Need a book? Engineering books recommendations...

Return to index: [Subject] [Thread] [Date] [Author]

[SEAOC] Re: [SEAOC] Conventional construction provisions

[Subject Prev][Subject Next][Thread Prev][Thread Next]
RLFOLEY(--nospam--at) wrote:
> I have just read your letter regarding the 1994 UBC conventional construction
> provisions.
> While I haven't focused as much attention on the subject as you obviously
> have, I would like to discuss this.  I noticed that a couple of connections
> have been added to the nailing schedule table.  Sorry I dont have all the
> code section references as I am home without my code as I am writing this,
> but one is nailing from rafter blocking to top plate and the other is from
> rim rafter to top plate.  These connections may represent the shear transfer
> from roof diaphram to shear wall you were concerned with.
> I'm sure you are familiar also with the testing that was done last year as
> UCI by Seb Ficcadente and others which sought to explain why, although a lack
> of shear transfer nailing from roof to walls was routinely lacking,  roofs
> were not sliding off the walls.  It seems the toenails from the rafters to
> the top plates alone was capable of transfering loads at least as great as
> the allowable shear capacity of an unblocked diaphram.
> I don't feel that the conventional construction provisions themselves were to
> blame for the damage we saw after Northridge as much as poor construction
> practices, inspection, and lack of structural observation.
> Keep the thread going.

Thank you for your response. The code does call for nailing of rim joist 
and blocking to the plate. It does not, however, call for connection of 
the diaphragm to the blocking/rim joist. To make matters worse, there is 
no provisions for connection of the diaphragm to braced panel on the 
interior of the building. The code is clear about blocking at perimeter 
walls, but not so clear on connections at interior walls used to resist 
developed shear.

I did read the two studies that you mentioned in the SEAOC Proceedures. 
Although I find this very probable, until it is voted and accepted as 
code governing, I would not place much merit in it. The one interesting 
case in point from the study was the placement of blocking outside of 
the plane of the wall - used typically as a stucco molding. The study 
concluded that the minimum nailing of the diaphram to the rafters was 
sufficient to transfer roof shear to the top plate of the wall - while 
the blocking reduced or eliminated rafter rotation.  I think that this 
needs further clarification as to capacity of the diaphragm. If the 
diaphram capacity of minimal (ie, unblocked), I doubt that I would 
questions this. However, if the line of shear occures in a blocked panel 
zone or in the center of a diaphragm where the wall's demand may exceed 
the capacity of the diaphragm connection - a development (drag) must be 
designed and the results of this study do not necessarily reflect this.

Simply put, basic number crunching of interior braced panels does not 
work when the panel is installed in a prescriptive manner - as I noted 
in my original letter.

The purpose of my concern is the lack of construction quality found in 
both damaged wood structures and other buildings where the basic 
engineering principles are not understood by the builder. 

Ben Schmidt SE is currently working on SEAOC to author a change in the 
code allowable values for Gypsum and Stucco shear walls. Heavy damage 
was sustained during Northridge (and Loma Prieta) in Gypsum and Stucco 
walls due, primarily, for improper connection of the stucco lath to the 
studs and general poor performance of gypsum products.
If Ben is successful, then this discussion is moot since changes in 
Stucco and Gypsum vaules will essentially negate conventional framing 
and nullify the section or force it into rewrite.

Construction quality by framers in my area are very poor. Framing and 
shearwall installation, unless clearly detailed, is not understood by 
the framer and is therefore not properly done in the field. Inspectors, 
too, are not trained to spot these problems and so, are left unresolved.

Finally, the question is not really collapse or failure. The real 
question is the cost of repair and how it affects the pockebook of the 
home owner and insurance company. With better control on construction, 
financial loss can be minimized. Is this not the responsibility of our 
professional community - above and beyond the protection of life.

Dennis Wish