The three-ply built-up girder is made from 2 by 10 material, and in one bay the butt joints of two of the three plies land together roughly 3 feet from the nearest pier, out in the middle of the span. How should the inspector rule?
- AApprove it, because nailing the plies tightly together transfers load across any joint in the members, so the two continuous plies carry the broken one past the joint between the piers.
Nails hold the plies side by side and share vertical load between them, but they cannot carry bending across the end grain of a butt joint. The interrupted ply contributes nothing at midspan.
- BApprove it, because only the outer plies of a built-up girder have to run continuous between supports, and both of the outer plies are unbroken in this bay.
All plies count toward the capacity of a built-up girder, so a break in an interior ply is just as much a loss of section as a break in an outer one, and here two of the three are broken together.
- CApprove it if the joint is covered with a plywood scab nailed to both faces of the girder, which splices the interrupted plies back to their full section again.
A scab may be part of an engineered repair, but choosing its size, material and nailing in the field substitutes for the design nobody has done. The prescriptive answer is to move the joints over a pier.
- Write a correction, because butt joints in the plies belong over a support, and stacking two of them away from a pier leaves the girder short of section where it is bending hardest.
Why D is correct
Joints in the plies of a built-up girder are made over supports, where bending is minimal, and are staggered between plies. Two joints stacked at the same point 3 feet out into a span leave only one ply continuous through the section carrying the most bending, so the girder has to be rebuilt or the joints relocated.
What this question is testing
Whether the candidate understands how a multi-ply member develops capacity and can identify the correct joint location by reasoning about where bending is greatest and least.
On the job
Built-up girders are cut from whatever lengths came off the truck, and joints migrate toward the middle of a bay when a crew is trying to avoid waste. Sight down the girder and mark every joint, then check that each one lands over a pier and that no two align. Correcting it while the girder sits on open piers is a matter of sliding members; correcting it after the floor is loaded means shoring the whole line.
Memory technique
Joints belong over piers, never over air.
Exam tip
For any multi-ply beam question, ask where bending is smallest; that is where the code puts the joints.
Where to look it up
IRC Chapter 5, the girder and beam provisions in R502 together with the built-up girder details and their span tables.