Glued Laminated Timber columns are often used to support Glued Laminated Timber beams and other framing in wood frame structures. These `Glulam’ columns provide structural support for moderate and large loads and as well may match the `timber’ architectural appeal of the structure itself. Occasionally Glulam columns are used where aesthetic appeal is less or unimportant but where such columns are used to match Glulam beam sizes while accommodating the beam reactions. Glulam columns typically support axial (axial compression) loads but may also be called upon to carry transverse and eccentric loads. The use of a Glued Laminated Timber `column’ to support axial loads is the subject of the present article.
The design of any Glulam should start with the evaluation of the following conditions.
1. Determine the load(s) to be supported (in this example, axial).
2. Determine the column height.
3. Evaluate the bracing conditions.
4. Evaluated or determine load eccentricity.
5. Determine column visibility and appearance requirements.
In this article we will determine the required size Glulam column to support two `in-series’ Glued Laminated Timber beams, each 40 ft long, carrying 1300 plf uniform Snow plus Dead loads. The beams splice across the column; it is visible and unbraced in both potential buckling directions for the physical height of 18 ft (floor to bottom of beams). An Architectural Appearance Grade is desired. The beam splice over the column is centered, the beam reactions are equal, and thus a single centered axial load equaling the sum of the beam reactions will be used in the design.
In this example a column with a width matching the supported beams will be sought. The beams supported are each 6-3/4 x 36 Douglas fir Glulams; thus it is intended that one dimension of the column will be 6-3/4 in. For a column to be suitable structurally, the Design Check for Compression parallel to Grain must be satisfied;
(is) fc ≤ Fc’,
where
fc is the applied axial stress parallel to grain, assumed uniform,
and
Fc’ is the Allowable Compression Parallel to Grain, obtained by
Fc’ = Fc CC CM Ct CP,
where
CC, the Load Duration factor, is 1.15 in this case, for Snow,
CM and Ct are not applicable as the service conditions for the column are assumed to be `dry’ and `normal’ temperature, and
CP is the Column Stability factor,
CP = (1 + FcE/Fc*)/2c + √{[(1 + FcE/Fc*)/2c]2 – FcE/Fc*)/c},
where
FcE = 0.822 E min’/(le/d)2,
where
E min’, le, and d are the minimum Modulus of Elasticity, effective length (in buckling), and cross section dimension associated with the critical direction of buckling,
Fc* is the Design Value for Compression Parallel to Grain multiplied by all applicable Adjustment factors except CP, and
c = 0.9 for Glulam.
In our example (as, I would venture to say, most examples of Glulam columns) the effective length shall be taken to equal the physical length, and assuming the critical buckling direction is parallel to the 6-3/4 in. section dimension,
Le = 18 x 12 = 216 in.
d = 6-3/4 in.
le/d = 216 / 6.75 = 32, which does not exceed 60; good!
The grade column to be used will be assumed to be Douglas fir (DF) Comb. 2, for which
Emin = 830,000 psi, and
Fc = 1950 psi (assuming 4 or more laminations).
Emin’ = Emin = 830,000 psi, and, thus,
FcE = 0.822 (830,000 psi)/(32)2 = 666 psi.
Fc* = 1950 psi (1.15) = 2243 psi.
FcE/Fc*= 666/2243 = 0.297, and
CP = (1 + 0.297)/1.8 + √{[(1 +0.297)/1.8]2 – 0.297)/0.9} = 0.286.
The Allowable axial stress is, thus,
Fc’ = 2243 psi (0.286) = 641 in.
The total load to be carried by the Column is
P = W/2 + W/2 = W, where
W = w L = 1300 plf (40 ft) = 52,000 lb (each beam), thus,
P = 52,000 lb (Snow plus Dead).
By setting the applied stress under design load equal to the Allowable stress the `other’ dimension to the column may be found;
(let) fc = P/A = 52,000 lb / (6-3/4 x d other) = 641 psi, or
d other = 52,000 lb / (6.75 in. x 641 psi) = 12.02 in.
So, this suggests that a 6-3/4 x 12 column is suitable.
Since le and Emin’ are the same for both directions of buckling, the assumption that the 6-3/4 in. depth governing is indeed applicable.
Also, a 12-in. deep column will be manufactured using 12 / 1.5 = 8 lams, so the use of 1950 psi for Fc for 4 or more lams is also applicable.
The question arises, however, “Will the narrow profile of the column look good?” (The column is about twice as deep as it is wide.)
The 6-3/4 in. dimension was used to `match’ the beam size. Wider column sizes could be investigated, but they will not frame flush with the 6-3/4 in. width beams…. unless wider beams are also investigated.
It turns out that a column size of 8-3/4 x 9 in the DF Comb. 2 has plenty of capacity with regard to the above conditions, and `looks’ squarer. It `could’ be framed under 6-3/4 in. wide beams, or 8-3/4 in. wide beams could also be investigated.
Note that even though we are discussing matching widths for column and beams, the column will be specified using a `Combination Number’ associated with timbers intended for axial loading, whereas the beams for the same project will be specified by a Combination Symbol (or Stress Class) associated with timbers intended for flexural loading. A Glulam beam could be used as a column, in this case, but with different design values. In either case, if used as a column, the member must be straight (not cambered), if used as a column.
It turns out that a 6-3/4 x 13.5 24F-1.8E `beam’ will also work, as long as it is straight, as well as an 8-3/4 x 9.
References
National Design Specification for Wood Construction (NDS) and Supplement – Design Values for Wood Construction, 2005, American Forest and Paper Association, Washington, D.C.
