Pre-Engineered Metal Building Roof Rod Bracing 4

[Rabbit12]

I've designed a few foundations for metal buildings but I don't claim to be an expert on the load path the PEMB manufacturers use. My question is specific to the rod bracing you commonly see in the roof of these structures. They installed some rod bracing that is bent pretty bad. I got a comment from a PEMB supplier's project manager that says these are non-structural elements and are only used to square the building. I had always thought these braces created a diaphragm. Am I wrong?

I'm going to challenge the statement and want something in writing from the PEMB manufacturer but wanted to check here with the experts in case they try to pull a fast one.

 

[hokie66]

retired,

Not to answer for JAE, but your diagram leaves out the compression struts. These are either discrete members, typically CHS, or in the case of smaller buildings, the roof purlins serve as bending/compression members.

Some small sheds, whether of hot rolled or cold formed steel, use the roofing as a diaphragm, and that is a consideration for the design engineer. Personally, I consider roofing to be just that...to shed water.

 

[r13]

hokie66,

That's my understanding too. Without purlins and roof panels, the lateral load is almost impossible to pass from one frame to the other, given there is rod in between only.

 

[r13]

JAE,

I'll accept your explanation, if I was told that in high wind area, and in seismic design, where buckling of the roof diaphragm is a great concern, then the rod can be used to complement the roof diaphragm in resisting the excessive lateral load, and to maintain the building/roof in squareness shape.

Edit
If the roof diaphragm does not have adequate stiffness as you've stated, then the rod needs to be adjusted periodically, as it inevitably would bend after repeat cycles of elongation (by tension), and shortening, or recover to the original length (by compression). In the end, a rod is usually a tension only member.

 

[human909]

And even with purlins and roof panels you can still readily buckle the roof if you don't have compression struts.

I had the chance to examine a lightly built portal frame industrial building that had its front wall blown in. There were no compression struts. The purlins bent, the nominally pinned columns on the facing wall deflected and cracked their foundations. The building was probably saved from further damages as the larger roller door blew off its tracks relieving a significant amount of pressure.

I had the feeling that the building was built and then signed off by a pliable structural engineer. Everything was excessively light.

 

[r13]

Internal pressure/suction might have been overlooked.

 

[rowingengineer]

Yes the compression doesn't disappear, the purlins in the diaphragm need to have the capacity for the mullion loads and bending.

http://www.nceng.com.au/

"Programming today is a race between software engineers striving to build bigger and better idiot-proof programs, and the Universe trying to produce bigger and better idiots. So far, the Universe is winning."

 

[hokie66]

Forget diaphragm action with metal roofing, in buidings of substantial size. With steel roof deck, you depend on diaphragm action, but the deck is not the roofing.

Some of the confusion here is because the initial question was posed with the diagonals in the roof horizontal truss being rods. They can be any section, and can be tension-compression or tension only.

 

[JAE]

...then the rod can be used to complement the roof diaphragm in resisting the excessive lateral load,

PEMB metal roofing, as hokie66 states, is NOT decking and has zero diaphragm capacity. Well OK maybe 1 plf capacity.
The rods do not complement the roofing, the serve as the only lateral diaphragm entity in the roof planes.
There is zero attention paid to the roofing during PEMB lateral design.

...the rod needs to be adjusted periodically,

This, I'm certain, never happens.

The rods are tension only members and do not ever get stretched into their inelastic range (either compression or tension) so any temporary wind force would have the rods simply come back to their original condition/length.

It seems as if you are suggesting that when the rods go into compression, they are somehow bent permanently. My experience is that the rods can simply bend out of the way to avoid the compression that may be introduced. For a 3/4" diameter rod about 17 ft. long (half of a 25 ft bay X brace length), a 10 kip load deforms the rod axially only .16 inches so it doesn't take much to have a rod bend "out of the way" of the load to avoid almost all stress.

 

[r13]

JAE,

If you are convinced the rod alone can create diaphragm, its fine. If you want to see distorted rod, just visit a few industrial buildings and look around, especially on walls.

 

[JAE]

Yep - have seen them...most always due to forklift impacts either directly or via pallets slamming into them. Never from wind.

 

[Ron247]

Only half of the tensions rods work at any given time. There is no attempt for the rods to take compression. When the wind changes direction, the other half works. If you can X-brace an open steel platform system that has no wall or roof panels, why can you not X-brace a PEMB?

When a PEMB is designed with rod bracing, the wall and roof panels do provide some diaphragm or shearwall ability but it is not part of the design for longitudinal wind. Installing the panels provides some ability, it is just not part of the design. The panels do prevent the rods from taking load up to some point because the loads are not capable of bypassing the roof and wall panel stiffness. Even with this you have to install the rod bracing properly tightened. If you do not, the roof and wall fasteners can loosen or slot the panels thus causing leaks. The fastener pattern is based on wind uplift and not on shear forces from the possible diaphragm.

While I agree there is some diaphragm ability, it does not negate the need for the rod bracing. Some codes use to specifically state you could not use panel diaphragm as a longitudinal brace system on PEMB style buildings. I am not familiar with all codes, but at one time I know they would not allow it.

Panel diaphragm is used at times, but they are typically small buildings and/or light wind loads (code permitting).

Also, even if you used the braced bay to square up that bay, how does it square up 3 bays down from it?

 

[RPMG]

In the Northridge earthquake, the rod bracing in PEMB's failed due to the hillside nuts on too thin of webs. The rod bracing, which is the PEMB's MFRS, failed at the connection. I don't know how bent the rods can be, but I can't believe that engineers are saying that they're not required once the building is erected.

I have seen calc's for PEMB's using the rods for the MWFRS. I have never seen a PEMB do a diaphragm analysis that uses the roof and siding as the load path. I have even designed foundations prior to the PEMB releasing calcs for schedule, based on an e-mail stating which bays would be braced.

 

[r13]

Yes, Ron's correct that some codes don't allow roof diaphragm as bracing for steel buildings.

Photos below just food for thoughts.

 

[Ron247]

I do recall a chart from the 80s where it showed how many lineal feet of a particular wall panel was needed to be able to remove the wall rods only. You still had to have the roof rods. There was a footnote about confirming the code would allow the substitution of wall panel for wall rods. This was not used for high loadings.

Years ago, several manufacturers reversed the wall panel material in their rollers to make a totally new offering without spending any money. It was generally referred to as a "reverse rib" or "shadow rib". Problem was that the original panel was their standard offering where you fastened the flat of the panel to the girt and therefor you did get some shear wall ability. The reverse rib fastened the rib of the panel, not the flat to the girt. This type of panel had no shear wall ability. They still used braced rods on either panel systems.

 

[r13]

“A metal building won’t be fully supported until all the roof and wall panels are actually on, including the permanent bracing. The temporary bracing is necessary to hold everything in place, but also to help plumb and square the building.”

Back to OP's question. If the construction is ongoing, the quote above seems apply.

 

[Ron247]

retired13: The 4th picture you posted must be in an area with really high gravity loads or the frames must have to support a lot of hanging loads. That building is only about 70' wide and 12' tall. While shorter buildings tend to have more gravity load outward thrust than taller buildings of the same width, those frames look really beefy for 70' wide.

 

[Ron247]

retired13: Part of the quote you noted relates to using the roof purlins and wall girts to help strut the rod loads from the center of the roof to the foundation. The purlins/girts have no significant axial capacity in compression unless the panels laterally brace them.

going back to your picture 4, the wind on the bare frames is probably much higher in magnitude than the fully sheeted building. Add to that the purlins being weak in compression and the temporary bracing definitely becomes much more necessary as noted in your quote.

 

[r13]

Ron247,

Yes, I think your assessments are correct.

 

[SteelPE]

Here is some metal building bracing porn for you:

I have always assumed that the roof bracing distributed the load to the wall bracing. 90% of the time the roof bracing aligns with the wall bracing.... the other 10% of the time the roof bracing is in the bay adjacent to the wall bracing.

 

[Ron247]

The wall bracing can be several bays away from the roof bracing but you still have to have a load path (the eave strut on smaller buildings) that can strut the load from the roof rod bay to the wall rod bay. They generally call that "offset bracing".

I agree SteelPE, that is bracing porn. Anytime you have to rotate you head 90 degrees to figure out what is going on, "porn" is the correct term. That is definitely not standard rod bracing. That must be a hanger in the picture.