Load and Resistance Factor Design (LRFD)

Transcription

1 53:134 Structural Desg II Load ad Resstace Factor Desg (LRFD) Specfcatos ad Buldg Codes: Structural steel desg of buldgs the US s prcpally based o the specfcatos of the Amerca Isttute of Steel Costructo (AISC). -- Curret Specfcatos: 1989 ASD ad 1999 LRFD AISC Specfcato for Structural Steel Buldgs Allowable Stress Desg ad Plastc Desg. (ASD) AISC Load ad Resstace Factor Desg Specfcato for Structural Steel Buldgs. (LRFD) -- LRFD Maual: Maual of Steel Costructo, Load ad Resstace Factor Desg, 3 rd Edto, Maual of Steel Costructo, 13 th Edto, 2005 (combed ASD ad LRFD provsos). Steel brdge desg s accordace wth specfcatos of AASHTO: -- Amerca Assocato of State Hghway ad Trasportato Offcals Ralroad brdge desg s accordace wth specfcatos of AREA: -- Amerca Ralway Egeerg Assocato. Desg Phlosophes: 1. Smlar to plastc desg, LRFD focuses o lmt state desg, where stregth or falure codto s cosdered. 2. Factored load factored stregth, or (Loads load factors) resstace resstace factors 3. I geeral, load factors ( > 1) amplfy loads, whle resstace factors ( < 1) reduce stregth. Structural Safety Requremet: φ Resstace factor R The omal resstace, or stregth, of the compoet uder Q cosderato A load effect (a force or a momet) J.S. Arora/Q. Wag 1 LoadadResstFactor.doc

2 53:134 Structural Desg II γ R A load factor φ s the called the desg stregth γ Q s the summato of all the load effects φr γ Q Resstace Factors φ : Teso members ( yeldg state) Teso members ( fracture state) φ = Compresso members Beams ( flexure ad shear) Fasteers Load Combatos: D Dead load E Earthquake load L Lve load L r Roof lve load R Ra load S Sow load W Wd load γ Q 1.4D 1.2D 1.6L 0.5 L or S or R 1.2D 1.6 L or S or R 0.5 L or 0.8 = 1.2D 1.6W 0.5L 0.5 Lr or S or R 1.2D± 1.0E 0.5L 0.2S 0.9D± ( 1.6 W or 1.0E) ( r ) ( r ) ( W) ( ) Note: γ Q s the summato of all the load effects. J.S. Arora/Q. Wag 2 LoadadResstFactor.doc

3 53:134 Structural Desg II The safety requtemet may be wrtte as γ Q Load effects or φr Resstace If more tha oe type of resstace s volved, ths ca be used to form the bass of a teracto formula. Note that the sum of the load-to-resstace ratos must be lmted to uty. For example, f both bedg ad axal compresso are actg, the teracto formula would be Pu φ P c Mu φ M b P u factored axal compressve load φ c P compressve desg stregth M u factored bedg momet φ b M flexural desg stregth For baxal bedg, there wll be two bedg ratos: Pu φ P c M φbm ux x M φ M b uy y where the x ad y subscrpts refer to bedg about the x ad y axes. Glossary (Part 16 of LRFD Specfcatos ad Code): Lmt State: a codto whch a structure or compoet becomes uft for servce ad s judged ether to be o loger useful for ts teded fucto (servceablty lmt state) or to be usafe (stregth lmt state). J.S. Arora/Q. Wag 3 LoadadResstFactor.doc

4 53:134 Structural Desg II Nomal loads: the magtudes of the loads specfed by the applcable code ( Q ). Load factor: a factor that accouts for uavodable devatos of the actual load from the omal value ad for ucertates the aalyss that trasforms the load to a load effect. ( γ ). Factored load: the product of the omal load ad a load factor ( γ Q). Resstace: the capacty of a structure or compoet to resst the effects of loads, as determed by computato usg specfed materal stregths ad dmesos ad formulas derved from accepted prcples of structural mechacs or by feld tests or laboratory tests of scaled models, allowg for modellg effects ad dffereces betwee laboratory ad feld codtos. Resstace s a geerc term that cludes both stregth ad servceablty lmt states. Nomal stregth: the capacty of a structure or compoet to resst the effects of loads, as determed by computato usg specfed materal stregths ad dmesos ad formulas derved from accepted prcples of structural mechacs or by feld tests or laboratory tests of scaled models, allowg for modellg effects ad dffereces betwee laboratory ad feld codtos ( R ). Resstace factor: a factor that accouts for uavodable devatos of the actual stregth from the omal value ad the maer ad cosequeces of falure (φ ). J.S. Arora/Q. Wag 4 LoadadResstFactor.doc

5 53:134 Structural Desg II Desg stregth: resstace (force, momet, stress, as approprate) provded by elemet or coecto; the product of the omal stregth ad the resstace factor ( φ ). Compact sectos: capable of developg a fully plastc stress dstrbuto ad possess rotato capacty of approxmately three before the oset of local bucklg;.e., local bucklg s ot a ssue. Nocompact sectos: ca develop the yeld stress compresso elemets before local bucklg occurs, but wll ot resst elastc local bucklg at stra levels requred for a fully plastc stress dstrbuto. Stffeer: a member, usually a agle or plate, attached to a plate or web of a beam or grder to dstrbute load, to trasfer shear, or to prevet bucklg of the member to whch t s attached. R Examples J.S. Arora/Q. Wag 5 LoadadResstFactor.doc