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Re: Load Capacity of a Holddown Post as a Compression Member

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If you have "long" posts, then the suggestions to use
Roark's formulas or computer-based P-delta
calculations will give you the appropriate critical
(elastic) buckling force.  

If you have "intermediate" or "short" posts, the
critical elastic buckling force calculated using those
methods would be higher than the actual critical
buckling force (i.e., would be unconservative.)  Take
a look at Figure 8-7 in the FPL "Wood Handbook" or Fig
3.6 of the "Wood Engineering and Construction
Handbook" by Faherty and Williamson. 

In steel columns, this difference (between elastic and
inelastic buckling) can be addressed using the "tau"
factor method suggested in the SSRC Guide.  I'm not
certain what to do with wood.  

One possible approach would be to multiply the elastic
result (for the column loaded with the triangular
distributed axial load) by the ratio of...

critical force using the NDS column formula

to

critical force using elastic buckling formula

(both calculated for a column of similar proportions,
but axially loaded at the ends only.)

d a v e  e v a n s

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