Design of Cold Formed Girt Systems 1

[SteelPE]

I have a question regarding the design of cold formed light gauge metal girt systems.

All of my projects to date have been covered by the IBC code system. I design mostly structures 3 stories or less in height. I design 2-3 warehouses per year. Sometimes these building have corrugated metal siding other times insulated metal panel (IMP). These systems often required cold formed girts placed at regular intervals to support the panel system (anywhere from 5’-0” o.c. to 7’-6” o.c.).

The girt systems are governed by the AISI spec (which I find to be a very difficult spec to get through). When designing these girt systems, I am typically confronted with unbraced length difficulties with regards to lateral torsional buckling (LTB). I know the AISI has some exceptions listed towards to the back when it comes to LTB and the corrugated metal siding. However, when I am tasked with designing IMP systems, I typically default to installing some type of LTB bracing on the girt (at the 1/2, 1/3 or 1/4 span points). My detail is a lt gauge strap that loops over the girt and is attached to the inside face of the metal panel (not the best system). We call these girt straps. We have designed the IMP systems this way for the past 10 years and occasionally have some slight push back from the clients.

This week I had one client, a steel fabricator/metal siding installer, completely blow up at the installation of these girt straps. He is complaining that these straps are cutting into his profit margin (which I DGAF about). He is looking to eliminate the straps… I told him no.... explosion.

The question I have is in regards to the design of these system.

Is it the correct approach to assume that IMP systems do not brace the metal panel the same way corrugated panel does?
Is there a better way to increase the strength of these systems without having to install the straps?

 

[Ron247]

I have not done any wall girt design in years and the more modern code requirements are hard for me to follow at times because I originally learned ASD, not LRFD type design. Are your wall girts "outset and continuous span", "outset and simple span" or "inset and simple span"? That alone makes a design difference WRT unbraced lengths.

 

[BUGGAR]

In the old times, we used sag rods, threaded rods spaced as you said but located in the center (neutral axis) of the girts. What keeps your strapped girts from rolling when people climb on them or hang stuff from them?
Maybe your siding installer doesn't think his panels have enough beans to carry the girts? How big are your girts? Are they worried about thermal penetrations in the panels?

 

[SteelPE]

Ron,

These are outset simple span girts. This is a conventionally framed building. We tried using a continuous span girt system years ago and is was a disaster to get the bolt holes lined up properly with the column girt clips (due to a W10x33 being slightly different than a W10x45 etc). Ever since we have used simple span girts.

Buggar,

The girts are 10CSx3.5x12ga girts (10" C with a 3.5 inch flange). People are not suppose to climb on them, so if they do, they run the risk of damaging the girt. I don't know of a single provision in the code that has a person climbing loading case.

The installer is a 30yo bean counter who thought I required the straps to increase the strength of the IMP. He threatened to send my design to the panel manufacturer. These straps don't increase the strength of the panel but rather the strength of the girt system itself (he doesn't have a clue).

I'm not sure about thermal penetrations, the straps just attach to the thin inside face of the IMP. So no thermal bridges.

 

[ajh1]

Normal design practice following AISI is to allow the wall panel to brace the outer flange of the girt. This is assuming that the panel has sufficient strength, is connected reasonably directly to the flange (no 4" of rigid insulation in between panel and girt for example), and has some level of panel to panel integrity. One of the specific AISI provisions is intended for normal wall panels where the fasteners to the girt are at no more than 12" centers (a few other rules in AISI, but that is the main one). You then get to use an adjustment factor that is typically in the 0.6 - 0.7 range depending on girt shape (Z or C), simple or continuous span, and depth of girt for your suction design without going to the trouble of using strapping on the inside flange. If you need to exceed that capacity then you need to add strapping on whatever centers you need and design the suction forces for the unbraced length between the strapping. You will NEVER fully get to a 100% braced scenario unless you add a liner panel to the inside flange. The outside flange under pressure will generate have full capacity, i.e., very short brace length. Alternatives to straps would be sag rods near the inside flange, girt braces of some depth to keep the girt from rotating, or angles in lieu of straps. If you use some sort of bracing that has some compressive stiffness then you would not need to run the braces to the ground. One of the other issues with flat strapping on the inside flange is a safety aspect for occupants of the building. The edges of the straps can be quite sharp if you run into them so it would be desirable to terminate them at the 7'6" high girt to avoid potential issues. (Except for high wind conditions, the PEMB guys usually position the first girt such that it clears the top of any personnel doors in the wall.

 

[ajh1]

One of the other challenges when looking at IMP explosive conditions is that the typical cold-form girt is not a doubly symmetrical member and thus takes a bit of a hit in the explosion equations because it is more subject to rotation of the member. They can be used for low level explosions, but for higher levels you are probably better off looking at W-shaped sections.

 

[SteelPE]

ajh1,

The sag rod idea isn't a bad one, but depending on the girt spacing you either:

-have the sag rod to be really large (to keep kl/r<200)
-anchor the sag rod to the top of foundation wall

Also, with our spans, we typically require multiple rows of girt straps, so we would need multiple rows of these sag rods to replicate what is happening with the girt straps.

All of the other methods you suggested were vetted years ago and the girt strap method for bracing the girts against LTB was selected.

I seem to remember asking a similar question 10 years ago and JAE saying that they intentionally force a sag on the girt forcing it buckle in a given direction therefore only requiring sag rods in tension (I did not really like this idea). FYI, that thread was started after an argument with the father of the client who started this thread (true story).

 

[Ron247]

clips (due to a W10x33 being slightly different than a W10x45 etc)

In a PEMB system, the column size/depth would not matter at all since they do not use centerlines. They would locate exterior columns based on the offset from Steel Line to the outside face of column so column depth does not matter like it would if they do a centerline location. Not using continuous spans is causing you take a bigger hit in the unbraced length arena. Not using Zs instead of Cs keeps you from lapping the sections.

I am not sure what if any sag strap research was ever done but thinking about how the building is erected, the following seem like something to review if someone was researching it:
[ul]
[li]When the girt is installed, the inner flange already wants to buckled downwards due to gravity.[/li]
[li]Once that initial direction is set, how likely is is that the direction would reverse under wind rather than try harder in the same direction, thus keeping the sag strap in tension not compression. I do not know the answer but that may be why sag straps are used successfully to brace an inner flange.[/li]
[li]Fastening the sag strap to the floor removes the buckling problem but does create "dang, I cut myself on the strap again" problem. So the bottom 7'-6" would have to be ground smooth at the edges. That is a lot of work.[/li]
[/ul]

I feel comfortable saying, the PEMB companies probably have the more economical system for both fab and erection. The like continuous outset Z-girts. They tend to use sag straps unless they are in a really high wind load area. They connect the sag strap the eave strut (sidewall) or the gable angle (endewall) and then let it extend down to the inside face of girt.

 

[Ron247]

Ever think about using a military grade nylon flat webbing? Easy to install. I would guess it would not cut you easily so it could be fastened at the bottom. I am not sure about life expectancy. I don't think it is real expensive. Would not be desirable in some environments but that is also true of steel at times. Fasten it from eave strut to base angle.

 

[SteelPE]

Ron,

The project in question is in a 130 mph wind zone, so it is getting up there in terms of wind pressures.

Your first response with regards to sagging is similar to what JAE was referencing years ago. Again, I understand the though process, but not sure I would fully buy it.

Also, you argument about using face of column vs center line of steel is what we learned the hard way years ago. Would be nice to use outset continuous girts, but that is a detailing nightmare.

Nylon straps don't eliminate the concern of the client (which is I don't want to install anything). The client concern is not in regards to material but rather the time it takes to install the straps. I am also not sure how you would "prove" they work.

 

[Ron247]

concern of the client (which is I don't want to install anything)

We can't work miracles. Are you sure you didn't get one of my Clients? Because all of mine think that way. If you figure out how to get them to change their mind on that, PLEASE post that for all of us to learn from. You will get the first thread ever with 4,000 stars.

The straps (metal or webbing) work mostly because the loads are so low. Fastening straps top and bottom turns them into tension members. They can allow the girt to buckle very slightly and still work.

Well, to do what he wants, I know of only one method, put a LOT BIGGER girt. One so big/thick it does not need bracing. But I am sure he will complain then about cost.

Oh yeah, just for your info. I remember an erector telling me it was easier to bolt the continuous girts together in sections that were 2 bays long, lift them onto the girt clips and then fasten them to the clips rather than doing them one bay at a time. I do not know if he is correct, but that was what he was doing.

Here is another possible issue with our design of what appears to be a PEMB but we are not using their "minimalist" logic. Their girts clips are gauge material that is actually a bent plate. They buy them by the thousands. Easy to field ream and correct fitting issues. We tend to to use thicker connectors, not so easy to ream and force. 1/8' thick in the PEMB world is a hoss when it comes to connectors. I know I have made the comment many times that I could not design at all if they ever get rid of the 3x3x1/4 angle. I see that used so often when it is evident something much thinner would probably work. We do not have the luxury of buying 1000 at a time and therefore making fab cost less.

 

[SteelPE]

Ron,

Thanks for the help. The thread was started to explore the possibility of inefficiency with the design process (since I am welcome to expanding my knowledge). Between your responses and those of Ajh it appears as if I am at least "in the correct zip code" with regards to the design process.

I think the client actually uses bent 10 ga plates for the clips to the columns, so they are one step ahead of you. I wonder if we used tek screws vs bolts we could switch to continuous Z girts?????? Bolt each end of the lap splice and then connect with tek screws in the middle. I bet that would lead to field issues as they would then need to take more time to make sure they were in the correct position?????

 

[Agent666]

Regarding the suggestion to use Nylon, Nylons not going to have a 50 year design life, it breaks down in sunlight over time so I would consider it a no go. Also how do you get it tight on site, I don't think that aspect is too practical given low labour skill levels.

In my mind, the insulated panel usually has the same metal thickness against the girts as a normal cladding, so I wouldn't expect things to be too fundamentally different. Overall stiffness of the insulated panel is probably higher than the normal cladding. From a point of evaluating buckling/restraint then surely this is at least equivalent if fixed appropriately to the girts?

 

[SteelPE]

Agent666

The AISI has a very specific set of requirements for using the reduction factors to determine the LTB capacity of the gifts. One of the big requirements is attachment of the panel to the girt every 12 inches o.c. The other is the rib spacing on the panels. IMP system have attachments ever 30” o.c. And the backside of the panel is not corrugated. So it’s a no go in accordance with the AISI.

 

[Agent666]

Yeah I can see how that would be quite limiting then! We don't have such requirements in our own cold formed section code (AS/NZS4600).

In practice though given you have loading in both directions and only one flange and web rotation is only restrained for inwards loading, I always end up with at least 1-2 braces unless you have extremely small spans for the outwards loading case or a grossly over-designed section. So presumably for one direction you need some braces/restraints, then if you can prove these work for the other direction also you can't really say you are overcooking anything because the cladding does nothing for one direction and surely that person can even see that, and cladding has some effect in practice in the other even if your code says to ignore it because of the rules you stated.

Most of the time in this part of the world the design of girts and purlins is simply done directly from manufacturers design tables that relate you back to a maximum line load that can be taken with a certain number of discrete braces.

Most designers don't delve into the cold formed code. I used to work for a place that produced these standard design tables for a supplier, I wasn't involved directly but there is a reasonable amount of conservatism built in for certain cases as you need to factor in quite a few things into a single table like varying lengths of adjacent spans.

 

[Agent666]

To be fair, I'd add that in my opinion in most cases it's distorsional buckling that seems to govern the design, rather than lateral torsional buckling. You can brace your section to hell and back and you can't improve that aspect.

 

[Ron247]

Agent666: The life expectancy is definitely an issue. Getting it tight does not seem that difficult to me. If it is attached top and bottom, the bottom could be wrapped around the base angle and pulled. The base angle would be fastened good at strap points. This would be true for nylon or steel straps. I do not think the strap has to be prevent very minor buckling, sag straps and most any strap type connection will have some slack in them. The low labor aspect, I cannot do anything about. If someone uses low-end labor, what aspect of the building would you trust?

 

[ajh1]

For insulated panels we have used a rivet connection that would swedge out inside of the panel on centers closer than the width of the panel as needed. Typically for the US that would only be at the corner wind zones. We have deemed that the insulated panel viewed as a whole would meet the 12" rule for ribs due to the combination of two layers of metal plus the rigid insulation. The rib rule is basically to limit deflection between the girts to some reasonable level so we are looking at a thin plate type deformation response. Actual rules in AISI are for the most part limits based upon the actual sample configurations that were tested to develop that provision.
The key to making much of this would without straps are the provisions in AISI that give us the reduction factor noted above that gives suction capacities higher than what you would get simply designing it as discretely unbraced on the inside flange. I don't believe that AS4600 has the same allowances. This AISI provision was developed prior to the distortional buckling implementation but has been permitted to continue as is since the values are based on testing. One of the nice things with tested value provisions is that they inherently cover all of the various limit states ( defined and undefined) as long as specific efforts haven't been made during the testing to constrain the sample beyond normal operating conditions in the field.

 

[SteelPE]

ajh1,

I understand where you are coming from. I buy the logic, but from time to time I am required to "prove it" and ultimately engineering judgement doesn't fly sometimes.

The fasteners you are referring to are called Fab-Lok fasteners in the US. You can find a sample here:

http://www.elcoconstruction.com/Products/Metal-Fastening/Rivets/Fab-Lok-R

If there is one thing this client absolutely hates, it's those fasteners. He get so mad when they need to be used. I have resorted to having him get the panel engineered by the manufacturer to avoid those fights.

 

[ajh1]

Extra fasteners are not for the panel, but to make the girt check and use the AISI factors based on the 12" fastener spacing. The full width spacing fails the 12" check so you can't use that provisions and thus you need to design with strapping, bracing, etc. to get the unbraced lengths of the inner flange down to a workable range. Nothing you do in panel design is going to solve that problem.