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.

 

[msquared48]

No. Forget it.

Mike McCann
MMC Engineering

 

[KOTOR1]

So the engineer assumed the live loads incorrectly then?

 

[JAE]

The UBC used to have a two-option approach to roof 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

 

[msquared48]

JAE:

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

 

[KOTOR1]

Mike, the reduction was applied for the live load. There is no snow load on the building. The building is in Florida.

 

[msquared48]

Sorry. Roof "live load" here in Washington, unless it is a mechanical load or assmbly, is always governed by snow load. I have never even thought of it in that way...

Well, ya learn something new every day. Thanks.

Mike McCann
MMC Engineering

 

[msquared48]

If you do use the slope reduction though, I would base the design of the rafters on their true length (slope length) and not the horizontally projected length.

Mike McCann
MMC Engineering

 

[swivel63]

the building is in florida and you're using the UBC 97? what happened to the SFBC or the FBC? or is the verbage similar?

 

[KOTOR1]

Swivel63, the building was built in 1998. We are doing a TI for the building. I think the verbiage for the SFBC or FBC then (in '97) and the UBC '97 was the same.

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.

 

[msquared48]

I believe that the reduction still applies. Check ASCE7, section 4.9.1, and note "h" of table 4-1.

Mike McCann
MMC Engineering

 

[JAE]

I don't have a UBC 97 handy right now but I don't recall a slope reduction in RLL. But typically you use the horizontal projection of a beam to determine RLL moments and shears, not the slope length. What Mike is pointing out above is that the slope reduction to 16 psf "might" be due to simple geometry where they are taking a vertical 20 psf and determining the orthogonal component to the beam and then determining shears and moments....I'll see if I can check in a UBC 97 later.

 

[kslee1000]

Since this project was designed in 1997, and built by 1998, this is an "existing structure", for which, uless it needs upgrade that either adding new loads, or involving some major changes, the original code and design stand valid.

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.

 

[hokie66]

What's a TI?

 

[msquared48]

Tenant Improvement

Mike McCann
MMC Engineering

 

[apsix]

Thursday Island

 

[beton1]

1.Tell the owner the code has changed. You have to conform.

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.

 

[ajh1]

UBC 97 Method 1 has a step function for both tributary area and roof slope. Note that live load reductions for roof and floors are different.
<= 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.

 

[kslee1000]

I don't know how many building was designed and built prior to 1999 that were taken the advantage of live load reduction, I afraid that a TI can simply say to a clint, "Hi buddy, rule has changed, you will need a new roof for your 10/20/30 years old house to be in compliance." If the change was based on credible failure events (not just we want do better), the code official shall pledge with the public and the government agency make it a well known fact, then it is at the hand of the TI & the owner.

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?"

 

[JAE]

ajh1 has it right - I forgot about the slope portion of that table (see attached).