Platform Live Load Vs Dead Load Calculations

[CSCPE24]

I would like to pick the brains of you experts out there about the proper calculation method of a support structure for platform.

For discussion purposes, let's say that I have a platform 3' wide x 10' long (19W4, 1" x 1/8" grating) that is simply supported by tube steel at the ends. (Think coffee table)The grating span is reinforced by 5"x3"x1/4" angle, toe-up.

If I was interested in the values of the reaction at each support for base plate design, it would be a simple statics calculation based on the dead weight of all the steel involved.

Now here is where I need clarification:

If the platform is required to handle 100psf of live load, am I supposed to ensure that I factor the additional total 3,000 lbs into the reaction for base plate calculations?

What is the proper method of such calculations to ensure that the live load capacity is always met?

The reason I ask is because in many facilities with platform grating, I noticed that even though the platforms are rated for 100psf or 125psf, the support steel are nowhere close to what they should be if the total live load is factored into the calculations for each base plate.

Thank you in advance for your comments and insight.

 

[hokie66]

If the live load is required to be 100psf, then all components of the structure need to be designed to carry their share of the load. The exception is where live loads reactions are allowed by code to be reduced for large areas, based on the unlikely event of the live load being over the entire area at any one time. This is probably not the case for a grating platform.

You could, however, have grating which is capable of resisting a greater load than required, and in that case the supporting structure need only carry the required load. The selection of grating is often controlled by deflection rather than strength.

 

[CSCPE24]

hokie66, what if like you said, that it is unlikely that the live load is present over the entire area at any one time?

Does the BOCA National Building Code or IBC contain information showing when and how much to reduce live load reactions?

Thanks for the clarification about the deflection of the grating.

 

[hokie66]

As I practice in Australia, I am not sure where the live load reduction factors are contained in your codes. But live load reduction is only used for large floor areas, and many engineers do not choose to reduce live loads except for on the columns.

 

[JStephen]

First off, are you sure the installations you're seeing are really rated for 100 psf uniform live load? I know in the applications we get into, there is a concentrated load requirement, but not a uniform live load requirement, and that could explain having similar grating but reduced support details.

 

[CSCPE24]

JStephen

Yes. The reason I'm sure is because the installations are based on customer specifications that require either 100 psf or 125 psf ratings on platforms. Most of these setups are typically for elevated equipment structures or equipment mezzanines if you will. We have concentrated load requirements too and normally that is approached by deflection of L/240 or L/360, etc.

With that said, we normally calculate and size for the dead load but that's what is confusing. Live loads of 100 psf and 125 psf to be used accross the board for platform erection does not make sense because the reactions to the supports would be astronomical especially when the platforms are normally like 30'-0" wide be 60'-0" long.

 

[frv]

The IBC does contain provisions directing you on how and when to reduce live load.

I don't have it on front of me, but it's based on trib area of the member in question and what type of member it is. For example, interior vs. exterior columns are reduced at a different rate.

Be aware that per code you may not reduce the live load if it is over 100 PSF.

I do not know what type of loading is likely on this platform, but I would suggest using your judgment on whether it should be reduced. Remember that a reduction is allowed, not required.

 

[doct9960]

CSCPE24 (Mechanical),

I assume you're a mechanical engineer. I strongly suggest that a structural engineer supervise you in the design of your platform. The structural engineer should be proficient in the building code that the platform needs to be designed for.

You don't just design a platform for dead load and live load. You also design them for other applicable loads like wind, earthquake, snow loads, etc. After determining all the applicable loads, you need to apply them in load combinations and load factors as specified in the applicable building code. You then design your platform and other structural components based on the worst load.

 

[CSCPE24]

doct9960,

You're correct. My area is mechanical. However I find my scope spanning across structural issues quite a bit lately.

We normally have a structural engineer to sign off on such designs prior to erection but I normally have to do a preliminary design layout for R.O.M. purposes.

BTW, all these platforms in question are indoors. Therefore, the only other "external" forces other than dead and live loads to consider are earthquake loads.

Thanks for the feedback.

 

[gwynn]

Some jursidictions will allow both a psf rating and a total load rating. In other words, a platform not accessable to the public may be rated at 100psf or say 750lbs live load. This is the only thing I have seen that is not mentioned above and may explain the low total loads used in design.

 

[IFRs]

This post went on way too long. If the live load is 100 psf then that is it. Design for it. Trying to weasle out of it is not what engineers should be doing. If the load is not 100 psf, fine - document what the loads are and move on. If that means 60kips on the support columns then the columns need to be disigned for it. Not really that big a deal. For large spans there will be bending in the beams and columns depending on the connection type. There may be sideways loads also - earthquake, random forklift hits, etc. A good thorough professional job is required. If the previous platforms were under designed it is soneome's responsibility to re-rate them for proper loading before someone gets hurt. If the owner does not like your design, they can go somewhere else. Sorry for the rant.

 

[CSCPE24]

Gwynn,

That makes sense as most of these platforms we deal with are strictly for maintenance personnel only.

IFTs - Thanks for your viewpoint. However, in our profession, it's not the point of trying to weasle out of things but taking the best approaches to solutions both from a safety and economical point of view. Besides, based on what I noted before, if it was a 30'-0"W x 120'-0"L platform rated at 125psf, many of the courier commercial buildings do not have concrete slabs that can withstand 22.5kips per reaction assuming I'm using 20 supports over the whole platform. They would either have to add more supports or increase the base plate sizes and both would prove undesirable under normal forklift traffic operations.

Last but not least, I think IFRs has brought up a good point. I think this discussion has gone a little too long but thank you everyone for your very helpful comments.

 

[civilperson]

A base plate of 6" square could support the 22.5 kip load with sufficient subgrade strength or slab capacity. Why do you assume that the plates are undersized?