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Re: wood screw design info (contractors substitution)

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Thanks for the comments Thor. As mentioned in my comments Simpson only had negative things to says..such as:
 
-screws are brittle. The heads would break off or a large chunk of wood would pull out.
-countersunk type screw driven into hole will bend hanger material and could lead to microcracks that would grow
-assemblies are tested with nails and not screws (even though load tables in the canadian lsd version related directly to individual nail capacities and not a fully assembled test case)
-they would not support screws in any way unless stated otherwise. They have a technical bulletin relating to this on their website
 
Dave
 
 
----- Original Message -----
From: Avicpeng
Sent: Thursday, March 31, 2005 2:17 PM
Subject: Re: wood screw design info (contractors substitution)

There are screw tables to be had.  I have a faded copy of tables generously sent to me some years ago by Larson Engineering.  My guess, and my own experience, on your breaking a wood screw is that screws sometimes insert and perform better if correctly predrilled.  I agree that retrofitting with nails is meaningless.  Try to evaluate the existing capacity, and if necessary have Simpson comment on your findings.

Thor Tandy P.Eng
Victoria, BC
Canada
vicpeng(--nospam--at)telus.net
----- Original Message -----
Sent: Thursday, March 31, 2005 11:09 AM
Subject: wood screw design info (contractors substitution)

Good afternoon all:
 
I have a situation where I spec'd Simpson hangers and the appropriate nails. The contractor thought he was doing a good thing by substituting wood screws for the nails. The floors are now complete with many, many hangers. The project is a rather large expensive home. Simpsons line is that the hangers were designed and tested with wood nails because failure mode for nails is ductile whereas it is brittle for wood screws.  The Canadian code does not mention capacities for wood screws, however, lag screws and timber rivets are covered. I would have thought that lag screws would be made of a similar material as wood screws.
 
I have a copy of the Wood Handbook from the USDA and it discusses the design methodology for screws and it does not say they cannot be used for structural connections. Simpson does use some screws but they are proprietary. In this document it mentions the NDS for Wood Construction published by the AFPA and LRFD for engineered wood published be ASCE.  I don't have either of these documents but it suggests that wood screws have design loads just like any other fastener.
 
There is no doubt that I would have preferred the contractor to use the nails, however, as usual, the Engineer is the one to either approve or say rip it out. Ripping it out in this case is just not realistic. Removing screws and putting proper nails in is also problematic because nail capacity will not be as good as if no hole was already there.
 
Any suggestions??
Do the US codes allow for the use of wood screws in this situation??
 
I was sure I read that a screws capacity would be at least as good as the same diameter nail. However, I have broken off wood screws before into maple and that is what makes me think that they are not as good.
 
The comparison is for 10d and 16d nails with No. 8 wood screws.  I do have the AFPA technical report 12 for the general dowel equations for calculating lateral connection values. I think that someone out there must have some helpful technical knowledge so that I don't have to dive into those calculations.
 
thanks
Dave