Live Load Reduction 1

[KOTOR1]

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.

 

[KOTOR1]

The new beam fails with a 20 psf live load. The owner is saying well the beam where a similar unit was added worked in 2000 even tough it was longer and now it should work easily with a shorter span. I can understand what his point is.

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.

 

[msquared48]

I don't understand why you are using 20 psf here instead of 16 psf in your calcs. According to ajh1, at 4:12 pitch, 16 psf should be ok at 200 Sq Ft or less. The same table is also in ASCE 7-05 as I mentioned above.

Am I missing something here? I apologize in advance for any misunderstanding I may have.

Mike McCann
MMC Engineering

 

[JAE]

It might be that under the current code, this slope reduction isn't used - I probably should have looked in the IBC also. Is that why there is a "problem", KOTOR1?

 

[KOTOR1]

JAE is right, the current code does not allow for slope 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.

 

[BAretired]

How much overstress are we talking about? If the beam "fails" with a load of 20 psf but "works" with a load of 16 psf then the design is too darn tight.

Have you tried using Limit States Design, or LRFD as you call it, and if you did, what were the results?

BA

 

[KOTOR1]

The beam is about 7% overstressed.

I have never really used LRFD w/ wood design. I will try it on Monday and let you know.

 

[BAretired]

Maybe I missed it somewhere in this thread but I was not aware we were talking about wood design. In Canada, we use Limit States Design for concrete, structural steel and timber. If you need any help with the timber part, let me know. So far as I am concerned a 7% over-stress in Working Stress Design (WSD) for wood is a tad more than I would like, but is not a big deal. The allowable stresses are not that well known.

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

 

[KOTOR1]

BA,
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