Tag Archives: gravity loads

Solid-sawn vs. Built-up Column Strength

Solid-sawn vs. Built-up Column Strength

This article was penned by my personal post-frame engineering mentor Dr. Frank Woeste, P.E. and appeared in Journal of Light Construction online (jlconline.com).
Q. Is a site-built column made with three pressure-treated No. 2 southern pine 2x6s and 1/2-inch plywood spacers added to produce a 5 1/2-by-5 1/2-inch cross-section the structural equivalent of a solid-sawn 6×6 post made of the same material? If not, will laminating the layers together with construction adhesive make the column stronger?

A. Frank Woeste, P.E., professor emeritus at Virginia Tech, responds: The short answer to the first question is no; a solid-sawn column will always be stronger than a free-standing, built-up column made with graded dimensional stock of the same lumber species. But it’s possible that the column can be built so that it is strong enough to function as a structural alternative. Don’t rely on glue, though; while it might help and certainly can’t hurt, there’s no way to determine the structural benefit and assign any meaningful values for a field-glued assembly.

Columns are subject to axial compression from gravity loads (and tension from wind uplift). To find the compressive strength of a built-up column made up of three 2x6s, you can’t just look up the compressive strength of one of the 2x6s and multiply it by three. Instead, there’s a formula in the NDS (National Design Specification for Wood Construction) that can be used to find the column’s allowable compression stress. If the 2x6s are nailed together per the rules (see illustration, below), the reduction factor is 0.6, or a 40% reduction in allowable axial stress. It matters what material you use to build the columns. Fortunately, compared with some species, the design properties of 2×6 southern pine are very good.

In addition to proper placement, the size and quality of the nails are important. The NDS specifies quality 30d common nails, which have a shank diameter of 0.207 inch and are 4 1/2 inches long, and so will penetrate through all three laminations. As tested, there are no gaps between the 2×6 members of the columns, so any plywood or OSB spacers should be added to the outside of the assembly after the plies are nailed together. It may be possible to fasten the plies together with structural screws, though you would need to determine (likely from their manufacturer) the size and number of screws needed to equal the NDS-required nailing schedule. The purpose of the nails or screws is to transfer shear from ply to ply, as the column is prone to bend about the weak axis. The fastener’s task is to make the plies behave as one solid unit in an attempt to restore some of the bending stiffness and strength that is lost by not using a solid-sawn column.
The overriding principle is “column buckling,” which is the result of “column bending.” “Bending deflection” is the devil. Once an unbraced column starts to bend—let’s call the amount Δ—under an axial load P, a bending moment (P x Δ) is created. This in turn creates more Δ, which creates more bending moment from the same P until eventually the column fails … it’s a runaway process.
In short, there is nothing simple about substituting a three-ply column for a solid-sawn column. Some of the lumber that is now available is much weaker and has lower density, or nail strength, and less stiffness, or modulus of elasticity (E), than southern pine. Along with the design details discussed above, E and compression strength parallel-to-grain are the most important variables for column strength.




Size & Species of Purlins

From the Client Mailbag…

Lots of good stories out there and I have been in the mode to share stories lately.

Here is one which went through our Technical Support Department recently:

pole barn framing“Today I visited the building inspector. I was confirming my intentions of building a Hansen designed structure. The reason is I already have a permit for a building that I was designing and building myself. One of my questions was about snow loads. I explained the layout of roof trusses and purlins on your building design. The inspector asked about the type of wood being used as purlins. I did not know. He gave me a copy of R802.5.1 (3) Rafter spans for common lumber species. The table shows one 2 x 8 that will make a 15ft. span. Is this the code information the inspector should use, or is there something else? What species of lumber is to be used for the purlins?

 Thank you”

And my answer:

Thank you for your investment in a new Hansen Pole Building. The allowable lumber species are listed in the General Notes on Page S-0 of your plans. Attached are the calculations for the 2×8 purlins. The Rafter spans in the Code do not apply to your building, they are prescriptive for stick frame construction. Of interest may be: https://www.hansenpolebuildings.com/blog/2013/12/purlins-2/

Along with these calculations:



4:12 roof slope (18.435° roof angle)
Trusses spaced 15-ft. o.c.
Purlin span 14.75-ft.
purlin spacing = 23.75 in.
roof steel dead load = .63 psf steel American Building Components catalogue

roof lumber dead load = 62.4 pcf * .43 lbs/ft.3 / (1 + .43 lbs/ft.3 * .009 * .19) * (1 + .0019) * 1.5″ / 12 in./ft. * 7.25″ / 12 in./ft. * (15′ – 3″ / 12 in./ft.) / 15′ / (23.75″ / 12 in./ft.) psf in purlin weight based on .43 G NDS
total purlin dead load = 1.638 psf

Check for gravity loads

Bending Stresses
Fb: allowable bending pressure
Fb‘ = Fb * CD * CM * Ct * CL * CF * Cfu * Ci * Cr
CD: load duration factor
CD = 1.15 NDS 2.3.2
CM: wet service factor
CM = 1 because purlins are protected from moisture by roof
Ct: temperature factor
Ct = 1 NDS 2.3.3
CL: beam stability factor
CL = 1 NDS 4.4.1
CF: size factor
CF = 1.2 NDS Supplement table 4A
Cfu: flat use factor
Cfu = 1 NDS Supplement table 4A
Ci: incising factor
Ci = 1 NDS 4.3.8
Cr: repetitive member factor
Cr = 1.15 NDS 4.3.9
Fb = 850 psi NDS Supplement Table 4-A
Fb‘ = 850 psi * 1.15 * 1 * 1 * 1 * 1.2 * 1 * 1 * 1.15
Fb‘ = 1348.95 psi

fb: bending stress from snow/dead loads
fb = (purlin_dead_load + S) * spacing / 12 * cos(θ) / 12 * (sf * 12 – 3)2 / 8 * 6 / b / d2 * cos(θ)
L = 20 psf using the appropriate load calculated above
fb = 21.638 psf * 23.75″ / 12 in./ft. * cos(18.435) / 12 in./ft. * (15′ * 12 in./ft. – 3″)2 / 8 * 6 / 1.5″ / 7.25″2 * cos(18.435)
fb = 957.195 psi ≤ 1348.95 psi; stressed to 71%

Shear Stresses
Fv: allowable shear pressure
Fv‘ = Fv * CD * CM * Ct * Ci
CD: load duration factor
CD = 1.15 NDS 2.3.2
Fv = 135 psi NDS Supplement Table 4-A
Fv‘ = 135 psi * 1.15 * 1 * 1 * 1
Fv‘ = 155.25 psi

fv: shear stress from snow/dead loads
fv = (purlin_dead_load + S) * spacing / 12 * cos(θ * π / 180) / 12 * (sf * 12 – b – 2 * d) / 2 * 3 / 2 / (b * d)
fv = 21.638 psf * 23.75″ / 12 in./ft. * cos(18.435° * 3.14159 / 180) / 12 in./ft. * (15′ * 12 in./ft. – 3″ – 2 * 7.25″) / 2 * 3 / 2 / (1.5″ * 7.25″)
fv = 37.942 psi ≤ 155.25 psi; stressed to 24.4%


Δallow: allowable deflection
Δallow = l / 150 IBC table 1604.3
l = 177″
Δallow = 177″ / 150
Δallow = 1.18″

And from our gracious client:

“Thank you for the response.

Those calculations are awesome. I won’t pretend that I know how to work those out.”

I have to agree the calculations are awesome, as are the other 150-250 pages which entail the design of every Hansen Pole Building. Every structural member and connection is checked for the ability to resist the applied snow, wind and seismic forces. It is this type of attention to details which made me want to have my own Hansen Pole Building.

And I do!