The ICBO Tri-Chapter (jurisdictions in S.F. bay area) code committee has
just approved a new guideline for distribution of lateral forces in wood
diaphragms. The majority of cities in the area are expected to adopt it.
There will eventually be a matrix posted on the web site that will indicate
which jurisdictions have adopted it. City of San Jose will adopt its use
Excerpts from the guideline are as follows:
I. Basic Static Force Procedure
Notwithstanding other specific requirements of the
1998 CBC, provide calculations in accordance with the following highlighted
1630.2.1 - Design Base Shear
1630.5 - Vertical Distribution of
1630.10 - Story Drift Limitation
(calculations per UBC Standard 23.223)
1630.6 - - Horizontal Distribution of
Shear (calculations per UBC Standard 23.222)
Under this method the lateral forces are distributed
to the various elements of the vertical lateral force resisting system in
proportion to their rigidities, considering the rigidity of the diaphragm.
II. Simplified Procedure
This method is applicable to structures of occupancy
categories 4 or 5 in CBC Table 16-K, of light wood frame construction three
stories or less in height. The following conditions for assuming flexible
diaphragms shall apply.
Conditions For Assuming Flexible Horizontal Diaphragms:
Wood horizontal diaphragms may be considered flexible and
the lateral load may be distributed based on the tributary area method as
prescribed below. Drift calculations are required for lines of resistance
with relatively flexible lateral force resisting systems, such as
cantilevered columns and narrow pre-manufactured wood panels in accordance
with Sections 1630.10 - Story Drift Limitation.
1. One and two dwelling buildings may be assumed as having flexible
horizontal diaphragms when deemed appropriate by the engineer of the record.
2. Wood structures of three stories or less in height, other than one
and two dwellings, may be considered as having flexible diaphragm when
deemed appropriate by the engineer of the record and provided the following
conditions are met:
a) The calculated redundancy factor of the building does not exceed
b) The capacity of the shear walls including holdowns and collectors
exceeds the design load by at least 20%.
Nothing in this guideline relieves the engineer responsible for the
structural design from applying sound engineering practice in the design of
the structure and achieving the structural performance as intended in the
California Building Code. The structural engineer responsible for the design
of the structure shall apply rational analysis in design of elements of the
project and shall consider suitability of Method II to the specific
structure based on the mass, geometric and stiffness regularity of the
structure. In addition, when specifically required by the building official,
the engineer of record shall conduct a more rigorous analysis to anticipate
the proper distribution of the load to lateral resisting elements."