Pre-engineered Metal Building Design Loads

[ron9876]

I have a question about the gravity loading to use for pre-engineered metal buildings.

I don't have a copy of the MBMA manual. Does anyone know if it allows different loading and related structural criteria than the governing building code (Florida Building Code in this case)?

What are the typical superimposed/collateral gravity loads that would be used for a church?

 

[nutte]

Are you the engineer specifying what the metal building manufacturer needs to design the building for? If that's the case, prepare to get resistance to whatever you specify. The collateral load is the one gray area that they can skimp on, reducing the building weight and thus their cost. We specify 10 psf collateral load for churches, but the metal building guys want to use 5 or even 3 psf. Add up the components that are going to be hanging from the roof (drop ceiling, ductwork, electrical, sprinkler system) and see what yours ought to be.

Alexander Newman discusses this in his book "Metal Building Systems - Design and Specifications." He concludes that with the real-world behavior of purlins loaded with concentrated loads, such as from sprinkler mains, not being fully understood, that it's best to err on the side of caution and go with a higher collateral load.

 

[msquared48]

Insulation, sprinklers, roofing, suspended ceiling, electrical, mechanical - it all adds up and 10 to 15 psf is normal.

And this is not to mention the temporary construction load.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ron9876]

I called for 16 psf.That included the roofing and insulation which they include in their dead load. I fine tuned the loads and agreed to 10 psf (not including deck and insulation).

They have responded that they use 5 psf for offices/churches, etc. and 3 psf for warehouse type structures.

I am curious if the MBMA manual has a reference for recommended loads.

Also ASCE7 has a minimum allowance of 4 psf for mechanical ducts. Do you guys read this as a requiremnt or a recommendation?

 

[ToadJones]

Personally, I see ASCE 7 for exactly as it claims to be:

"Minimum Design Loads for Buildings and Other Structures"

I take the key word to be "Minimum".

 

[Ron]

Ron9876...MBMA Manual does not supercede the code. Apply the code-mandated loads under the Florida Building Code or ASCE 7.

If you are using ASCE 7 as your criteria base, then all of its provisions apply, and as TJ stated...they are "Minimum".

Regarding another issue with purlins in PEMB's, when you apply a concentrated gravity load such as to hang sprinklers or mechanical equipment, you risk taking the purlin out of plane for its design properties. For wind loads on the gable end, this can create a buckling problem for the purlins...Check it.

Also, keep in mind that if you have large doors on the building, such as a hangar, you should design as partially enclosed rather than enclosed, because of the gaps at the doors (see ASCE 7 Commentary C6.5.11.1)

 

[nutte]

Regarding ASCE 7 and 4 psf for mechanical ducts:

There is a table of various building components/materials and their weights. That is where this 4 psf comes from. This table is located in the Commentary, which is not part of the ASCE Standard Minimum Design Loads for Buildings and Other Structures. It is provided for information purposes only.

So, 4 psf may be a good value to use, but it is not mandated that you use at least that.

 

[mbullism]

I'm sure it's not the same in every state... Re: dead loads in MA under the current code, they require you use the ASCE loads as a minimum...

 

[msquared48]

ost of the time, the "Dead Load" I have seen metal Building manufacturers spec is the weight of the metal roofing, pirlins and mainframes that DOES weigh 3 to 5 psf.

They, and others, obviously need to up the anty here and apply the actual dead loads that are going to be seen by that particular structure, not what they feel is normal in the industry for that type of structure.

You need to DEMAND the 16 psf, and document the loads seen by the structure that totaled to that amount. As the supplier, they MUST comply to your specs.

Possible additional problem: Did they already submit a bid based on the 3 to 5 psf value to design, fabricate and erect the structure?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jheidt2543]

msquared48,

I have to disagree with some of what you said. It is the EOR who has to determine the building loads to be addressed by the metal building manufacturer; they don't know the details of the building the way the EOR does. If you give them the loads to design for, they will do it.

Secondly, the Metal Bldg. industry differentiates between dead load (the weight of the building componets) and collateral load (the weight of attached items in the building treated as dead load).

On a recent job, one method of handling this for a light industrial building was to call out a DL = 4 psf over the entire roof area (the weight of the actual building components), plus 4 psf collateral on purlins and 10 psf collateral on main frames. The result was that purlins were designed for a total "dead load" of 8 psf and main frames 14 psf; realistic values for this particular structure.

 

[ron9876]

Yeah Mike that is the problem. Their bid was based on "standard of the industry" not the drawings. I don't know how they can be comfortable taking that risk but so be it. I know that I can require them to design based on my drawings but I want to be sure that I am being reasonable. They think these loads are double what is reasonable. Any input?

Ceiling..............1 psf
Sprinklers...........3 psf
Ductwork.............3 psf
Lights...............1 psf
Misc.................2 psf

I guess I am starting to get to the point that I am an old fart but this business that is my chosen profession is not so much fun anymore.

 

[msquared48]

Jheidt2543 brings up a VERY applicable point though with the collateral load comment. That fact had slipped my mind.

If you look on the calcs and add their dead load value to the collateral load value, what is the total? If it is 16 psf or greater, I think you may be OK if the loads are correctly applied with the proper load combinations.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[ron9876]

Like I said above this supplier included the roof deck and insulation in the dead load. So that would reduce my 16 psf. Collateral load is everything else.

Got an email from the supplier with their standard dead loads:
Suspended ceiling....1 psf
Lighting.............0.1-1 psf
HVAC ducts...........1 psf
Sprinklers...........1.5 psf (dry)
3.0 psf (wet)

So that is how they arrive at 5 psf. Only 1 psf for HVAC. If the owner wants to add anything in the future they would have to add framing.

 

[Ron]

ron9876...exactly...this my comment about the purlins. Even a large fan for air movement could be an issue of hung from the wrong location.

Their dead loads are a bit light, but then they will argue that they are only supplying a building shell with a minimal allowance for anything.

I am also of the opinion that these buildings typically have way too many air leaks to be considered enclosed (if large doors of any type, particularly rolling doors). I did a check on one just before going to trial on it about 6 months ago....windward gaps exceeded 4sf allowable around doors (for a 12x12 door, you only need a 1-3/8" gap all around to exceed, assuming a seal on the bottom)

 

[msquared48]

He pulled these values from Table 1.1c of the MBMA. You might point out to him, however, that the title of this table is "Typical Collateral Loads". Note tue use of the word "Typical" here.

This implies that there are exceptions. The manual also states that the local BO can alter these load values if he or she sees fit. This includes you as the approving engineer of record.

For the record, collateral loads are all those dead load weights of materials that are not supplied by the metal building manufacturer, such as mechanical, electrical, suspended ceiling, sprinklers, etc. The decking does not see these loads, but the purlins might, and the mainframes will.

Section 1.1 Introduction, at the end of the first paragraph, an interesting statement is made. After referring to the IBC and ASCE7 as the source documents, it states that "The user should refer to the source documents for a complete presentation of the loading requirements and only use this Manual as a review and commentary."

Sounds pretty explicit to me. Stick to your guns.



Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[jheidt2543]

Ron's point about locating an RT fan in the wrong place has happened. However, I don't think it is resaonable, nor do most owner want a building designed for every conceivable load. We design to a resonable standard to keep buildings affordable and safe, not indestructable.

Roof top HVAC units should have supplemental framing to carry the load to main framing members. I think this is one reason a dual load system is good, every purlin is not going to see an RTU but any main framing member may.

 

[abusementpark]

I've always gotten the impression that the collateral is simply the additional "fat" we like to put in the dead load. Thus, this is why it is not included in the uplift load combinations.

 

[jheidt2543]

I do think collateral loads are real and I thinkd that's the way the metal building manufacturers look at it too. They split the two (DL and CL) to make the distiction between the building componets' weight and the building contents.

Regarding up lift, I suppose everyone has their own way of looking at it, but I think collateral loading should be added with DL for the uplift calculation. If they are supposed to be real loads, they should be included. And, they still would get factored down, .6(DL+CL).

Just my two cents.

 

[hokie66]

Collateral loading may or may not be there, so should not be included in wind uplift calculations. Collateral load is really live load, as it can change with time.

 

[ron9876]

Thanks for the input. I have decided going forward that for a church type of structure I am going to use 8 psf collateral load and require that any equipment and larger sprinkler lines be specifically designed for.