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RE: IBC First Draft Comment

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Steve, please see my comments in the body of your message.

Dennis Wish PE

|-----Original Message-----
|From: Steve Privett [mailto:eqretrodr(--nospam--at)earthlink.net]
|Sent: Monday, March 02, 1998 5:31 PM
||
|Since at least 1979, (when I started engineering) there has been the
|statement under conventional framing that stated that the provisions
|didn't apply to buildings of "unusual shape, size or split levels.  In
|the 88 UBC, Tables for structural irregularities were added.  I've used
|the argument that the items defined in the structural irregularity
|tables are considered the elements of unusual size, or shape and have
|been able to educate the owner of the property enough that he/she is
|willing and desires to pay for the structural design the building should
|have.
|

Dennis Responds:
Yes, I did realize this when I compared the difference between "irregular
shape" and "unusual shape". There is not much difference in these two codes
with regard to shape.


|When using the conventional framing provisions, one must rely on the
|span tables for rafters and I think they have been limited to 15 psf
|dl.  There are tables for 30 psf ll for roof, which it appears will be
|eliminated in the new code.

Dennis Responds:
In table 23-I-V-R-5 of the 1994 UBC a dead load of 20 psf and a live load of
20 psf is used in the span tables. The confusion with this section of the 94
code is that the conventional framing section of the code does not prohibit
this span table from use, but does not mention the table in section 2326.1
or 2326.5.2 which only provides instructions for loads which exceed those in
the other tables. Therefore, the decision to allow inclusion of 23-I-V-R-5
is generally left to the discretion of the building official.
The IBC is much clearer on this issue and expressly state so. This is
reduced in the IBC to 15 psf dead load which greatly restricts the design of
Tile Roof structures in high seismic regions.

|With regard to trusses, I've used the
|argument that they are not outlined in the conventional framing section
|therefore their use is not conventional framing.  And with the longer
|spans, and therefore higher reactions, the bearing conditions need
|examined.  This is especially true where you have a carrying truss at a
|change in direction or change from trussed to conventional framing as is
|common in so many prefab truss systems.

Dennis Responds:
I've used the same argument until the publication of the '94 code slipped
the following statement in section 2326.1 "Other approved repetitive wood
members may be used in lieu of solid-sawn lumber in conventional
construction provided these members comply with the provisions of this
code". Other approved repetitive members may have been meant as Strand
materials or plywood web joists, but it is generally interpreted to include
plated trusses.
My problem with this is not the quality of construction of plated roof
trusses, but the question of responsibility for assurance that the shear
transfer from roof to exterior walls is properly transferred.
The semantics allow for poor translation of the code and create potential
problems in construction quality as questions remain open as to how shear is
transferred since the truss manufacture (at least in the 94 code) take no
responsibility for anything other than gravity loads applied to each truss.
|
|I am glad however to see the wording change to specifically address the
|irregularities as it  makes it easier to convince a stubborn contractor
|who thinks we overdesign buildings.  This also gives the plan checker
|something in black and white as to when an engineer is required.  I've
|always taken the oppurtunity to "educate" plan checkers regarding my
|interpetation of this first paragraph of the general construction
|provisions and usually with success.
|--
|Steve Privett
|mailto:eqretrodr(--nospam--at)earthlink.net
|
|
Dennis Responds:
I am not trying to drum up business but my concern is based upon visual
inspection of homes in my neighborhood that are allowed to be constructed
under conventional framing traditions, but whose city inspectors and
officials do not have the experience and training to track shear transfers
from roof to foundation.
By limiting the span of a truss, the builder either raises his cost of
construction by providing intermediate bearing, or is forced to hire and
engineer or architect to assure that these homes are properly constructed to
resist wind and seismic.
My neighborhood may not be typical of others, but the bottom feeders that we
discuss in this list are active in my area and have little concern for the
prospective owners capability to afford the repair of inevitable damage from
poor or missing connections.

I applaud the committees that worked on this section of the IBC because they
have clearly defined the sections so, as you agree, the building official
can make a decision with less debate on semantics.

Dennis