Deflection Load Combinations - ASCE or IBC 3

[Eren Jaeger]

Just want to ask about the load combinations you use for the deflection. Initially, my knowledge is that when computing for the deflection, you consider the ASD load combinations defined n ASCE 7-10 24.1

1. D
2. D + L
3. D + (Lr or S or R)
4. D + 0.75L + 0.75(Lr or S or R)
5. D + (0.6W or 0.7E)
6a. D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R)
6b. D + 0.75L + 0.75(0.7E) + 0.75S
7. 0.6D + 0.6W
8. 0.6D + 0.7E

but when I started looking for the deflection limits to be used for the beams, I came across the IBC 2012 Table 1604.3 which enumerates the deflection limits based on the type of loading. That is,

1. L
2. S or W
3. D + L

My questions are:

1) Which one do you use? The load combinations from ASCE or IBC?
2) If it is IBC, are the defined load combinations really enough?
3) If it is ASCE, what are the deflection limits you are using?
4) Just to clarify. Is my knowledge, that in computing deflections you only consider the gravity loads, right (assuming for beam only)? because lateral loads are considered only for column drifts?

I appreciate any inputs. thanks in advance!

 

[Settingsun]

I don't use either code but the ASCE combinations you posted are not enough. It is typical for live load to be considered in isolation as that is what the people on the floor feel. Consider a prestressed element that hogs under dead load: D + L may be small/zero/still hogging even though the element is too springy.

 

[JAE]

The listing of ASCE 7 load combinations are intended for strength calculations, not deflection/serviceability.

Your second listing of three combinations are more in line with the deflection limits that are generally understood by engineers to be of most importance - i.e. how does the structural react, deflection-wise, under loads through its lifespan. Specifically you'd want to look at live, seismic, wind, snow, etc. that affect the in-place structure.

The D+L combo is also looked at to help control total deflections. We look at Dead only as well.

1) Which one do you use? The load combinations from ASCE or IBC? [red]IBC references ASCE 7 so you can use either[/red]
2) If it is IBC, are the defined load combinations really enough? [red]see comments above[/red]
3) If it is ASCE, what are the deflection limits you are using? [red]We use L/360 for live or snow. L/240 to L/36 for dead alone. L/240 for D+L H/300 to H/500 for drift under wind depending on cladding. Seismic is its own beast and ASCE 7 provides limits on overall building drift in the seismic sections.[/red]
4) Just to clarify. Is my knowledge, that in computing deflections you only consider the gravity loads, right (assuming for beam only)? because lateral loads are considered only for column drifts? [red] Gravity loads will mostly affect beams and slab systems for deflection. Wind/seismic limits are about the overall frame system..not just the columns[/red]

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[Eren Jaeger]

Thank you steveh49 and JAE!