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Live Load Reduction

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

No.  Forget it.

Mike McCann
MMC Engineering

RE: Live Load Reduction

(OP)
So the engineer assumed the live loads incorrectly then?

RE: Live Load Reduction

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


 

RE: Live Load Reduction

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

RE: Live Load Reduction

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

RE: Live Load Reduction

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

RE: Live Load Reduction

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

RE: Live Load Reduction

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?   

RE: Live Load Reduction

(OP)
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.

RE: Live Load Reduction

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

Mike McCann
MMC Engineering

RE: Live Load Reduction

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.

 

RE: Live Load Reduction

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.   

RE: Live Load Reduction

What's a TI?

RE: Live Load Reduction

Tenant Improvement

Mike McCann
MMC Engineering

RE: Live Load Reduction

Thursday Island

RE: Live Load Reduction

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.

RE: Live Load Reduction

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.

RE: Live Load Reduction

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?"

RE: Live Load Reduction

(OP)
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.

RE: Live Load Reduction

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

RE: Live Load Reduction

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?

 

RE: Live Load Reduction

(OP)
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.
 

RE: Live Load Reduction

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

RE: Live Load Reduction

(OP)
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.

RE: Live Load Reduction

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

RE: Live Load Reduction

(OP)
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

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