Beam Deflection / Serviceability Limits using ASD
Beam Deflection / Serviceability Limits using ASD
(OP)
I have a question on what load factors, if any, should be applied for design of wood beams by ASD for Live Load Deflection and Total Load Deflection.
Using LRFD, load factors increase the loads but deflections are calculated using un-factored loads to represent actual loading and actual deflection. Using ASD however, loads are either kept constant or reduced (ie combo 4, snow and live loads are reduced by a factor of 0.75).
When designing wood beams, should the un-factored loads still be used for deflection, even if that deflection would actually be pushing the beam past its flexural / shear limit states as defined by the factored loads?
My opinion is that it is overly conservative to apply un-factored loads to find deflection of wood beams by ASD, but I may be missing something.
Using LRFD, load factors increase the loads but deflections are calculated using un-factored loads to represent actual loading and actual deflection. Using ASD however, loads are either kept constant or reduced (ie combo 4, snow and live loads are reduced by a factor of 0.75).
When designing wood beams, should the un-factored loads still be used for deflection, even if that deflection would actually be pushing the beam past its flexural / shear limit states as defined by the factored loads?
My opinion is that it is overly conservative to apply un-factored loads to find deflection of wood beams by ASD, but I may be missing something.






RE: Beam Deflection / Serviceability Limits using ASD
BA
RE: Beam Deflection / Serviceability Limits using ASD
RE: Beam Deflection / Serviceability Limits using ASD
Aside from creep, there are architectural considerations to long term dead load deflections. As for live load deflections - rain water ponding on relatively flat roofs should be checked against the combination of live and long term dead load deflections and roof slope (make sure the water can run out of the low spots). If your roof finish is tile or shingles with a pronounced pattern, it's easier to see long term deflections in the rafters and ridge beams. Finish material considerations should also be considered with live loads (L/360 or stiffer for plaster and other brittle finishes).
I've found when designing wood members, 'acceptable observable' deflections govern my member selection more than stress. I've made my share of embarrasing deflection screw-ups that actually met code requirements but failed the 'real life' test. Now that I'm retired I can admit it.
LonnieP
RE: Beam Deflection / Serviceability Limits using ASD
DaveAtkins
RE: Beam Deflection / Serviceability Limits using ASD
Dave - When designing with ASD, loads can be reduced by combination 4 found in ASCE 2.4.1 / IBC Eq 16-11.
"4. D + 0.75L + 0.75(Lr or S or R)".
RE: Beam Deflection / Serviceability Limits using ASD
RE: Beam Deflection / Serviceability Limits using ASD
RE: Beam Deflection / Serviceability Limits using ASD
- The serviceability calculations should yield the same result for both ASD and LRFD; the user's perception of deflection won't change in response to your design method.
- There is no fundamental reason not to factor loads for serviceability checks, but the acceptable limit used must be appropriate for the load combination. In my jurisdiction deflections due to permanent loads, equal to dead + a portion of live load, are often checked.
- Creep must be accounted for with wood.
RE: Beam Deflection / Serviceability Limits using ASD
SLS is about every day. ULS is about never, ever, having a collapse day.
RE: Beam Deflection / Serviceability Limits using ASD