Existing Steel Beam
Existing Steel Beam
(OP)
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.






RE: Existing Steel Beam
Look into moving it slightly and sharing the load with a nearby beam or bar joist.
RE: Existing Steel Beam
RE: Existing Steel Beam
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?
RE: Existing Steel Beam
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.
RE: Existing Steel Beam
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?
RE: Existing Steel Beam
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Existing Steel Beam
I have no clue how to apply the provisions of LLR? What is it?
RE: Existing Steel Beam
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.
RE: Existing Steel Beam
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.
RE: Existing Steel Beam
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.
RE: Existing Steel Beam
RE: Existing Steel Beam
RE: Existing Steel 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.
RE: Existing Steel Beam
Mike McCann
MMC Engineering
http://mmcengineering.tripod.com
RE: Existing Steel Beam
RE: Existing Steel Beam
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
RE: Existing Steel Beam
RE: Existing Steel Beam
RE: Existing Steel Beam
RE: Existing Steel Beam
In the US codes, (IBC) you can reduce roof live loads based on tributary area similar to floor live load reductions. In the older codes (Uniform Building Code) there was a simple method:
0-200 sf - 20 psf roof live load
201-600 sf - 16 psf roof live load
>600 sf - 12 psf roof live load
The new codes are similar but provide a formula for the actual RLL to be used.
You don't reduce snow loads of course.
RE: Existing Steel Beam
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.
RE: Existing Steel Beam
I think DST148 said pretty much the same thing in an earlier post.
RE: Existing Steel Beam
If there is a 200 k-ft moment in the member due to some arrangement of loading, and a new point or uniform load is added to the member, the new moment can't exceed 210 k-ft. Same goes for shear and axial.
RE: Existing Steel Beam
RE: Existing Steel Beam
RE: Existing Steel Beam
The IBC and ASCE 7 definition for dead loads include fixed service equipment, such as . . . ventilating and air conditioning systems.
If the mechanical unit is tall, I would take a quick look at the wind load reactions (from overturning) from the mechanical unit to see if it adds a significant load to the beam.
RE: Existing Steel Beam
Jake
http://www.pelicensemanager.com
RE: Existing Steel Beam
RE: Existing Steel Beam
RE: Existing Steel Beam
RE: Existing Steel Beam
RE: Existing Steel Beam
@shobroco: I acknowledge your experience and observations, however,.....
For snow loadings, the American code uses Canadian code as a point of reference for thermal, aerodynamics effects etc. As per ASCE commentary : The combined consideration of exposure and thermal conditions generates ground-to-roof factors that range from a low of 0.49 to a high of 1.01....., all regardless of their thermal condition.
I have worked in Ontario Canada on projects related to pharmaceutical industry. This is what we have done - for new projects full snow load on the units; for checking adequacy of existing members, we started with full snow load on the units, reduced on few occasions, but never less than 2/3 of the roof snow load.
I agree with you, these are subjective.
RE: Existing Steel Beam
RE: Existing Steel Beam
I suppose you could have a 1/50 year snow storm with a large drift against the RTU and then get another significant snow storm with no wind which would load the top of the RTU (all while the RTU is not putting out enough heat to melt the snow)...but I think that is very unlikely.
Granted I am very new at this, but with that logic I have never considered snow loads on top of the RTU.
RE: Existing Steel Beam
RE: Existing Steel Beam
BA