Is it necessary
to place splices in built-up lumber beams directly above the
A. The easy answer to this
question is Yes, but it’s not entirely true. What is true
is that you can’t run into trouble locating all splices
directly over support posts.
In reality, the most efficient location for splices is at
points of inflection. The sketch at right shows the expected
deflection of a uniformly loaded beam without any splices
spanning from wall to wall across a center post. Note how the
beam sags near the centers of the spans, while the deflection
curve turns upward over the post. The points where the
curvature of the beam transitions from concave down over the
post to concave up between the posts are the inflection points.
At those points, stresses in the wood due to bending are lowest
— in fact, they are zero. Unfortunately, shear stresses
won’t be zero at these points, so if you spliced all the
members of a built-up lumber beam at inflection points, you
would still need some type of steel or wood shear plates nailed
or bolted across the splice to transfer the loads from one
section of beam to the next. That’s a trick that’s
common in commercial steel construction, but that becomes a
pain for wood framing.
A second problem is that wood beams aren’t flexible
enough to see the shape of the curvature and reveal the
inflection points; their locations must be calculated. Since
the location of each inflection point depends on the relative
length of adjacent spans, the number of spans, and the
variations in load along the beam, there is no easy rule of
thumb for locating the inflection points and hence the best
location for splices.
So my suggestion is to take the safe route and set all your
splices in multiple span built-up beams directly over the