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Strap and Tie-Down Systems Part 2 by Alfred Commins
- To: SEAINT Listservice <seaint(--nospam--at)seaint.org>
- Subject: Strap and Tie-Down Systems Part 2 by Alfred Commins
- From: WISH DENNIS <dennis.wish(--nospam--at)verizon.net>
- Date: Tue, 20 Nov 2007 14:55:10 -0800 (PST)
In the November issue of Structure Magazine, I read Alfred Commins article on strap-tie down's commonly used for tracts and lower income residential almost esclusively in my area of California. With a few exceptions, I think Commin's is right on the mark with his comments and recommendations to retire strap systems such as HPAHD produced by Simpson and others (I think this is the right number although I do not specify these types of holddowns for the same reason's Commins points to). I live in an area of highly mineralized soils and with builders to cut corners by eliminating templates when embedding strap-ties for hold down systems. In addition to the spalling of the concrete, a majority of these straps are left exposed sufficiently to become wet through irregation systems as simple as lawn sprinklers. I once discussed with the Simpson's R&D who estimated the life span of a strap-tie
at less than 5 years in a highly mineralized region.
It appears to me that Commins comments are on the mark and possibly many of have known this for many years. What strikes me is that developers of high-end homes have still used these ties in $1M plus homes and their comments have all been the same; The code allows it and they have the right to use it under a free enterprise system. However, if they retire these straps, the damage done will likely become the damage to be done and what will we do before the costly effects of deterioration and rust take its toll in a high seismic region.
I am not sure that the concentric holddown connector that Commins recommends is the answer because of cost and installation, however, Northridge and Loma Prieta seemed to draw early conclusions as to eccentric holddown placement. My position on this was always to recommend a Hold down system that was screwed rather than bolted since it seemed to provide easiest solution at the best price when looking at the Simpson Catalog (which I assume is comparable to USP and/or Silver (if they are still around)). Bolted problems with the eccentric HD's have proven to be somewhat of a problem, but I am not sure that I would recommend reduction in capacity where screwed connections such as the SDS1/4 are used. The deflection of the bolted holddown might also have an alternative solution if one can be presented such as the use of tandem bolts and screwed connections or possible a bent press-pin in the flange of the HD or an
epoxy connection between plates (with holes added) that would create greater friction between the bolted HD plate and the surface of the wood.
Has anyone given thought to the thousands of homes that have used straps in high seismic regions and considered the potential ramification for failure of the straps due to enviornment and a necessary retrofit scheme if any part of the strap is visible from outside the concrete?
This sounds like a highly charged subject that I have been aware of for many years and have done everything down to refusing work from developers who are adament about the use of these straps unless the project is borderline Prescriptive (UBC 2320) that does not require hold downs for braced panels.
Dennis S. Wish, PE
California Professional Engineer
Structural Engineering Consultant