IBC Deflection limits

Question

IBC Deflection limits

mudturk37 (Structural)

(OP)

18 Jun 12 10:22

I am referencing the 2012 IBC but I have checked the commentary for the past couple code cycles and it was the same in each.

Table 1604.3 Deflection Limits

The commentary states "In computing deflections to verify compliance with Table 1604.3 limits, the loads shown in the column headings of Table 1604.3 are the only loads that must be applied to the member......It is not necessary to use the load combinations of Section 1605.3 for verifying that the deflection limits have been met."

Lets assume a floor joist for example. Per the table, Live Load deflection is L/360 and Dead Load plus Live Load deflection is L/240. The column heading S or W simply has a "---" for the deflection for floor members.

Is it safe to assume that if you have a floor joist that may be supporting a wind or snow load, that you don't have to check deflections under those loading conditions, only strength?

For example, a floor joist(s) underneath an interior shear wall with an overturning moment applied midspan on the joist. In the past, I have calculated the joist with live and dead loads from floor loading in addition to the concentrated load from the OTM. In these cases, deflection is normally controlling. Have I been too conservative in the past?

Is it necessary to check the deflection limit against the deflection from a wind load?

woodman88 (Structural) · Answer 1 · 2012-06-18 10:48:37

Maybe. If the floor is going to have a snow (or w/s) load applied, I would be checking the roof Live Load deflection requirements. I do this as I consider the snow load as a Roof Live Load for deflections. I also consider any deck loading to also need to meet the roofing requirements. This is because the deflection requirements are to keep the interior finishes from cracking due to deflection. Technically, you may not have to. I have never looked into to not doing this, so I could not say.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

JAE (Structural) · Answer 2 · 2012-06-18 10:57:11

Deflection limits in the code are set but I always use a lot of engineering judgement and discretion in applying them to my structures.

mudturk37 (Structural) · Answer 3 · 2012-06-18 11:06:00

woodman88, are you saying that if you check a floor joist with a roof load, you would use a 240/180 deflection limit instead?

mudturk37 (Structural) · Answer 4 · 2012-06-18 11:11:06

I've always found it interesting that load duration is not factored into deflection requirements.

As I've understood it, something like a floor deflection limit is based more on human comfort and feeling the vibration of a floor. Deflection on a floor joist from a wind event is much less likely to be felt at the instance of maximum deflection since it is a short duration load.

The commentary to this table would seem to imply that this can be accounted for by ignoring the deflection from short term loading.

I would think that in the case of a shearwall joist, any instantaneous joist deflection would not adversely affect the wall finish since it could be assumed that the wall is stiff enough to resist this deflection if designed as a shearwall.

woodman88 (Structural) · Answer 5 · 2012-06-18 12:43:14

Like JAE states, you have to use engineering judgment. But when interior floor live loads are applied per the load cases I use the floor criteria. Thus as in the case where the floor joist supports a exterior wall, with snow loading on one side and floor live loading on the other.. But where the joist for it's full length is exposed to snow loading, I would apply the roof deflections for the load cases that include the snow load. In any case, I would never exceed the roof deflection limits.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

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IBC Deflection limits