# (Canopy problem) Threaded rods.



## WilDV

The canopy is 51'x10', the 3/4" threaded S.S. rods are connected at the top end of the canopy. there are four of this rods. At the other end of the canopy, a 6"SQ. HSS is used as a support.

When there is an uplift force, the rods are going to be under compression. I have never checked a rod for compression, only for tension. Has anyone ever checked a threaded rod for compression?


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## ipswitch

WilDV said:


> The canopy is 51'x10', the 3/4" threaded S.S. rods are connected at the top end of the canopy. there are four of this rods. At the other end of the canopy, a 6"SQ. HSS is used as a support.
> 
> When there is an uplift force, the rods are going to be under compression. I have never checked a rod for compression, only for tension. Has anyone ever checked a threaded rod for compression?


I'd imagine it's already slender. I'm not sure. Can you design it as a purlin?


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## WilDV

Yes, its slender. The rod is 9.6 feet long. The drawing shows that they decided to cover it using a hot dip galvanized steel tube, the steel tube is about 9' long. I'm not sure if that is just for aesthetic design or so that it wouldn't exceed L/r (300). But anyway lets say that slenderness is not an issue. Is it acceptable to treat it like a column under compression?


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## McEngr

I am at work right now working on bar joist reinforcing. The issue is all covered in chapter E of the AISC.

You may omit (usually) the k factor and set it equal to 1.0.

Then, you can use a radius of gyraction of D/4. For a 3/4" rod, r=0.1875. For instance, a 24" long 3/4" rod has a kl/r = 24/0.1875=128. Then you would use table 4-22 for 36 ksi or 50 ksi depending on your grade.


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## dakota_79

"lets say that slenderness is not an issue".

Slenderness will be your controlling limit state by a country mile. Don't ignore it, if that's what you're getting at. It sounds like you may be dealing with architects. Don't let them convince you to ignore it, either. Those cowboys will try to convince you to do lot's of inappropriate things! :O (whoa get your minds out of the gutter...inappropriate _engineering_ things)

"Is it acceptable to treat it like a column under compression? "

Don't get hung up on terms like "columns" (or "beams", or whatever). There's a reason the various codes and standards barely contain these terms, if at all. You'll notice, as McEngr correctly noted, that AISC Chapter E is titled "Design of _Members for Compression_", not "Design of Columns". So therefore, if a member is in compression, use this chapter no matter what. You can call it a SuperAwesomeAntiGravityDevice if you want...it's still designed by Chapter E. Don't miss the forest for the trees. Terminology doesn't matter. Load path is the only thing that matters.


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## kevo_55

For an uplift case, why not just treat the rods as a tension only member and simply resist the uplift as a moment at the connection at the base of the canopy?


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## bmc846

ASCE 8-02 will also have information on compression in stainless elements..


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## dakota_79

What kevo said.


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## Phalanx

Unistrut has a pretty slick way to stiffen threaded rods. Essentially, a piece of strut is bolted to the threaded rod using a cradle clip assembly. With the cradle clips installed, the system is usually controlled by buckling of the strut. If the steel pipe is used as a compression member, it should be connected to the threaded rod at regular intervals.

http://www.unistrut.us/Unistrut-General-Engineering-Catalog-Number-17/#?page=70


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## WilDV

I used chapter E.


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## WilDV

I also change the spacing and added more rods. Thanks for the help.


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## dakota_79

The key is what did you do to account for the slenderness, since a 9.5-ft long, 3/4" dia rod on it's own has a KL/r = 610? Not to mention the significant 2nd order effects. Can't imagine you can get any capacity out of them without major stiffening along the lines of what Phalanx mentioned above.


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## WilDV

dakota_79,

I don't really want to go into detail, but the rod is covered by 1-3/4" or maybe 1-7/8" steel to limit the vibration and amplification (if I remember correctly). The weight of the canopy is pretty heavy, furthermore the sides and back of the canopy is anchored to the wall. The canopy should be able to withstand wind loads during an event of a hurricane level 2.

Doesn't the provision/local building codes consider most of the order effects? I thought that was why we use provisions. Anyway is it really that important to have it stiffened? I've seen canopies anchored to the sides and back, and I have seen cable used.

I'm glad I finished it, almost 60 pages of hand calculation, pretty much no program only used justfor 1 member.


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## dakota_79

WilDV, I'm not trying to sound like a d**k here. Really not. You said in another post you're a young engineer - we've all been there and understand there's a learning curve in translating our education into real world experience. That said:

1. This calc should not take more than a page or two, including accounting for 2nd order effects and slenderness. 60 pages is alarming. Quantity does not equal quality.

2. After 60 pages, you should know what the dimensions and properties of your stiffening elements are, since that is the most critical feature of this design as you've laid it out, and as noted above.

3. "hurricane level 2" is not language found in ASCE 7 wind load criteria. Unless this is in Florida and is something addressed in their state code (which I'm not familiar with), not sure how you translate that into load on the structure, and it therefore has no meaning in this context.

4. "Doesn't the provision/local building codes consider most of the order effects? I thought that was why we use provisions." Not quite sure what you mean. Assuming you're designing to IBC &amp; the applicable state building code (all of which reference IBC, many with amendments), IBC adopts AISC 360 by reference, and this problem would clearly be subject to AISC 360 Chapter C requirements (stability and 2nd order effects) in addition to the Chapter E requirements. More plainly, when your canopy sees net uplift, the rods will no longer be "taught" and will therefore have a sag to them due to their self-weight. The sag will cause your compression load to become eccentric, creating a bending moment in the rods which will cause further "sag", causing further eccentricity, larger moments, more sag, etc. If this 2nd order effect does not converge to a steady state, your rods are unstable for compressive loads and therefore have no compression capacity. Note this is a separate issue from pure buckling, which is covered by the Euler equations in Chapter E and nominally accounts only for P-"little delta" effects (the sag eccentricity issue is a P-"big delta" effect). The alternative to all of this is to treat the rods as tension-only and, as kevo noted, design the canopy as a cantilever for the uplift case, or to make the canopy heavy enough that there is no _net_ uplift.

5. Just because you've seen something "designed" one way, doesn't mean it's correct. I deal with a lot of old existing structures, and everyday we see things that we would never design as new construction, whether it be just because of more stringent current code requirements or because the old design was more generally inadequate or even incompetent. Unfortunately this is too often also the case on new stuff we see from other vendors, too (first time you see someone spec A325 anchor rods, or give you loads from their structure in a high seismic zone that don't include seismic loads, you'll know what I mean!). I'd guess most canopies we see that only have slender rods either have no net uplift, have been designed for cantilever action under net uplift, or have been significantly stiffened as Phalanx mentioned.

I just hope you're being truly overseen on this by competent licensed structural engineer(s). If not, get in their grill with lots of engineering questions and _force_ them to pay attention to what you're doing, and run away from that place fast if they don't.


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## WilDV

dakota_79,

Ok, that wasn't very nice, I appreciate you participating but you're nitpicking a little to much. My main goal was to find a direction for the rods. And my intention was never to reveal everything. My calculation was checked by more than one license engineer and they gave me a list of what to check for. It was 60 pages, possibly because their where other various type of canopies not involving rods. I would really appreciate it if you stick to the rods and uplift forces, since I can't discuss everything. You are limited to the information that I posted, you have not seen my calculation, the plans, know the location, nor work with the license engineers that I have work with, so please hold you're criticism on that.

And yes, you guys know more than me. I am not challenging anyone, I just want to know as much as you guys and the licensed engineer I work with.


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## dakota_79

Hey man don't get defensive. Honestly trying to help you in your growth. We've all needed that at some point. I realize I might have come off a little condescending there, but that's not the intention. Apologies if that's the way it was taken.

However, you're the one who said the 60 pages of calcs were for this single member, if I may quote you directly. And I also only dealt with the issues you directly presented. Take the guidance or leave it.


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## ipswitch

WilDV said:


> dakota_79,
> 
> Ok, that wasn't very nice, I appreciate you participating but you're nitpicking a little to much. My main goal was to find a direction for the rods. And my intention was never to reveal everything. My calculation was checked by more than one license engineer and they gave me a list of what to check for. It was 60 pages, possibly because their where other various type of canopies not involving rods. I would really appreciate it if you stick to the rods and uplift forces, since I can't discuss everything. You are limited to the information that I posted, you have not seen my calculation, the plans, know the location, nor work with the license engineers that I have work with, so please hold you're criticism on that.
> 
> And yes, you guys know more than me. I am not challenging anyone, I just want to know as much as you guys and the licensed engineer I work with.


Dude. Whatever work you do you need to be able to defend to several other PEs at all times and take criticisms. People's lives are at risk if the work I do is wrong. I'm constantly "beat upon" by my colleagues when I have them review my work and talk about ideas with them. It's why we get paid the big bucks


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## bmc846

Quite a turn this thread took. Criticism is going to be part of this career path and has to be taken with a grain of salt. Especially if you end up working on an OSHPD controlled job where other firms justify their review pay by picking apart your calculations.


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## WilDV

Yeah, I guess I got a little sensitive when he mentioned the people I worked with. I should of explained in the beginning that their are anchors on the sides and back of the canopy, and a beam at the middle.

When it got checked the first time, they gave me a list of members to analyze. It was a lot of members but the licensed engineer (SE) thought it would be a good practice for me and it would help see the load path.


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## kevo_55

We almost never get into fights down here.

Let's not start now, ok?


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## jpardee

Code:


To address the technical aspect of this discussion, speaking as someone who has designed/signed/ sealed a lot of canopies and various structures for wind loads in Florida, threaded rod or any type of rod bracing would almost never be allowed to carry even small compression loading. 
Under 1990's AISC code( and I believe these are still good), all members used for compressive loads must have a max slenderness ratio kL/r less than 200.  Any tension member is limited to max kL/r of 300.  So your rod sounds like it might just barely get by or be close to a kL/r=300.
If the dead load of the canopy does not exceed the uplift, then the rods would be subject to compression, but as they only have kL/r of 300, they cannot be allowed to go into compression.  Other structural provisions must be made.  Most structural engineers would laugh at someone using a threaded rod, even large diameter, as a compression member.  That is not what they are made for, the fittings and so on are not designed for compression or reversed loading. 
You can tie the canopy down with some members going downward, or create a fixed connection in the other members (easier said than done, and always too expensive.)  The rods or other struts or even cable bracing could be used to secure it down against uplift.
Wire rope or cable-bracings are still used in many buildings, and they of course will be more slender than members.  There are other code requirements for cables, and you should know something about rigging before specifiying wire rope as structural elements. 
To meet code you will need to meet the slenderness ratio of kL/r &lt; 200.  A good option is to use pipe.  For 9 or 10 ft, in tension only, you must use something with r &gt; .576, such as 1.5 inch sch 40 pipe.
Forget about bracing the rods and what is covering the rod, unless it is structurally attached.  If the galvanized tube covering the rod is not welded or bolted or otherwise secured, then it is not there.  You could make a structural element where the threaded rod is tightened against the sleeve to make a specialty member, but this would be more used for a mechanical orfurnishings application. 
The dimension of 51 ft sounds too long for just 4 rods, that is a spacing of 17 ft??


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## WilDV

You hit it right. The rod was spaced no more than 9 feet and the rod was tightened with steel sleeve (bolted). The calculations were passed and checked by SE licensed engineers. Thanks.


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