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Existing Steel Beam

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

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.

RE: Existing Steel Beam

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.

RE: Existing Steel Beam

(OP)
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?

RE: Existing Steel Beam

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.

RE: Existing Steel Beam

(OP)
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?

RE: Existing Steel Beam

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

RE: Existing Steel Beam

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.

RE: Existing Steel Beam

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.

RE: Existing Steel Beam

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.

RE: Existing Steel Beam

I agree

RE: Existing Steel Beam

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

RE: Existing Steel Beam

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.

RE: Existing Steel Beam

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%.

RE: Existing Steel Beam

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
 

RE: Existing Steel Beam

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.

RE: Existing Steel Beam

(OP)
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.

RE: Existing Steel Beam

(OP)
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....

RE: Existing Steel Beam

hokie66,
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

Maybe this is why the 2009 IBC Section 3403.3 and 3404.3 has been changed to state "...an increase in design gravity load of more than 5 percent..."

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

engr567...I think you need to analyze the beam in its existing condition...get your maximum factored moments, shear, reactions...then analyze your beam with the RTU. If the ratio of new/old (for moment, shear, or reactions) is greater than 1.05 then you need to start looking at the entire load path and reinforce the members as necessary along the way.

I think DST148 said pretty much the same thing in an earlier post.

RE: Existing Steel Beam

You can't increase the force.  They are referring to the internal forces in the member, shear, moment, and axial.  It doesn't matter how the external forces are arranged on the member, the internal forces can't be increased by more than 5%.

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

(OP)
IBC 2009 and 2012 are clear. Makes sense now, 5% is based on gravity load. Thanks woodman88.

RE: Existing Steel Beam

Back to the live load thing, I suggest that the mechanical unit is a live load, so the portion of code required live load in the area of the unit is in fact the unit.  Does that help?  Not if snow load controls.

RE: Existing Steel Beam

Hokie,
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

Be careful with the 5% provision.  I've recently done some work in the telecom industry (adding antennas and equipment shelters to rooftops), and on a few occasions have come across sites where engineers have used the 5% rule of thumb on one retrofit after another.  Sure, it may be within the 5% over the existing condition, but is it within 5% over the original design condition?  Just make sure  additional loads beyond the initial design condition weren't already added to that beam before you say it's within 5% and further analysis is unnecessary.

Jake
http://www.pelicensemanager.com

RE: Existing Steel Beam

My point about the loading is that you won't have both the mechanical unit and another load in the same place.  Whether you call the mechanical unit live or dead, there is only one superimposed gravity loading on that area...except for snow load.

RE: Existing Steel Beam

I'll stick my neck out again (I forget who but someone disagreed with me about this in another post) & I'll say the snow load on a mechanical unit is insignificant.  I'm guessing you don't have a lot of experience with it in Australia, hokie, but we get a bit here in Canada.  If snow is building up on your RTU, you've got more immediate troubles than weight.  Your building must be getting pretty cold with the heat shut down.  Yes, I know not all RTUs are for heat, but the vast majority are producing or exhausting heat in some way & they don't accumulate snow on them.

RE: Existing Steel Beam

I like the idea shobroco, but what happens if the building is vacant and unrented for 6 months and all the mechanical equipment is turned off?

RE: Existing Steel Beam

It's unlikely that the heat is completely off unless the building is abandoned, in which case it may be a benefit that the roof comes down.  It saves demolition costs (that's a joke, okay?).  I've had discussions before over too many what-if scenarios & I figure at some point you have to decide if you're comfortable with the probabilities.  That's all we're working with anyway, isn't it?

RE: Existing Steel Beam

@jdgengineer - Ref ASCE 7-05 commentary - Brief power interruptions and loss of heat are acknowledged in the Ct = 1.0 category. Consequently, heated structures need not be designed for this unlikely event. Some dwellings are not used in the winter. Although their thermal factor may increase to 1.2 at that time, they are unoccupied, so their importance factor reduces to 0.8. The net effect is to require the same design as for a heated, occupied dwelling.

@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

Thanks for the insight DST. I don't use snow loads in my part of the country s a lot of this I am not familiar with. Seems to me not using any sow load on an area is different than using a Ct of 1.0 though.  

RE: Existing Steel Beam

If you get enough wind to get the worst case drift against the RTU, what are the chances that there will be snow left on top of the RTU?

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

Sorry to bring this thread alive again but what about the condition where you are within the allowable 5% increase but the beam is now overstressed? Can the beam be reinforced to the original loads and code or does it now need to be reinforced to the new loads and code?

RE: Existing Steel Beam

Drifted snow around the RTU is frequently more than the weight of the unit itself. If drifted snow contributes to the load on your beam, it must be considered.

BA

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