The Glued Laminated Timber Industry provides design aids to assist users in the proper selection of Glued Laminated Timber (Glulam) products. Examples of such aids are the Column Capacity tables provided by the American Institute of Timber Construction (AITC). Design aids are also provided by the American Plywood Association – The Engineered Wood Association as well as by individual manufacturers.
To illustrate the use of such design aids we will use AITC Table DF-2-1_15.pdf available at the AITC website, www.aitc-glulam.org under Technical Information, Column Capacity Tables. We will use this Table to `size’ the Glulam column necessary to accommodate a total load of 52,000 lb; the column height is 18 ft; and total load is from snow and the dead weight of the roof itself. The column under consideration is in the `West’ where Douglas fir (DF) is the generally available glulam material (species). The Table provided indicates a `Combination 2′, which, we would anticipate is the commonly available column manufactured. Hence, likely also, the `2′ in the Table file name.
Where snow loads govern the design of a roof a `Load Duration factor’ (CD) of 1.15 is used, reflecting the varying strength of wood depending on the duration of loading. (Snow loads are taken to have durations of two-months; other loading conditions are taken to have different durations.) And, hence, also likely, the 1_15 in the Table file name.
The Table is further limited to `Dry Use’ and the commonly manufactured sizes available. The footnotes also indicate that the Table is based on the ends of the columns being prevented from translation. This is typical in structural wood design where connections or attachments are not expected to prevent rotation (hold the ends rigidly straight).
In an earlier article it was stated that column sizes of 6-3/4 x 12 and 8-3/4 x 9 were found adequate by detailed calculations for the above conditions.
The Table is arranged in terms of `Capacity’ in lb (pounds) by size and length (height). For the example in question we will use the Table to select a size that, for a height of 18 ft, has a capacity of (at least) 52,000 lb. And we find that NO column size is adequate at a height of 18 ft, not even the 8-3/4 x 9! In fact, the 8-3/4 x 9 has only a capacity of 41,930 lb at a height of 18 ft. By detailed calculations it can be shown (though not here) that the 8-3/4 x 9 has a capacity of approximately 82,700 lb. WHY?!!!
The dilemma is betrayed by the wording `eccentricity’. The Table allows for the column load to be `off center’. In particular, the Table is cast to allow an eccentric (off-centeredness) of up to 1/6th the column width or depth. This is actually not much (off-centeredness, e.g. 1-1/2 in. for the 9 in. depth), but it results in the significantly lower than expected capacities in the Table. The structural wood industry recognizes that in wood framing `things’ often frame off center legitimately. Another way of looking at the 1/6th dimension eccentricity is that the load is assumed to act in the `middle third’, either direction (width or depth). That is still `pretty close’ to center. It does NOT accommodate conditions such as hanging a load off the side of the column, such as by a bracket or hanger. Those conditions need to be examined carefully. The reason the 8-3/4 x 9 column was found by calculation able to handle 52,000 lb, in fact way more than 52,000 lb, was that the framing conditions justified using exact centeredness (no eccentricity). This exact centeredness was justified by two equal loads acting on each half of the column section. With the load centered, the capacity of the column is roughly DOUBLE that of a column allowing limited eccentricity. The `trick’ in design, particularly wood design, is to know where centeredness, or off-centeredness, can or should be assumed, or not. Proper detailing of a connection or beam-column junction may allow the assumption `perfect centeredness’, though such detailing is not common in a lot of wood construction. In NO case should the Table be used where the column load locations are outside the middle third, or uncontrolled.
References
Design of a Glulam Column, Associated Content, Jeff Filler.
Table `Combination 2 (DF L2)’, American Institute of Timber Construction, Centennial, CO.
The American Institute of Timber Construction, Centennial, CO, www.AITC-GLULAM.org.
Timber Construction Manual, Sixth Edition, American Institute of Timber Construction, Centennial, CO, John Wiley & Sons, Hoboken, NJ.
