Existing Steel Beam

[engr567]

I have an existing roof beam, and a new RTU is to be supported by this beam. I was told that an existing beam can be overstressed a maximum of 10% and still be within code allowance? Does anyone know about this? If so, where can I find this info in IBC 2006? Thanks in advance.

 

[MiketheEngineer]

I think it is 5% without having to do do a re-calc of the whole structure.

Look into moving it slightly and sharing the load with a nearby beam or bar joist.

 

[dcarr82775]

I have used 10% as a rule of thumb for whether or not to fix something. Depends on the piece I am looking at. That being said, the IEBC gives 5% over.

 

[engr567]

Mike,
Given the dimensions of the unit, there is not much we can do to relocate the unit and reduce the load on the beam. Based on my calculations, I came 9.5% overstressed. Also, do you remember a section in IBC that talks about this provision?

 

[jdgengineer]

In California it is in Chapter 34 - Existing Buildings Chapter, but I believe this chapter is specific to California's Code. For the IBC code, I'd bet it's in the IEBC (International Existing Building Code).

However, it's important to interpret the code language correctly. The code is not saying that you can have a beam that is overstressed by 5%. It is saying that you don't need to check the beam etc. if the loading to the beam has not increased by 5%. There's a difference. i.e. if you are increasing your beams loading by 25% but are only overstressing the beam by 5% that in my opinion is not the intent of the code.

 

[engr567]

jdgengineer,
Good point. However, I am little bit confused. As you mentioned, the loading increase by 5% is allowed. Does not it matter if the loading (in kips - equivalent to point load) increase is due to the additional point load at mid-span of the beam vs. the same load increase as uniformly distributed load entire length?

 

[msquared48]

Have you tried applying the provisions of LLR to the existing beam?

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[engr567]

Mike,
I have no clue how to apply the provisions of LLR? What is it?

 

[woodman88]

The California code is based on the IBC. The IBC also has chapter 34 Existing Structures. The 5% increase in gravity loads are noted in section 3403.3 and 3404.3.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[DST148]

Ref IBC 2003, Chapter 34 Existing structures, section 3403.2 - Additions or alterations to an existing structure shall not increase the force in any structural element by more than 5 percent...... My intepretation is that the ratio of the demand on the existing element due to revised loading to the demand due to existing loading shall not be greater than 1.05. This would apply to every limit state.
If the ratio mentioned above is > 1.05, but the existing element has the capacity to meet the demand requirements of the revised loading based on the present day codes, then the existing element would still be acceptable.

 

[steellion]

Agreed with jdg and DST, an increase in the load of 5% requires re-analysis for the beam to meet current code. This does not allow a 5% overstress.

What you can do is check the beam using LRFD, which will generally get you a little extra capacity beyond the ASD which the beam was likely designed for if the building is 10+ years old.

 

[MiketheEngineer]

I agree

 

[JAE]

I think Mike was referring to live load reductions (LLR) to get some extra capacity usage out of the structural element.

 

[bookowski]

I agree with the others here, the intent is not that you can overstress a beam.

Can you do anything to get more capacity such as reducing the unbraced length or refine your loads? I didn't see any mention of what type of beam you are talking about, I'm assuming steel - LRFD should help.

 

[msquared48]

JAE is correct. Sorry - I was out most of the day.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

But you can't reduce roof live load, can you? I would just sharpen my pencil. Haven't seen a case yet where I can't rationalize away 10%.

 

[msquared48]

I don't know why not, unless it is a snow load.

What is your live load for the roof? And where is this project?

I will admit that I do not normally even using LLR except for larger structures and beams/columns supporting a lot of area. For me, it has always been something I could use in my hip pocket to help solve a problem.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

Maybe I am confusing requirements in different countries here. We don't reduce roof live loads in Australia, and I thought that was a fairly uniform thing, but perhaps not.

 

[engr567]

Thanks all for your input. This gives me plenty of options to think. BTW, this project is in East Coast with snow load (live load) 30 psf and flat roof. I do not know if there is anything I can do to reduce the live load and still be within code provisions.

 

[engr567]

A question though, how do you interpret IBC provision of "---- shall not increase the FORCE in any structural element by more than 5%....? Does it mean by LOAD, OR REACTIONS, OR MOMENT, OR ...? Does it matter if the original design was based on UDL and, new POINT LOAD (load increase in Kips is still within 5%) is applied at the center of the span or vice-versa? Seems confusing....