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Re: 3x3x1/4 plate washers in IRC vs. 2x2x3/16 in IBC

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At 06:13 PM 12/15/2004 -0900, you wrote:

Everyone:

I know people have talked about plate washers for wood sill plates a bit on this list but was hoping someone could spark my memory. Anchorage is currently going through the 2003 code adoption locally. 2003 International Residential Code Section R602.11.1 requires 3x3x1/4 plate washers for foundation sill plates in Seismic Design Category D.  2003 IBC 2308.12.8 requires 2x2x3/16 plate washers for conventional light frame construction in Seismic Design Category D and IBC 2305.3.10 requires 2x2x3/16 plate washers for engineered designs.

Is there a change to require 3x3x1/4 plate washers in an accumulative supplement or errata to the IBC?  The question from the local homebuilders has been when a one or two family dwelling is engineered can they use the 2x2x3/16 plate washers in the IBC?

Thanks.

Scott M. Haan P.E.
Deputy Building Official

Municipality of Anchorage
Development Services  Department
Mission Statement:  Guide safe construction and responsible development for the community.

Scott,

Here's my take on this: If you have an engineer design a residence, it no longer falls under the IRC provisions. The IRC is a prescriptive code. It SHOULD be far more conservative than a sharpened-pencil design, principally because there is no review by a licensed professional.  If you want to save the money by not hiring a professional to create a compliant design, you'll be "stuck" with a very conservative approach (3x3x1/4 washers). If you are willing to spend the money to have the design checked and verified by a professional who is going to calculate actual loads and trace the load paths to the foundation soils, then more latitude is offered on the minimum sizes (2x2x3/16 washers).

This is, in fact, one of my beefs with the IRC - in most cases it's too non-conservative.  Example (my apologies to regular readers): If I design a reinforced concrete wall,  I MUST include 0.2% horizontal reinforcement, and cannot space bars farther than 18" o/c.  If I design a 12" concrete wall, I must use two curtains of steel.  In the IRC, a builder can "design" a 12" concrete wall with a single curtain of steel, spacings of up to 72" o/c and NO horizontal steel whatsoever.  Not only that, but the code's allowance of below-grade unreinforced CMU is appalling.  I do a reasonable number of residential surveys, and I have yet to enter a basement more than 15 years old that doesn't have a crack in the mortar which would completely eliminate any tension bond in the CMU. Most unreinforced CMU basements 20+ years old with more than 6' of backfill have at least one crack that is 1/16" wide and spans more than half the length of the wall.  Another example? Collar ties.  IRC allows the use of a non-structural ridgeboard on a stick framed 3.5 in 12 vault, as long as there is a collar tie 1/3 of the way down from the ridge fastened with 3 - 8d nails each end.  That collar tie isn't going to do squat.  You're probably going to need more capacity than you can get with nails to hold the walls together in that case. But it's allowable under the IRC.

Sorry for the rant, but it's one of those things that makes me look like the "bad guy" every time a builder walks into my office. He can build it "with less" and be code compliant than if I give him an accurate engineered design. I have no sympathy for having to go get a 1/4" plate washer.

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