ACI 318 Chapt 17 Wood Crushing to limit anchor forces

[BR0]

In Chapter 17 of ACI 318-14 Section 17.2.3.4.3 has the requirements for tension loaded anchors in concrete. The commentary for option C mentions wood crushing of the sill to limit the force that can be transferred.

Question:
- In that same vein, is it reasonable to use the wood crushing at the SDS screws of the hold down to limit the force that can be transferred to the anchor bolt?

- Is it reasonable to reduce the force that can be delivered by a hold down by reducing the amount of screws that the Hold Down has?

Thank you in advance

 

[dhengr]

Boblriley:
They are talking about crushing, bearing or compression perpendicular to the grain in the wood sills, the usual weak link in a situation like this. Drive a screw into wood and you can normally pull the head right through the outer piece. Put a washer, some sufficiently stiff hardware, or a large plate washer under a screw or a nut on a bolt, and you will as likely get a screw pull-out failure from the holding piece or screw breakage and likely A.B. pull out. You have to look at all of these conditions at any fixing to determine which controls. My guess is that ACI is just warning that this can be a problem with A.B’s. without a sufficiently stiff and properly sized washer. In fact, there are details and requirement in some codes that sq. pl. washers be used under the foundation A.B. nuts to assure proper uplift (tensile) load application to the sills. This also helps account for the fact that the A.B. is not always centered under the sill pl. I believe the NDS requires these in some conditions. Any hardware or hold down strength should not usually be controlled by wood crushing caused by the screws.

 

[BR0]

dhengr,
Thanks for response. I might have missed your point, but I don't think the intention is to warn you about this type of failure. It actually is used to reduce the force that is required to be designed for. As an example the sill plate crushing usually is significantly less than omega * shear force.