Pole shed diaphragm? 2

[jayrod12]

I have a 60'x150' pole shed project (19' to underside of truss) where the client does not want to provide an OSB or plywood diaphragm.

The system the client wants is pre-eng trusses at 4' o.c. (To match the poles) with 2x6 strapping at 24" o.c. and metal sheeting. This construction method is to apply to both the walls and the roof. The metal sheeting is NOT steel deck but more of a VicWest style architectural sheeting.

Has anyone here designed 2x6 strapping (or strapping in general) as a form of diaphragm?

We've determined that the sheeting may provide some form of diaphragm however we think that the weak link in the system will be the strapping, specifically the connection of the strapping to the trusses/poles.

Does anyone have any experience with this?

 

[connect2]

Long ago...I too had the task of looking at Pole Farm buildings such as hay sheds etc. And found myself saying what the 'F' are you guys doing?!? That doesn't work.
The city slicker headed into the country.
Of course that lead on a trail to the American Society of Agricultural Engineers, ASAE. Now Ag Engineers are different than Structural/Civil Engineers. A whole breed unto themselves.
Search them out you'll find papers on such subjects as;
Diaphragm Design of Metal Clad, Post Frame Rectangular Buildings ASAE EP484.1...this is actually a standard.
Characterizing Diaphragm Shear Stiffness for Diaphragm-Frame Interaction Amalysis ASAE Vol 32(5).
Pole Barn Wind Resistance Design Using Diaphragm Action ASAE Paper No: 75-4035
Strength and Stiffnes of Screwed Roof Panels for Pole Barns ASAE 1983.
Strength and Stiffness of Metal Clad wood freamed diaphragms CSAE...a Canadian equiv.
An Experimental Investigation of Diaphragm Behaviour of Farm Buildings ASAE 84-4511
Evaluation of Wood Frame and Metal Cladding in Roof Diaphragms ASAE 40(2).
Well there's a lot more like the above, but also,
Improving Resistance of Poles to Lateral Loads, ASAE Transactions 1981.
Design of Pole Foundations for Lateral Loads, Transactions of the ASAE 1981.
Performance of Pole Anchorages Under Gravity and Withdrawal Loads Transactions of the ASAE 1966.

All worth reading. But you would be best to head to the American Society of Agricultural Engineers, all the papers I have are old. Like me.
No doubt there is a lot of newer and more valuable updated stuff.
Good luck.

 

[jayrod12]

Thanks for all your comments so far. Something that we are looking at right now is modeling the frame in RISA (consisting of the two posts and the wood truss) as fixed on the bottom with a spring (representing the 29ga diaphragm) at the underside of truss. We are doing this because the 100plf of allowable shear resistance (the average of the numbers in the NFBA Post-Frame building manual for 29 Ga steel siding) is not enough to account for to take 100% of the lateral load as a diaphragm. However we believe it may be too conservative to ignore completely.

The spring stiffness would be adjusted (by iteration) to the point where the reaction at the top of the columns (The spring reaction) maxes out the shear capacity of the sheet metal. Our analysis goes as follows, the reaction at the springs would then be applied to the entire eave as a UDL. Then, the reaction on the end walls would be divided by the width of the building to determine if the linear shear exceeds the allowable load of 100 plf. Does this sound reasonable like a reasonable approach?

We think that this may be the most realistic way of looking at this as the sheet metal obviously provides some resistance and therefore acts as a semi-pinned support(maybe?). We also think that if the building starts to sway the purlins/girts (strapping) will provide some lateral resistance (perhaps not a quantitative amount but some none the less).

Further I have a couple other questions:

OHIOMatt/Everyone
What is the fastener and fastening pattern to achieve the 300 plf previously stated (roof cladding to strapping, strapping to truss)?

Everyone Else
How does the American code look at slenderness in this case (noted above)? In our initial runs in Risa we did not encounter any slenderness errors; we had expected to run into issues in this regard. In the Canadian wood code column slenderness cannot exceed 50. Assuming a fixed-free connection conditions (k=2) the slenderness ratio for a 4-2x8 going 19' works out to 63. Assuming a partially restrained-in-rotation top connection (k=1.5) and a fixed bottom connection the slenderness ratio is 47 and therefore it appears we can get a reasonably sized column to work.

Any of your thoughts and suggestions are much appreciated.

 

[msquared48]

Spaced at 4 feet instead of the 8 to 12 normal, an effective 6X8 column arrangement could work at a higher stress grade, possibly with knee braces, and considering the higher allowable lateral drift/deflection ratios allowed for pole buildings.

Sorry, but I will never rely on the 29 gage material at the roof.

Mike McCann
MMC Engineering

 

[SlideRuleEra]

jayrod12 - Sometimes brute force methods are not unreasonable. Have you considered driving treated timber piling? Then you have your cantilevered "poles" projecting 19' above ground.

1. The building is fairly large, the one-time cost of pile driver mobilization can be "absorbed" more easily than on a smaller structure.

2. Installation of the "poles" will be quick which will hold down labor costs when compared to a more sophisticated design.

3. You should be able to accommodated the clients "special" requirements.

Perhaps pile driving is not common in your area... in which case please ignore. Where I am, driven piling would really be a potentially viable solution (both technically and financially).

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