Live Load Reduction
Live Load Reduction
(OP)
I was looking at the calculations for an existing building done in the office by another engineer using the 1997 UBC. The slope of the roof was 4" inches vertical every 12" horizontal. The live load used was 16 psf. Was there a reduction in live loads in the UBC 97 for that slope? I just need to verify that since according to the IBC 2006, the load no longer can be reduced. I am doing a TI for the job.






RE: Live Load Reduction
Mike McCann
MMC Engineering
RE: Live Load Reduction
RE: Live Load Reduction
Both were based upon tributary area. The first option used a formula where you could calculate a live load reduction directly. The other approach used a small table that had this:
Tributary Area Roof Live Load
0-200 sf 20 psf
200-600 sf 16 psf
>600 sf 12 psf
RE: Live Load Reduction
I believe he is talking only about a slope reduction here, irrespective of any area redcuction that you mention, which I would never take anyway for snow due to the nature of the animal.
Mike McCann
MMC Engineering
RE: Live Load Reduction
RE: Live Load Reduction
Well, ya learn something new every day. Thanks.
Mike McCann
MMC Engineering
RE: Live Load Reduction
Mike McCann
MMC Engineering
RE: Live Load Reduction
RE: Live Load Reduction
Mike, I have a horizontal beam under the roof trusses (sloped). About seven years ago, they added a new mechanical unit on one of the beams supporting the sloped trusses. UBC'97 governed then too. This beam worked with the 16 psf LL. Now they are planning to add another mechanical unit on the roof, at a beam that is about 0.5' shorter than the one we had to look into in 2000. Using the 16 psf LL and 200 lbs additional the beam is working. Now if I use 20 psf, the beam doesnt. We are going to find it hard explaining this to the owner.
I will be talking to the building official to see if I can use the older code, but I doubt it.
RE: Live Load Reduction
Mike McCann
MMC Engineering
RE: Live Load Reduction
RE: Live Load Reduction
However, if you can justify, or has doubt, there is odds that the roof would be experiencing the newer load in its service life, for such case, redo the calculation and strengthen the roof if necessary. But, before that, inform and consult with your supervisor for proper actions.
RE: Live Load Reduction
RE: Live Load Reduction
Mike McCann
MMC Engineering
RE: Live Load Reduction
RE: Live Load Reduction
2.Therefore, reinforcing of structure may be required.
Is this a wood or steel roof structure? 200 lb is not a heavy mechanical unit. Access for work would be the issue,
depending on ceiling finishes.
RE: Live Load Reduction
<= 200 sqft 201 - 600 > 600
< 4:12 20 16 12
4:12 to < 12:12 16 14 12
>= 12:12 12 12 12
Method 2 does the same sort of thing with a straight line interpolation between the corner points of the above table.
ASCE7-05 has similar equations for the straight-line interpolation, no step function.
RE: Live Load Reduction
But, if this is a home improvement project, then go ahead, let the client know that the code has changed due to some concerns, and ask. "Do you want to live with the risk, or else?"
RE: Live Load Reduction
RE: Live Load Reduction
My boss is still thinking about the whole matter. I don't know how we will approach the matter further. I will keep you posted.
I guess, the original engineer should never have used the 16 psf.
RE: Live Load Reduction
Am I missing something here? I apologize in advance for any misunderstanding I may have.
Mike McCann
MMC Engineering
RE: Live Load Reduction
RE: Live Load Reduction
We had to go to Florida (we are based in New York) and talk to the owner and the city representative regarding the beam. We just got back today. The owner does not want to budge with regards to the beam and the city is fine only if we allow it. My boss is possibly going to make a decision on Monday.
RE: Live Load Reduction
Have you tried using Limit States Design, or LRFD as you call it, and if you did, what were the results?
BA
RE: Live Load Reduction
I have never really used LRFD w/ wood design. I will try it on Monday and let you know.
RE: Live Load Reduction
On the other hand, designing to such light live loads is not something which I would recommend. The slight saving in material is simply not worth the worry when something changes. I realize, of course, that you are stuck with an existing situation and cannot change it...but remember this in the future. It is a valuable lesson.
BA
RE: Live Load Reduction
I waiting for my boss to give me the direction on where to go on the wood beam. I will suggest the LRFD method to him. If he says yes, I may be needing help.
Thanks,
Kotor