Skip to main content
Calcs.com checks deflection separately for each segment of a beam between supports, not just over the overall span. That means a short overhang past a bearing point is treated as its own segment with its own deflection limit, and it can fail its own check even when the main span passes comfortably.

​
How the L in the deflection ratio is measured

When you see a deflection result reported as something like L/240 or L/360, the L refers to the length of the segment being checked, not the total length of the member. For a beam that runs across two supports with a short piece hanging past the second support, there are two segments: the main span between the supports, and the overhang beyond the last support. Each one is checked against the applicable limit using its own L. Because deflection limits scale with segment length, a very short overhang has a very tight absolute deflection budget. Even a small tip deflection can exceed that budget and be reported as a FAIL, while the longer main span, with a much larger allowable deflection, comes in well under its limit.

​
How to recognize this in your results

If you’re seeing a deflection FAIL that doesn’t match a hand check of the overall span, look at the deflection ratio the calculator is actually reporting:
  1. Note which segment the failing ratio belongs to; the results table or diagram will identify it.
  2. Read off the L value used for that segment. If it’s much smaller than your full member length, the check is on an overhang or an interior sub-span, not the whole beam.
  3. Re-do the hand check using that segment length and its own tip deflection, and the numbers should line up.

​
If the overhang wasn’t intentional

A common cause of a surprise overhang segment is a support that’s been placed slightly inboard of the beam end without meaning to. Open the support layout and confirm each support is at the position you intended. If the beam should end flush at its last support, move the support to the end of the member, and the overhang segment (and its failing check) will disappear.
This is also worth checking whenever a joist or rafter shows a small, unexpected deflection FAIL; a stray overhang at one end is a much more common cause than an actual under-designed main span.