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.

 

[Aesur]

I agree with you, it was my understanding that these rods provided a horizontal truss to take the diaphragm (lateral) loading to the braced frames.

 

[hokie66]

Project managers lie about lots of things. You should require a full certification of the as built structure.

 

[JAE]

Yes - part of the load path of PEMB's.

If they are bent up, then they are less effective, but since many PEMB companies design to L/80 for lateral drift (I'm joking but not by much) they don't really care if there is a kink in the rods.

 

[r13]

The PEMB guy is probably right, the rod here is more on service function than load resisting element. Similar to bridging for truss roof.

 

[Ron247]

The rod bracing is not to square the building with. It can be used for that purpose (but shouldn't) but it is generally installed to stabilize the building for wind/seismic that is perpendicular to the direction of the main frames. It is longitudinal bracing. The braces do not create a diaphragm, they create a truss system. The rafters of the rigid frames serve as the top and bottom chords of a truss. The roof rods serve as diagonals of the truss while the roof purlins nearest the connection of the rod to the rigid frame are the verticals of the truss.

The columns also serve as top and bottom chords in the wall while the wall rods serve as the diagonals. The eave strut is also a vertical in the truss system. Basically, if you looked down on the roof and rotated the walls to the same plane as the roof, it would look like a truss system.

You need a stamped letter from the PEMB engineer if the rod bracing is not going to be corrected.

As far as using the rods to square with, they should not overload the rod trying to square with it. If they tighten the rod to draw the building square, they are pre-loading the rod. Later when the wind or seismic occurs, the added load could break the rod. The braced bay is usually erected first, but that is to supply a stabilized bay the rest of the building is attached to.

When someone does not want the wall rods, a portal frame is generally installed. You cannot use a portal frame to square a building or at least it is VERY difficult to use it for that. So if the rods are only to square with, why do you have to have a portal frame when they are excluded? Answer-the rods are not to square with.

 

[Ron247]

JAE, you are not joking about H/80, you are actually conservative. I have seen limits of H/50 and H/60 regularly.

 

[canwesteng]

I've heard similar things from PEMB guys before when I questioned them. Ultimately, since something is damaged in this case I would ask for a stamped letter or drawing stating it is ok as is, I doubt they will give you any kind of calculation for review.

 

[RPMG]

We usually receive calcs to size the foundations, and we get to see wind load reactions. I have never encountered that position. I have only seen PEMB designers assume tension-only x-bracing.

 

[Ron247]

Very good point RPMG. If the rods were only for squaring, you would not be given wind and seismic reactions from them for the foundation design. If they were allowed to be used for squaring the building, you would be given a magnitude for that also but I have never seen a squaring reaction.

 

[r13]

A roof diaphragm needs in plane stiffness to transfer loads, rods seem too weak to meet such requirement.

 

[Ron247]

Tension only rods are the most common bracing system of a PEMB. Cheap and easy to install. PEMBs rarely use roof diaphragm as their longitudinal bracing system. There are several roof panels that are on sliding clips for thermal expansion. This type of panel cannot supply any diaphragm strength. Even when a panel can supply a diaphragm ability, it is rarely used. The standard roof panel offering of most PEMB suppliers can offer diaphragm ability but rod systems are used for longitudinal bracing.

A 20 psf wind on a 100' wide and 30' tall building is about 15 kips lateral force to each sidewall. A building that size would typically have 2 braced bays to make it 7.5 kips to each braced bay sidewall. It does not take that much rod to take 7.5 kips even when you take into account the angle of the rod.

 

[r13]

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 think this guy said exactly the function of the rod - to maintain the squareness of the frame bays (bracing). If the roof beams and panels have not in place yet, distortion is anticipated, otherwise, the rod is either undersized, or the arrangement is inadequate (number of rodded bays, pattern, etc.)

 

[r13]

Since we are on the topic, I would like to point out a lesson I learnt long time ago - no bracing system will work if there is no source to resist the drag force in demand. Thinking this way, if the frames are free to swing side way, without positive tie down mechanism in the immediate point of encounter, the braces will simply take the ride, and the frames fail as dominos.

 

[Settingsun]

How is sagging of the rods typically controlled if they aren't tensioned? Do they tie them to the purlins? Or just leave them sagging?

 

[human909]

I would be a weeks paycheck that they are more that just for squaring the structure. What do installers and project managers no about the nitty-gritty of structural engineering? Sure the GOOD ones know what 'doesn't look right' or 'this is how we normally do it' or 'it is easier to install it this way. But it is pretty rare to find somebody who thinks about load paths etc...

Without knowing the exact framing of the structure being discussed it is hard to comment further about what the load paths are. But I'm happy to bet that the rod bracing is part of the wind resisting structure.

How is sagging of the rods typically controlled if they aren't tensioned? Do they tie them to the purlins? Or just leave them sagging?

It isn't 'controlled' and there isn't a great need to control apart from aesthetics. If fabricated and erected correctly then installation should mean that the bars tension by self weight. Or put it another way, the steel workers tension them with their pry bars to get the bolts holes to line up. In my early days in the industry I saw quite a few structure built like this, though I've never designed one myself. Non-tensioned rods really aren't a whole lot different to angles used as cross bracing.

For these sorts of structures a little movement isn't the end of the world. If you want stiffness then pretension or using rigid struts for bracing sounds like a far better solution.

(I'm just speaking off the cuff here. I'm sure there is academic research into this kind of thing.)

 

[JAE]

retired13. I believe you are very mis-guided here.

ALL pre-manufactured metal buildings use X-braced rod bracing to create a diaphragm and transfer lateral wind/seismic forces to the vertical X-braced rods in the side walls.
Almost every (99%) of PEMB's do NOT have a metal deck diaphragm. The metal roofing they use is perched upon flimsy Z-girts spaced at about 8 ft. o.c. and cannot, and do not, serve as a diaphragm.

The rods have very little to do with squaring up the bays. The major moment frame bents in one direction do not need rods to "square them up".
In the other direction the bents are spaced by bolted Z purlins and wall girts so the space between bents is kept constant.

 

[jimstructures]

The quoted PEMB Guy is not telling the correct story. The brace rods are there to brace the building. If you need to square the building the simplest way is to add additional cables and come-alongs in multiple locations and gradually bring the building square. If the building is only marginly out of square then you might be able to square it with the brace rods, but it is easier to do with additional cables and come-alongs.

Jim H

 

[rowingengineer]

JAE, don't want to get into a fight, but depending on the size of the building, the sheeting with purlins can provide bracing capacities to a value higher than one would expect. However not suggesting pemb guys would use it as much as our cold formed shed in aust do. I have witnessed testing on Australian roofs which are not detailed for these actions which far exceeded my expectations,

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."

 

[r13]

JAE,

ALL pre-manufactured metal buildings use X-braced rod bracing to create a diaphragm and transfer lateral wind/seismic forces to the vertical X-braced rods in the side walls.

Is this what you mean (rods in compression)?