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[SEAOC] 1994 UBC Conventional Framing Section - Opinions Wanted[Subject Prev][Subject Next][Thread Prev][Thread Next]
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- Subject: [SEAOC] 1994 UBC Conventional Framing Section - Opinions Wanted
- From: Wish <wish(--nospam--at)cyberg8t.com>
- Date: Tue, 21 May 1996 13:47:26 -0700
The 1994 UBC grants provisions for non-professionals to design and construct light framed wood structures for residential use. This section of the code provides a prescriptive measure, which when followed, assumes that the structure is adequately tied together. Cities such as Los Angeles, adopt similar measures and provide typical construction sheets (Typical Type V) which tell the builder / designer which gravity load members to use for specific plans and how to sheath and anchor braced panels in place. Most of the larger cities have, since the Northridge Earthquake, opted to be more discriminating in the definition of conventional construction. This is done by providing a degree of ambiguity into the definition of "conventional" framing. The 1994 UBC, however, has removed some of the a ambiguity by tightening up the definition of the code as well as expanding it?s interpretation to multistory, unusually shaped structures with non-stacking shear elements(Section 2326.5.4). In the many smaller municipalities throughout each state, the question of the building official?s authority is of great concern. The code does not identify the political cycle that often ties the hands of the building official against their better judgment. Specifically, new construction brings revenue to each city in the form of building fee?s, utility, infrastructure and school taxes. A developer of individual home-sites is an asset that most towns try to protect - right down to the city council. Without support from city council members, building officials often can not over-rule published codes, even if written incorrectly. Section 2326 - Conventional Light-Frame Construction Provisions of the UBC (1994) is one such poorly written code. Although the section adequately covers gravity load members, it does not clearly define lateral load connections from roof to foundations. Cross references to Chapter 16 of the code are greatly deficient except where the structures do not conform to the provisions defined in "Conventional Framing". A brief summary of the provisions of this section is as follows: Please note that sections 2326.1 through 2326.4 define the preamble to the body of this section of the code - establishing which structures conform to "conventional framing" definitions. I start with section 2326.5 since it is specific to Seismic Zone 4 ; 1. 2326.5.1 defines the maximum spacing of the braced panel. 2. 2326.5.2 defines the maximum loading to gravity members (rafter spans) and indicates that if the loads exceed these values an engineering design shall be provided. 3. 2326.5.4 provides a far less restrictive definition of non-conventional and, in fact, extends the term conventional to include two story wood framed residential structures. 4. 2322.214.171.124 provides a means to attach a shear wall to a header, but makes no provisions for resistance to uplift 5. 2326.5.6 defines Interior braced wall supports - requiring a continuous foundation under the walls at interval not exceeding 50 feet with EXCEPTIONS 6. 2326.6 defines mud sill anchorage 7. 2326.7 requires beams and girders to be bearing and indicates the depth of girders in single story construction by opening width. 8. 2326.8.1 defines gravity load framing details, including blocking and framing around openings, sub-floor sheathing and horizontal wood structural panels (2326.9.2). 9. 2326.11 defines Wall Framing - stud size, spacing, maximum height for bearing and non-bearing conditions and plate lap splices. 10. 2326.11.3 defines bracing - with minimum requirements for panel width to height limitations, minimum sheathing, anchorage and hold down requirements. 11. 2326.11.4 provides a more severe ALTERNATIVE to the previous section by defining plywood shear wall panels - sheathing, nailing, and tie-down devices The remainder of the section defines roof and ceiling framing - with very little difference over the 1991 code. So WHAT IS THE PROBLEM? UBC does not provide links to establish adequate ties of the diaphragms to shear resisting elements. By this I refer to the load path of the diaphragm to foundation (ie; diaphragm connection to shear element (except at the perimeter bearing), drags and collectors, chords etc). 1. Braced walls are only defined to have connection to foundations, but fail to address the connection of the wall panel to the diaphragm above. 2. Where wind loads predominantly govern, discussions of drags and collectors to interior braced walls are deficient. For a one story home fifty feet in width - the interior braced panel will take almost fifty percent of the lateral load. Assuming a typical scenario with a 70 mph wind - assume a 16 psf load, the approximate load to the wall will be in the neighborhood of 2800 lbs. Following section 2326.11.4 for a minimum 2.67 ft panel 10 feet in height, the unit shear would be 1048 plf. The provisions in the section require a 3/8" thick CDX panel w/ 8d at 6" on center assumed blocked by vertical layout of the panel (top and bottom plate connections at the panel ends). This section of the code requires resistance to uplift of 1800 lbs, but the actual uplift where framing is parallel to the wall can be closer to 10,000 lbs if the dead load is ignored. 3. This provision assumes that the builder is familiar with this section of the code. A false assumption since current requirements for licensing of general contractors does not require nor test for the understanding, or compliance to conventional framing codes. I believe my point is clear. The UBC provides a means for the allowance of unacceptable building construction standards. In area's outside the large city, inspectors are not adequately trained to locate discontinuities of shear transfer. A building official who wishes to negate this section of the code faces an uphill battle with local administrators (city council, construction developers, mayors office etc.) who have more to gain from revenue developed through profitable construction than potential effects of improper construction technique. Developers threaten to move their projects to other, more liberal communities. Once this happens, the building official has very little ammunition to fight with since the provisions are published, and accepted by the building industry. In the May edition of SEAOSC?s newsletter, the President?s message addressed a somewhat similar issue. I was abhorred to read that Mr. Ranous considered the inconsistencies to be best left to the ethics and conscience of individual engineers rather than to be addressed by the structural engineering community as a whole. It is just these issues that need to be addressed by the professional community. We must remember that these codes are adopted by votes from many different professional communities - not simply the structural profession. SEAOC has a very limited number of votes within ICBO and it is therefore extrememly urgent that the community act professionally to communicate our concerns for the publication of codes such as this. I apologize for the length of this letter, but I am interested in your comments and have been asked (by two life members of SEAOC) to draft a letter to the state urging a united stand against inadequate code provisions. Please start a thread with this message and provide me with your thoughts, idea?s and any other comments you may have. If you disagree, please let me have your comments as well. Sincerely, Dennis S. Wish PE Editor SEAOC Online ...
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