Wood Laminated Posts - does this work?

[allisch]

There is a laminated post company which is manufacturing laminated 2x wood posts for pole barn type structures. We are in a 90 mph wind zone.
They seem to show that they can use a (4)ply 2x6 post for wall heights to 24' with posts at 8' oc. (#1SYP material)
30 psf LL, 7 psf DL, 90 mph, negligable earthquake
They specified a maximum building size of 32' width.

I see the design wind load for the "non-corner" part of the building as being 13.6 psf suction (per IBC Table 1609.6.2.1(2)), which gives approx 110 plf.
If the post is properly embedded in designed footing, then I would use a fixed base and pinned top support.
Max Moment = wl^2/8 where L = the sidewall ht=24'
M=7850 ftlbs applied due to wind


According to my NDS 2005, I see #1SYP 2x6's as Fb=1650, E=1700, Fc=1750
S=33in^3 if the post is turned the strong direction to resist wind.
(Assuming the post is laterally braced (weak axis) 2' on center by girts, but only braced at the floor and ceiling level (24') in the strong axis.
I see the Moment capacity =4540 ftlbs at 100% or 6040 ftlbs at 133% wind stress.
So the wind stress in bending is 173%

So I think the post fails without even adding the gravity loads.
I have that the post has a design axial load of 11.4k.
If the building is framed with trusses (and posts only on the perimeter) in the 32' direction, with 30 psf LL, 7 psf DL, and 90 mph winds, the applied axial load under combined wind + dead + live is approx 5.3k.
So the axial load is using 47% of the design stress.
(I use column Ke=0.8 for fixed base posts-pinned at the top)

I missing something? Do you see any way that they can make this engineering work?
I don't see how they can have engineering to make this work.

 

[allisch]

Dave-
Thanks for the info.

I think I can see possible locations where we may differ:
Cf looks like it is already incorporated into the tables for Southern Pine (according to the notes on page 37 of the NDS Supplement 2005)so I have a hard time factoring in Cf again.
Doesn't Cr have to do with multiple members spaced 2' oc or closer across the structure?
(That was my interpretation from p 37 supplement also.)
allis_ch@hotmail.com

 

[DaveAtkins]

We both made a mistake.

You are correct about the Cf--it is already incorporated into the values, so it can't be used.

However, you can always use the Cr factor for a post built up of at least three members. The Code says the members CAN be in contact with one another.

Bottom line--without the Cf factors, the 2 X 6 post doesn't work.

DaveAtkins

 

[allisch]

Dave-
Thanks again for the reply.
I think this solves the problem at hand.

 

[msquared48]

The lateral deflection limitation here is L/120.

No way two 6X6's will do this even at 10 psf minimum wind loading for an 8' bay spacing. No way. I calculate the deflection at over 15", and that is not considering the minimum wind load on a sloped roof. That's less than L/19.

Mike McCann
McCann Engineering

 

[JStructsteel]

If you are spec'n a laminated post from a company, can you not ask for engineering calculations before you use it? Otherwise, go down the road to supplier B who will show calcs, or design it properly. 50 engineers can design a 2x4 50 different ways, and all seem to justify it in their own head. I see this all the time.

 

[allisch]

jrisebo-
Thanks for the suggestion, and that was my original thought process (which would have been followed if I thought that the data I was getting from the manufacturer's engineer was correct.)
Common sense tells me that this does not work. I have requested that their engineer review his calcs and resubmit.

 

[GoCyclones]

If the columns are nail-laminated, IBC References ASAE EP 559 (1997) "Design Requirements and Bending Properties for Mechanically Laminated Columns". This standard has a higher Cr factor that can be used, if memory serves, something like 1.35.

However if your columns will be spliced (maybe, since they are 24'), there are other considerations that this standard addresses including reduced stiffness and bending strength in the spliced region.

24' seems pretty tall for 2x6's. In my previous post-frame experience 4-ply 2x8's or larger would be used for a height like this.

Deflection from the wind alone is probably borderline if you assume fixed base and pinned top. WL^4/185EI = 2.4" ~ L/120.

 

[allisch]

Gocyclones-
Thanks- that (4)ply 2x8 was more like what I was expecting for this height.
And that is the same as what I calced for the wind deflection (not counting Pdelta effects).