Bearing Perpendicular to Grain
General Discussion | Example
General Discussion
- Bearing stresses perpendicular to the grain occur at the ends of beams
bearing directly on:
- Another wood member
- Masonry wall
- Steel Hanger
- Steel Girder
- Bearing plates
- Column Caps
- Bearing stresses occur where there is a transfer of load from the
beam to its support mechanism.
- Bearing failures are due to the localized crushing of wood fiber in
the vicinity of the bearing load.
- These failures are generally not serious and do not lead to structural collapse.
- The allowable tabulated compression perpendicular to the grain values are based upon an average deformation level of .04".
- The governing equation is:
fc^£ Fc^', where:- fc^ = P/A
- A is the net bearing area.
- Fc^'= Fc^ CM Ct Cb
- Cb is the bearing area factor.
- Accounts for the resistance provided by surrounding wood upon deformation of wood directly underneath the load.
- According to the 1997 NDS, Cb is used for:
- At locations where lb < 6" and end distance is ³ 3", then
or in tabular form, Cb for various bearing lengths is:
lb .5" 1" 1.5" 2" 3" 4" ³ 6" Cb 1.75 1.38 1.25 1.19 1.13 1.10 1.00
- fc^ = P/A
- Bearing at an angle to the grain.
- For bearing situations that occur somewhere between perpendicular
or parallel to the grain use Hankinson's Formula to determine the
allowable stress value.
- For bearing situations that occur somewhere between perpendicular
or parallel to the grain use Hankinson's Formula to determine the
allowable stress value.
Example:
- This problem has been adapted and modified from Structural Design in Wood by J. Stalnaker and E. Harris.
- To prevent bearing failure, what length of connection bracket is required?
- These dry floor beams are No. 1 and better, DF-L, 4X16 joists spaced at 4' o.c.
- This is a heavy storage facility.
- Loads:
- Dead: beam dead wt + decking + misc.
Assume 35 pcf wood: 13 plf Assume 3/4" plywood subfloor with 2" reinforced concrete topping: 3(3/4)(4) + 150(2/12)(4)=109 plf Misc. at 2 psf: 2(4)=8 plf Total D 130 plf
- Floor live load: heavy storage = 250 psf
L = 250(4) = 1000 plf.
- Therefore, wtl = 130
+ 1000 = 1130 plf.
- Examine fc^ £
F'c^
- Dead: beam dead wt + decking + misc.
