2015 IBC Table 1604.3 - Deflection Limits

[Deker]

The 2015 IBC has clarified the intent of Table 1604.3 footnote (d), which now says "The deflection limit for the D+L load combination only applies to the deflection due to the creep component of long-term dead load deflection plus the short-term live load deflection." In researching this note I was able to find the reason behind the original code change proposal, which was presented by the American Wood Council.

"Deflection limits for the load combination D+L, were taken from the UBC deflection limits. However, the intent of the UBC limits was not brought forward. The original intent of these provisions was to limit the total deflection based on the combination of live load deflection and the creep component of the dead load deflection. As a result, there have been several prior code cycle modifications to these provisions to re-instate the original intent, such as the addition of footnote (g) for steel structural members which effectively excludes steel from checking for the creep component of dead load deflection. As currently written and formatted, the D+L deflection provision can be misinterpreted to suggest that the total deflection due to dead load, D, including both the immediate and creep components fo the dead load deflection, should be used with the defelction limit in this column. Additionally, use of the 0.5D in footnote (d) is potentially non-conservative without clarification that the 0.5D load reduction approach is a numerically consistent alternative to the NDS provisions. Without this clarification, a potential misinterpretation is that the creep component of dead load deflection is to be calculated using NDS provisions and the reduced dead load (i.e. 0.5D). This change makes calculation of D+L deflection for comparison against the D+L deflection limit in Table 1604.3 consistent with the provisions in NDS 3.5.2 for long-term loading and consistent with the stated intent in the UBC and with similar provisions in ACI 318 as described in the ACI 318 Commentary."

Count me as one who has misinterpreted the D+L limit to apply to the total dead and live load deflections. I know every software package I've used has done the same. On a side note, footnote (g) finally makes sense to me. But if the deflection limits are based at least partly on the permissible curvature in finish materials (hence footnote (i) which allows cantilever members to use twice the span), why would the deflection limits be different for L and D_creep+L? I am assuming the creep deflection occurs after the finishes have been installed.

So I have two questions:

1. In your practice, do you check the D+L deflection limit using the total dead load or the creep component only? If you use the creep component only, what limit do you use for the immediate dead load deflection?
2. If deflection limits are based on permissible curvatures, why are there different deflection limits for L and D_creep+L?

 

[Deker]

Any thoughts?

 

[medeek]

The problem with these building codes is there is too many cooks in the kitchen, how do these technical errors happen.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jdgengineer]

Deker I agree with you I have always designed for full D+L when checking deflection. I've been aware that the table alluded to only creep checks, but have not checked against creep only.

I was always taught to check with full loads and I agree that is what software uses.

I agree it seems curvature would be most important to determining damage to finishes. This seems like it would make sense to be based on total deflection. I'm not sure why the building code only checks creep component, in reality it seems like you should be checking total deflection + creep (1.5-2.0 x dead load + live load) although we have not typicallly checked this for normal usage.

For the structures we design we never use code minimum deflection ratios anyways and use internal ones which are more strict. Absolute deflection tends to govern a lot of our larger members to avoid perception of sag.

 

[jdgengineer]

Here is a good article from Ghosh a few years ago.

http://skghoshassociates.com/SKGAblog/viewpost.php?id=35

 

[Deker]

Thank you both for your responses. Given that the ACI tables for permissible deflections also use different limits for L and D_creep+L, there must be something more to it than just curvature in the finishes. Just not sure what it is.

 

[jdgengineer]

Both MacGregor's book and Breyer's book has a little discussion on it that may be helfpul to review if you have the references.

MacGregor mentions that you are designing the finishes for the difference in deflection from the time of installation to total expected deflection. Based on this comment, it's my thought that the IBC assumes that all instantaneous dead load deflection has already occured at the time of installation of the finishes and therefore only requires a review of creep + live load.

This makes sense to me if:

1) Most/All of the dead load is in place at the time of installation of the finishes. This is probably the case in most instances, but may not be true in all cases. Living Roof's for instances are often one of the last thing installed in my experience.
2) The contractor is installing a finish that wouldn't be bent to the deflected shape but will form itself based on the deflected shape (i.e. plaster ceiling)
3) The contractor rips furring our other methods to flatten out the surface and then installs the finish material if it could be bent (like wood ceilings or gypsum board) to avoid excessive curvature.

Interesting discussion, but I don't think it will impact our typical approach. Luckily, we are not in the "economy" side of the market.

 

[rapt]

This has always been an interesting one for concrete in ACI318 where it talks about limiting LL deflection. In concrete design, especially reinforced concrete but also prestressed, adding LL causes increased DL deflection because it increases the amount of cracking and thus reduces the effective inertia. And then there is creep and shrinkage deflection. And creep deflection is affected by all loads. Shrinkage deflection is also but not to the same degree as it is affect ted by the degree of cracking.

What you should actually do is calculate expected Total Long Term Deflection allowing for all effects of all loads, cracking and long term creep and shrinkage effects. And then calculate the estimated initial deflection due to DL and any load applied at the time the attachment that is going to be affected by deflection is attached and subtract that from the Total Long Term deflection. This gives what we would call the "Incremental Deflection", i.e. the increment of deflection that will affect the attached item.

Same should be done for Timber.

Unfortunately, I think the initial wording for all of this came from Steel Design where the member is elastic an none of the peculiarities of inelastic behaviour need to be considered.

 

[jdgengineer]

The way rapt described is in more inline with how MacGregor stated it. I simplified a bit, his description is better.