5% Gravity Load Rule and Self Weight

[Insp-struc]

When reviewing an existing building and doing an alteration by adding equipment to the roof, would the 5% gravity load increase "rule"/exception account for the self weight of the roof members? So if the original design was for 50psf (dead and snow) and now i am getting 55psf, this will substantially affect the 5% if the roof was an open web steel joist and metal deck compared to a precast/cast in place configurations.

 

[phamENG]

I've always understood the % increase rules to relate to the lateral force resisting systems and primary framing a few steps beyond the area you're modifying.

If you're adding load to a floor or roof - analyze the directly effected framing and whatever is supporting it. Chase the load until the member you would try to analyze has an influence area much larger than the area where you're working. Only then will you reach a level of probabilistic load application that would make the allowable % increase a rational analysis/design decision.

 

[hawkaz]

The IBC allows a 5% increase in design gravity load. D+L are gravity loads, so I use them both.

I agree with phamENG that direct analysis is better, but unfortunately, original building info is not always available to do that

 

[dhengr]

Insp-struc:
For starters, you’ve already suggested a 10% increase (50 to 55lbs./sq.ft.) in the roof loading, and you should absolutely prove that that’s o.k. Furthermore, your new rooftop equipment does not add 5lbs./sq.ft. to the whole roof. It adds a fairly sizable load to a 20 or 30 sq.ft. area and then is initially carried by only a few members, which should be checked out. I’ve always understood the 5% load increase allowance to give you some permission to add a little load, after the fact, using some common sense, without having to redesign the whole structure. Our design loads and methods are generally not any more accurate than +/- 5%, we apply factors of safety to everything we do, we spec. min. strengths for all the bldg. materials, which are usually exceeded, and/so you can usually find 5% fat in most designs. We also know that loads, load paths and max. stresses can adjust and redistribute to some extent, but not if there is only one load path. This all has to be considered in the overall picture/context. Then it is up to the engineer to use some common sense, good judgement and experience in determining exactly where to draw the line. This should be interpreted with the same sort of logic (I guess, sort of the inverse logic) which allows us to start reducing some of the live load at some point, when it seems reasonable that every sq.ft., of a large enough contributing area, will not be fully loaded. I interpret this to mean that I can have a 5% overstress in most cases, members, connections, etc. and not have to strengthen these elements, even though that is not exactly the way it is stated. The code thought process was…, a 5% uniform load increase will cause a 5% stress increase in a simple beam or a column, period. It certainly doesn’t mean I can take 5% of the total cap’y. of a given element, or 5% of the max. allowable load on the element (a beam or a bar joist, for example) and apply it indiscriminately, as a point load, and not cause any problems. At the very least, you should be checking that this new load does not introduce any new local problems, excessive stresses, deflections, buckling, etc. in its application. Then, follow this out to the reaction points of the member and see that it has only changed things by a percentage point or two. The effect of the new load quickly peters out, and is less likely to be an issue, as you move further away from the point/area of loading. To stretch your example, only a little bit, you would/might take 5% of the weight of the fully loaded bldg. and apply that to a beam or bar joist, and think that met the letter of the bldg. code?

 

[Insp-struc]

Thank you for the replies.

In this case, as a global check, the 5psf load increase does apply.

If I was going to break out a simple example.

- Under old loading - design is for 50psf to the roof members
- This is in an area where snow load will control over Live (no live load reduction will help)
- Under new loading and the alteration by adding equipment there is a total of 55psf on the roof members
- lets assume the roof members are no longer manufactured and load tables/drawings are not available
- Consider everything a simple span
- This is not considering that the member is 5% over capacity in any way

If we just look at the moment equation and increase in loading:

wl^2/8 where w considers the self weight of the member in it
w = (D+S) and is now allowed for 1.05*W = (D+S)
Given the construction (if precast or concrete) the weight can be substantial

I understand that if the member is a substantial self weight it has already been designed with that in mind so something like 5psf gravity load (which was 10% increase to the member) comes to be 1% to 2% bending stress increase.

Thank you for your input, and I apologize if I have made this confusing