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Determining the size of a beam or column is
influenced by:
The load the member is to carry - Snow,
Wind, Earthquake, Building occupants, Intended Use, etc.
The span of the member - the distance from
one support to the other
The type of wood - different woods vary
greatly in their ability to carry loads
The spacing of members - Closer spacing of posts will reduce beam sizes as well as the size of the
post because it has a smaller load to carry.
Bracing - floor sheathing or other bracing
will reduce the tendency to bend and increase its ability to carry a
load.
If you are obtaining your post and beam frame from
a supplier, they may be able to work with you to properly size the
members. (i.e. not oversizing them)
If you are doing it from scratch, from wood
harvested in your back yard, it may be more productive to seek the
assistance of an engineer for sizing them for you. Question any proposed
member that appears out of bounds, and look at changing the factors above to
make it work for you.
I would be cautious about trying to determine the
size of structural members from rule of thumb observations of others because the
loads, spans, wood type, spacing and bracing may be different than your
conditions.
Wayne J. Bingham
Architect
1444 Michigan Avenue
Salt Lake City, Utah 84105
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