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How to access existing steel without original loads?

How to access existing steel without original loads?

How to access existing steel without original loads?

(OP)
On a current project we’re slightly increasing the point loads at a few locations on the existing steel (ordinary concentrically braced frame) at a few locations inside a power plant. I would normally use the IBC provision 3403.3 stating existing members carrying vertical load can accommodate a load increase of 5% without needed evaluation. Because the horizontal steel does not support any slabs, gratings, etc. the load increase on the point loads on the beams is in the realm of 10% negating the use of this provision. Without having the existing loads on these beams (the client doesn’t have them), how can I evaluate the existing steel for the increased loads? Intuitively I know that adding 5kips to a W21x211 with a 30ft span will likely be acceptable but how can I prove this without having the original calcs that determined the beam sizes?

RE: How to access existing steel without original loads?

Am I missing something or can't you simply use the beam's capacity as the "original loads" (i.e. the designer designed the beam to just work with a unity = 1.0).
Then with your added load you can evaluate the percent change in the various effects (moment, shear, etc.).

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RE: How to access existing steel without original loads?

If this is not a private power plant for a factory, it is probably regulated by FERC. They might either have the drawings or could recommend loads to use.

Mike McCann, PE, SE (WA)


RE: How to access existing steel without original loads?

Like JAE, I think you have to work from measured section properties and determine the capacity of the system.
Regarding the 5%, I understand the codes to apply it to forces in elements rather than loads on the member.
Additionally I've worked on remodel/retrofits where our determination of capacity based upon measured section properties led to a conclusion that the structure was not properly supporting the existing load, not to mention a 5% increase.

RE: How to access existing steel without original loads?

(OP)
JAE: That is a great idea, is there a code basis (IBC, ASCE 7, AISC 325, etc.) for using the beam's maximum capacity as the original load? It's very likely I would fall within the 5% allowable increase.

Msquared48: I have drawings for the plant, just no loads. These beams aren't supporting any vertical load. Would FERC perscribe loads to use that are in addition to ASCE 7?

Triangled: Are you recommending that I check the beams can take the load increase and assume there is no existing loads other than self weight?

RE: How to access existing steel without original loads?

I concur with JAE's approach. Since you have the drawings, don't just calculate the beam's original maximum capacity by assuming a uniform distributed load over the full length. You mention that your are "increasing the point loads". Point loading of beams is common in an electric generating stations. Calculate the beam's original maximum capacity by assuming the loads are applied at locations per the drawing (confirmed by field inspection). Then, add the 5 kip additional load at the appropriate points. Depending of the loading points, shear may be more of a concern than moment. Be sure to check the capacity of the beam's connections, too. Confirm that there are no addition loads added since the drawings were made. Power plants are modified often and not all changes are recorded.

www.SlideRuleEra.net idea
www.VacuumTubeEra.net r2d2

RE: How to access existing steel without original loads?

StructureMan44:

They could. Power plants are notorious for moving heavy equipment.

If these beams support no vertical load though, are they part of a moment frame?

Mike McCann, PE, SE (WA)


RE: How to access existing steel without original loads?

Also, if you can't track down enough information to be comfortable, there's a point where you just have to decide to modify and add new capacity equal to the new load.

RE: How to access existing steel without original loads?

StructureMan44:

"Triangled: Are you recommending that I check the beams can take the load increase and assume there is no existing loads other than self weight?"

No, if I understand what you're thinking in your question to me, I am definitely not recommending that. What I am suggesting though is that the 5% thingy maybe is not the way to approach this calculation. You mention you have the drawings, which i would suppose, have the member sizes and connections. I'd recommend that you determine what the new loading combination will be and check the structure for that combination. This eliminates errors associated with the observations of others here such as Msquared48 "Power plants are notorious for moving heavy equipment." If multiple engineers sequentially apply the 5% thingy (neutral) to the same structure, the last one in line could be signing off on something 50% overstressed.

RE: How to access existing steel without original loads?

Even if you had the information from the original design, you would still need a site survey to see what is on the beams now. Some of the "beams" may be struts/tie beams carrying wind or other horizontal loads (think duct loads) to the bracing system. The plant people often store tubes and pipes etc. in the boilerhouse, in areas with little equipment, i.e. along the edges of the building, set down on these tie/struts.

In the original design, it was common, but not universal, to leave a large margin for additional loads, this may work for you.

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

RE: How to access existing steel without original loads?

(OP)
Thank you for all the great replies folks - very intelligent and generous folks here in the structural forum with advice that is greatly appreciated for this young engineer.

Triangled & SlideRuleEra: I will perform a site visit to ensure the pipe supports (some of which will be increasing) shown in the drawings and photos are the only loads on the beam.

Msquared48 & Paddingtongreen: The structure looks to be all braced frame not moment frame. The larger secondary horizontal members look to be sized mainly for resisting horizontal load as they are 30ft long and they support the external wall of the plant.

TLHS: I would enjoy and feel more confident performing a full analysis of the structure to determine the loads in the primary beam (supporting the pipe) and secondary beams (supporting the primary beam) but unfortunately time and budget don’t allow this.

I’ll confirm the strength and connections of the primary beam are acceptable for the load increase. Should the secondary beams that support the primary beam have no additional loads (additional pipe supports not shown in drawings or photos, moved heavy equipment. etc.) beyond those from the primary beam, then it appears the most prudent approach is to verify the additional load from the primary beam is below 5% of the secondary beam’s nominal capacity and then write off the load increases as acceptable.

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