 5 years. Selec he secion. See Solion on Page 116, Texbook. 16