Design Example
General Discussion | Design Example
General Discussion
- Like any structural design situation, the design of beams is an iterative process that relies on the making of assumptions and then the checking of those assumptions.
- The design process for beamscan be shortened if one is able to correctly predict the controlling mode. The following simple
rule of thumb may help you decide which mode; bending, shear, or deflection; controls.
- If l/d £ 10: shear controls
- if 13 £ l/d £ 25: bending controls
- if l/d > 30: deflection controls.
- In many cases, however, bending controls. The following flow chart is a suggested design procedure for solid-sawn lumber
and glulams in bending controlled situations.
Design Example
- This example problem has been adapted and modified from the Commentary on the 1991 NDS and revised to reflect the 1997
version.
- Design a simply-supported roof beam that spans 32' and supports 5-5000 lb (1000 lb D + 4000 lb S) loads applied to the beam by purlins spaced at 8' o.c. The purlins laterally support the beam's compression edge. The length of bearing for the roof beam is 6". Assume dry conditions with temperatures generally less than 100°F, but with an occasional daily variation reaching 125°F. Use 24F-V1 SP glulams.
- Loading: Assume beam self weight = 40 plf.
- Bending:
- Getting trial size:
- Determine CL:
- Determine CV:
- Check bending:
- For this case where the actual self weight is slightly less than that assumed, we conservatively used the original
load and resulting M and V values.
- For this case where the actual self weight is slightly less than that assumed, we conservatively used the original
load and resulting M and V values.
- Getting trial size:
- Deflection:
- Shear:
- Bearing:
- 5 X 23-3/8 beam supports:
- At purlin locations:
- 5 X 23-3/8 beam supports:
