Pole shed diaphragm? 2

Is the owner attempting to patent a new type of pole building that flies? He is kidding himself here, miserably, for a structure that size. I would back run away from this one, and quickly, as he wants to be the engineer.

Mike McCann
MMC Engineering

Haha, That was my first thought. Unfortunately it wasn't my decision whether to accept the job or not, we have the contract to design the building.

From the first minute I looked at it I knew it needed some sort of proper diaphragm, We can get away with no wall sheathing by using bracing but the roof needs something otherwise the columns would need to be considered cantilevers. Which then they fail miserably in slenderness.

Anyone besides Mike have another opinion? (Not a knock of Mike's opinion as I tend to agree with him)

PEMB's have the X-Bracing in the roof structure, so, in all eventuality, it could be used here for wind against the 60' wide face, but for the longer face, with no shear walls, what would be the point as the lateral load still has to go to the poles?

However, with 19 foot eaves I think the forces will be too great to attain the economy of structure he is probably looking for, even with the poles at 4'.

Mike McCann
MMC Engineering

That's basically what I was thinking.

We don't mind the load going into the poles but we are more looking to be able to have the poles as fixed at the bottom and pinned at the top. For that I believe we need a diaphragm so the load can share across multiple poles.

If you solid block between the poles along the long side, they should equally share the end wall load just fine.

Mike McCann
MMC Engineering

Here's the pushback we're getting that I always love.

"We've been doing them this way for years! We're not putting sheathing on this thing."

They have jobs from years ago, with seals I might add, that have no sheathing and the columns are not overly sized. So either the previous engineers (yes plural) had a way of looking at the strapping as a pseudo-diaphragm or we're doing something wrong. Some of the older buildings that have been done were under the Farm Building Code (We're in Canada) which allowed for less stringent designs, however they removed that code from use and now these building fall under the National Building Code.

So can anyone think of a way that strapping could provide some sort of diaphragm action?

can you come up with some sort of Vierendeel truss with the strapping. I cant think of any other way to get it to work.

I think there are some pole building design guides out there. Each cantilevered pole might need to take its share.

I will try to dig up the reference. We looked at a couple of existing buildings recently, no (reliable) diaphragm, no strapping. Just buried / cantilevered poles.

I've got the reference from Sliderule's website. Still need to take a closer look at it.

We've done some of these before but they were never this large. Mainly 12-15 feet tall and reasonable plan dimensions.

I have looked at this two times. Neither ever came to an answer that worked. Basically my approach has been to look at a plywood diaphragm as a moment connection for each sheet of plywood. So you would have the moment connections at 8' oc one way and 4' oc the other. Comparing this to a situation with trusses at 4' oc with 2x6 flat at 2' oc, the moment connection would be 4' oc one way and 2' oc the other. In both of the cases I looked at I could not get a simple moment connection to work and the client did not like the connections I was coming up with.
This way was also causing a cross moment being transfer by the trusses. So I had to state a minimum size and grade for the top chord and that the wind condition needed to be checked at a 1.25 (you may need a higher or lower factor for your case) factor inplace of the code 1.6 factor to allow the truss top chord to transfer this moment.
But as I stated I never finalized my calculation on these. So I could not say if the final numbers would give an answer I could seal. They were also for much smaller structures.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

In discussions in the office I also said that if we came up with moment connections for the strapping we could consider it. But as you found the contractor does not want to do anything special, he figures a couple of nails will suffice, which it will not if we are hoping to use the strapping for lateral.

And I didn't think the truss mfr would appreciate the loads we would be requiring them to resist.

I have not weighed in on this as I think I misunderstood some of the terminology. You mention 2x6 strapping, is this a system of roof purlins and wall girts? This is actually fairly common. The roof and wall sheeting will have diaphragm and shear wall capacity. You just have to be very specific on the fasteners. On the side walls, you will need some form of strut that connects the roof and the walls. On the end walls, the end truss can act as this member.

Your issue is going to be finding the allowable capacity of the sheeting. It is not easy to get, but can be calculated. There is published literature available from the National Frame Builders Association.

The sheeting that is being spec'd is 29Ga (yes 29) architectural siding. So it's basically paper and has zero capacity.

the 2x6 strapping could be considered purlins and girts I guess. but it would be bending in the weak axis as the 2x6's are on flat.

It is generally just applied to give the siding something to be mounted to but we were looking into the probability of using it for some form of lateral load transfer/sharing.

If people think a sketch will help I can post one however it is basically a built-up wood column with a standard roof truss sitting on top. There is 2x6's on flat mounted horizontally on the wall between the columns at 24" o/c and on the roof perpendicular to the roof joists at 24" on centre. Nothing special.

Jayrod12, the system that you describe is used thousands of times a year. The 29 ga. material has much more capacity than you are giving it credit for. I know this, because several years ago, I was in your same situation. I have read the research and looked at the test results. Dr. James Leflar of the Colorado State University has done significant work on this. One of his published works is "A Mathematical Model of Steel-Clad Wood Frame Shear Diaphragms".

I have seen allowable capacities with 29 ga that go upwards of 300 plf. As stated earlier, this requires very specific nail / screw patterns and sizes.

I typically use 100 plf for 29ga as I don't trust that it will get fastened correctly.

I do not rely on it at all, ever.

Mike McCann
MMC Engineering

@Mike;

Unfortunately, if I don't rely on it, I can't rely on any builders ever using me again :>
There is plenty evidence in our neck of the woods that it is pretty darn effective as a diaphragm.

@ ExcelEngineering:

Professionally speaking, I would not rely on it because at the first crinkle from contractors walking on it wrong, there goes any possible limited diaphragm action. To me, the stuff is just like aluminum foil.

As a side not, if this is the only work you have, you need to branch out in the market. Don't take me wrong here, but from your last comment, you appear to possibly be compromising your professional judgment to get work, and that is dangerous, very dangerous. If clients do not have enough money to do a job right, they will be the first to seek remediation from others, you in particular, when it fails. Be careful. Personally, I have been there and have had to walk away. The professional liability is just not worth it to me.

Mike McCann
MMC Engineering

Thanks Mike.
I am pretty well diversified and walk away from plenty of jobs.
I don't feel that I am compromising my judgement. The 29 ga is not that bad. I always specify a screw pattern that is well in excess of that required for my limit of 100 plf. We just do not see these things failing very often here. I only do about one of these every couple of years and they are pretty small in height and footprint.
Also, how is a small crinkle going to affect the overall diaphragm strength? -we are talking shear, not standing on a beer can.

If it makes you feel any better, I do not do ANY work on mobile homes.