Wood Truss to Concrete Wall - Out-of Plane Connection

[dhoward26]

I'm having a debate with another structural friend over connections of wood trusses to concrete walls. I learned one way and he learned another and I'm not sure at this point which is the "correct" way. I attached a detail of the connection.

For out-of-plane wind and seismic forces when connecting a wood truss to a concrete wall, is it acceptable use Simpson A-35's (or something similar) or do you have to have direct connection to the anchor bolts in the top of the wall with a steel angle of some sort? I'm aware of the code requirements about not placing wood in cross-grain bending, etc. The way I was taught through my EIT years and first couple of PE years was to use Simpson A35's, but I'm questioning the validity of this now after doing more research.

 

[woodman88]

dhoward26

ASCE7-10 - 12.11.2.2.3 & ASCE7-10 - 12.11.2.2.1

Falls under the section for Structural Walls and Their Anchorage (ASCE7-10 - 12.11)

ASCE7-10 - 12.14.7.5.2

Falls under the section for Anchorage of Concrete or Masonry Structural Walls (ASCE7-10 - 12.14.7.5)

Both of these section seem to be mainly concerned with the use of a 2x10 or 2x12 ledger where the ledger bolts are placed at the center of the ledger with the diaphragm being nailed to the top of the ledger. The cross-grain bending in this case can rarely take the forces applied and can not take the minimum forces required by code for the anchorage of the masonry wall.

It is not an overall restriction against the placing of wood members in cross-grain bending.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

Et al:

I have posted in the past in the forums that the AITC specifically has stated since it's inception that "cross-grain bending is not recommended."

Although technically allowed by the use of the word "recommended", the implication is clear that the use of cross-grain bending is not a good engineering practice and should be avoided.



Mike McCann
MMC Engineering

 

[woodman88]

From page 71 of the ANSI/AF&PA SDPWS-2008

"The washer need not extend to within 1/2" of the sheathed edge where sheathing material unit shear capacity is less than or equal to 400 plf nominal. This allowance is based on observations from tests and field performance of gypsum products where sheathing fastener tear-out or sheathing slotting at fastener locations were the dominant failure modes. Other sheathing materials with unit shear capacity less than 400 plf nominal are included in this provision based on the judgement that the magnitude of unit uplift force versus sheathing type is the significant factor leading to potential for bottom plate spitting."

I guess that any good engineer would use the 3"x3" plate washers (within a 1/2" of the shear sheathing) for all shearwalls under 400 plf also.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.

 

[msquared48]

Agreed Garth.

Mike McCann
MMC Engineering

 

[RFreund]

I'm not really sure I follow Garth's post, however cross-grain bending does seem to be allowed in some cases -
Shearswalls design for combination shear and uplift. The nails at the bottom plate are allowed be used to resist uplift which would consequently put the bottom plate in cross-grain bending. See attached document.
But they also specifically say "Conditions effectively eliminate cross-grain bending as a failure mode...." (top of page 2)

EIT
www.HowToEngineer.com

 

[msquared48]

RFreund:

Your reference also states under "Methodology" that this is limited only to straps that can be nailed to the exterior sheathing that is nailed to the end shear wall studs. So the use of HD's is not allowed that attach to the stud and not the sheathing. The restriction of the straps would take the eccentricity out of the plate causing the cross grain bending condition, whereas the HD's and similar hold downs maintain the eccentricity.

This would not apply, however, to doubly sheathed walls though where the HD's would be the better option using the same logic.

Seems to me that the stiffness of the wall as a deep beam is not mentioned anywhere here, and should be considered. I am looking for a limiting set of aspect rations for the walls and cannot find it.... yet.

Mike McCann
MMC Engineering