NDS Appendix E

[shepherd]

NDS appendix E gives design guidance for the net section effect of wood connections for loading conditions parallel to the grain (similar to block shear checks for steel). The equations are based upon the allowable tension values parallel to the grain and allowable shear values parallel to the grain. In a case where you have a horizontal tie connection to a sloping member with steel side plates and bolts, it appears the correct failure mode would be a combination of shear and cross grain tension (wood's weakest direction). How do others handle this situation? I seem to recall that it is acceptable to assume the cross grain tension strength is equal to the shear strength; however, I can't find anything to back that up.

NDS section 3.8.2 indicates to avoid cross grain tension. I suppose I could wrap a light gage strap around the sloping member to avoid cross grain tension. Any thoughts?

Thanks,

 

[RARWOOD]

Based on your description it appears you are trying to make a connection similair to the attachement of web members in a truss to the top and bottom chords.

The simplest example would be an A-frame with the horizontal tie between the sloping members raised 1' t 2' above the bottom of the sloping members. A very common way of doing this is is to use two steel plates attached to the horizontal tie with two bolts and attached to the sloping member with a single bolt and 2 shear plates.

The horizontal tie applies its load at an angle to the grain of the sloping member. You then determine the allowable load that can be carried by the shear plates acting at an angle to the grain.

The important thing abouttension perpendicular to the grain, is the avoidance of know conditions that induce tension pependicular to the grain. The most common conditions are the notching of the tension face of a beam at its support and shear stress at a connection when members are supported by bolts or shear plates. See the 2005 NDS Section 3.4.3 pages 16 & 17.

Another condition to avoid is the hanging of heavy or medium loads below the nuetral axis of a beam. The single case of tension perpendicular to the grain over looked most often, is radial tension in curved beams.

Regarding allowable tension perpendicular to the grain, for Southern Pine it is taken as 1/3 the shear stress, for other softwoods it taken as 15 psi.

 

[shepherd]

Thanks, any idea where those cross grain tension values are published?

 

[RARWOOD]

The NDS doesn't published tension perpendicular to the grain values for solid sawn lumber. For glulam the NDS refers you to the AITC "Timber Constuction Manual". The TCM 5th edition refers you to AITC 117. The 4th edition of the TCM contains information on radial tension design values, in the section on curved members.