Beam Deflection / Serviceability Limits using ASD

[rudycwright]

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.

 

[BAretired]

Unfactored loads should be used to calculate deflection. What is conservative about that?

BA

 

[rudycwright]

The unfactored loads being applied to find deflection would be greater than the loads that would cause failure by flexure or shear under factored loads.

 

[LonnieP]

Rudy, There's not a simple answer for deflection, especially with wood members. First off, wood members creep under dead load. Wet lumber creeps more than low moisture material and seasoned lumber creeps even less. Even Glu-Lams and Web Joists creep. Most engineers apply a factor of between 1.5 and 2 to their instantaneous dead load deflections to account for this creep. You should discuss this 'factor' with your office to see what they recommend.

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

 

[DaveAtkins]

When are unfactored loads ever greater than factored loads? We are not following your logic.

DaveAtkins

 

[rudycwright]

Lonnie - Thank you for your response. Do you find that for wood beams with long spans (ie 10-20 feet) that an "observable" deflection will govern more often than the live load and total load deflection limits?

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)".

 

[JedClampett]

rudy, you cherry picked one combination that is taking advantage of the small likelihood of full snow plus full live load happening simultaneously. You would still have to check dead plus full live, dead plus full snow, etc. Most of us would pick one combination either through experience or running the numbers and check that. If you have some wacky snow drifts, you might have to check that case, also.

 

[Ron]

Factoring, whether done to the loading or to the material properties, is an attempt at stochastic optimization. Deflection is a true parameter that directly yields load-response characteristics. When you place an actual load of 100plf along a beam, the beam feels that whole load, not some factor of that load (neglecting load sharing with other members). For this reason one has to assume that full loading is necessary to adequately accommodate serviceability needs. Further, since there can be no "factoring" on the response side of deflection (E and I are not amenable to factoring), there is no reason to factor the load side.

 

[apsix]

A few points:
- 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.

 

[CELinOttawa]

Rudy: Practicing engineers virtually never check deflection for anything beyond D + L. For roofs it gets more complicated, but not much. Like Jed said, SLS is checked in one (sometimes two) combinations... These should be the MOST common combinations, not the least (as you have listed). Oh, and for the record, there are codes list which their SLS load combinations to be checked, just like the ULS ones. You may want to have a look at AS/NZS 1170.0 Cl 4.3 for an idea of how that code handles this issue.

SLS is about every day. ULS is about never, ever, having a collapse day.

 

[alhawk]

Hey Rudy!