Deflections
Deflections
(OP)
I'm fairly new out of school and have taught myself ASD, since we only learned LRFD in school. My question is this:
For LRFD, when calculating deflections, you use a FACTORED load. So you use 1.2D + 1.6L to get your loading, then plug in the simple beam formula to get your deflection.
For ASD, when calculating deflections, you use an UNFACTORED load. So you use D + L to get your loading, then plug in the simple beam formula to get your deflection.
Am I missing something, or is LRFD more conservative from a deflection standpoint? Thanks
For LRFD, when calculating deflections, you use a FACTORED load. So you use 1.2D + 1.6L to get your loading, then plug in the simple beam formula to get your deflection.
For ASD, when calculating deflections, you use an UNFACTORED load. So you use D + L to get your loading, then plug in the simple beam formula to get your deflection.
Am I missing something, or is LRFD more conservative from a deflection standpoint? Thanks






RE: Deflections
L+D = 0.67M --> 1.5(L+D) = M (Fb= 0.67Fy; M oc Fb)[ASD]
1.2D + 1.6L = 0.9M --> 1.33D + 1.78L = M [LRFD]
LRFD design methods reflect that the dead load is fairly certain (factor closer to 1), but that the LL is more variable (larger factor). ASD lumps the factor of safety together, but reduces the resistance instead (member stresses).
While the LRFD method yields larger deflections (since w & P have been factored), it is not usually used as a governing criteria (see AISC chapter L). ASD _does_ limit a design on the basis of deflection (see AISC 1.13.1).
From a practical point of view, the deflection should be considered, because too much flexing or spring will cause claddings or coverings to crack, and the structure might be considered unsafe because of the bounce. It depends where it's at and what its used for.
RE: Deflections
AISC Chapter L: The closest thing I could find to saying that "While the LRFD method yields larger deflections (since w & P have been factored), it is not usually used as a governing criteria (see AISC chapter L)." is this:
"Limiting values of structural behavior to ensure serviceability (max deflections, accelerations, etc) shall be chosen with due regard to the intended function of the structure.
Essentially, the IBC (or whatever code you are going by) controls deflections. You ALWAYS check deflections for the appropriate limit, whether it is L/240, 360, 600 etc.
RE: Deflections
RE: Deflections
LRFD designs yeild larger real world deflections because you can usually use smaller sections from a strength standpoint. Ie., if the ASD design says a W14x30 works, maybe the LRFD says a W14x22 will work (both purely from a capacity standpoint). Then you need to check servicability using unfactored loads -- the W14x22 will obviously deflect more, so it may or may not be acceptable depending on your servicabilty criteria.
If serviceability (deflection, vibration, bldg drift) is governing your design, then LRFD vs. ASD is immaterial -- you'll get the same answer when you're done. Only when strength is governing your design does the potential exist for LRFD to come out ahead.
RE: Deflections
LRFD 3rd edition, page 16.1-79, Section L3.1, "Deformations in structural members and structural systems dur to SERVICE LOADS shall not impair the serviceability of the structure."
RE: Deflections
L3.1 -- "Deformations is structural members and structural systems due to service loads shall not impair the serviceability of the structure."
Service loads, not factored loads.
RE: Deflections
RE: Deflections
Hg
Eng-Tips guidelines: FAQ731-376
RE: Deflections
dasman,
I agree with you that unfactored loads should be used for deflection calculations.
However, every time I use LRFD for strength calculations, I typically end up with bigger required member sizes.
Maybe this is only because I commonly work on industrial projects where live loads exceed dead loads by four or five times. The increased load factor for live loads ends up increasing the member size.
I am glad to hear that AISC is finally coming out with a new manual (unified LRFD and ASD) so us ASD types can utilize the more recent research easily.
We are supposed to be able to freely download the specification section of the new manual in April sometime.
Regards,
JPJ
RE: Deflections
RE: Deflections
Yes.