Presentation on theme: "2009 IRC Bracing Provisions 2009 bracing provisions are significantly different in organization, scope and detail from the previous version of the code."— Presentation transcript:
2009 IRC Bracing Provisions 2009 bracing provisions are significantly different in organization, scope and detail from the previous version of the code.
Sections 5537 and 6737 of the California Business & Professions Code: An unlicensed person may prepare construction documents for the following: Single-family dwellings of wood frame construction not more than two stories and basement in height. Multiple dwellings containing no more than four dwelling units of wood frame construction not more than two stories and basement in height on any lawfully divided lot. Garages or other structures appurtenant to single-family dwelling or multiple dwellings of wood frame construction not more than two stories and basement in height. Agricultural and ranch buildings of wood frame construction unless the Building Official deems that an undue risk to the public health, safety or welfare is involved.
However, if any portion of any structure exempted by Section 5537(a) of the California Business & Professions Code deviates from substantial compliance with conventional framing requirements for wood frame construction found in the most recent edition of Title 24 of the California Code of Regulations or tables of limitation for wood frame construction, as defined by the applicable building code duly adopted by the local jurisdiction or the state, the Building Official shall require the preparation of construction documents for that portion by, or under the responsible control of, a Registered Design Professional. The construction documents for that portion shall bear the stamp and signature of the licensee who is responsible for their preparation.
5 Determine climatic and geographic design criteria none85none D2 D2 V.H.CECnoneYESnoneCEC Exposure C assumed Very Heavy TODAY’S COMPLEX HOUSE With unusual or irregular shapes, along with high gravity or lateral(seismic) loads, many homes constructed in certain regions are engineered. For example, in Tacoma, Washington, 90 percent of all new houses constructed have an engineered load path. This is also true for new homes in parts of Florida, California, Oregon and other states. Page 43 CRC
For SDC E, the IRC contains provisions for reclassification of structures from SDC E to D2 (see IRC Section R301.2.2.1.2) under the following circumstances: Where the structure is in a SDC D region based on in the International Building Code maps, or All exterior braced wall panels are in the same vertical plane from foundation to roof, and Floors do not cantilever over exterior walls, and Building meets IRC Section R301.2.2.2.5 requirements for “regularity” For SDC E structures that do not meet these requirements, the prescriptive structural requirements of the IRC cannot be used and the IBC, or other referenced standards, must be used to design the structure. As discussed in CHAPTER 4, the IRC places additional limitations on irregular portions of structures in higher Seismic Design Categories, as they may have physical features that are more vulnerable to seismic forces.
R301.2.2.2.5 Irregular buildings The problem with irregular buildings is that the irregular shape of the structure can cause the lateral loads to shift around in the building in unusual and difficult-to-predict load paths. Such structures are difficult to design and almost impossible to effectively specify using the prescriptive provisions of the IRC. In order to permit some degree of building irregularity that the prescriptive provisions can safely accommodate, this section was developed. Those structures, or portions thereof, that are outside of these limitations are irregular shaped buildings and must be designed using the IBC to ensure that the building has adequate capacity and a load path to resist seismic forces. Buildings having a regular shape can be constructed by following the prescriptive provisions of the IRC.
(R301.2.2.2.5) IRREGULAR means: 1.When exterior shear wall lines or braced wall panels are not in one plane vertically from the foundation to the uppermost story in which they are required. But there are some exceptions: We are on pg 65 of the 2010 CRC
Exceptions to #1 Setbacks or cantilevers do not exceed four times the nominal depth of the wood floor joists and ALL of the following conditions are met: 1. Floor joists are nominal 2x10 or larger and spaced not more than 16 inches on center. (Note that some engineered I-joist products are manufactured at 9-1/2-inch depths. These can be considered within the scope of this condition.) 2. The ratio of the backspan to cantilever is at least 2:1. 3. Floor joists at the ends of braced wall panels are doubled. 4. A continuous rim joist is connected to ends of all cantilevered joists. 5. Gravity loads at the end of the cantilever are limited to uniform wall and roof loads and the headers in the wall must have a span of 8 feet or less.
(R301.2.2.2.5) IRREGULAR means: 2. When a section of floor or roof is not laterally supported by shear walls or braced wall lines on all edges. Exception: Portions of floors that do not support shear walls or braced wall panels above, or roofs, shall be permitted to extend no more than 6 feet (1829 mm) beyond a shear wall or braced wall line.
(R301.2.2.2.5) IRREGULAR means: 3. When the end of a braced wall panel occurs over an opening in the wall below and ends at a horizontal distance greater than 1 foot from the edge of the opening. This provision is applicable to shear walls and braced wall panels offset in plane and to braced wall panels offset out of plane as permitted by the exception to Item 1 above.
Exceptions to #3 For wood light-frame wall construction, a number of exceptions apply to irregularity #3. Provided that the building width, loading conditions and framing member species meet the requirements of IRC Table R502.5(1), and the entire length of the braced wall panel does not occur over the opening, the header sizes shown below may be used to exempt irregularity #3. Table 4.7
(R301.2.2.2.5) IRREGULAR means: 4. When an opening in a floor or roof exceeds the lesser of 12 feet or 50 percent of the least floor or roof dimension. (no exceptions)
(R301.2.2.2.5) IRREGULAR means: 5. When portions of a floor level are vertically offset. IRC Section R502.6.1 states that joist framing must be lapped a minimum of 3 inches and face nailed together with three 10d nails. As an alternate, a wood or metal splice with equal strength is permitted. These measures address the problems associated with floor-offset irregularities.
(R301.2.2.2.5) IRREGULAR means: 6. When shear walls and braced wall lines do not occur in two perpendicular directions. No exceptions allowed
Note that an angled corner as described in IRC Section R602.10.1.3 (illustrated below) is not such an irregularity, but rather an architectural feature found on a basically rectangular structure. This section permits the bracing that may be found on the angled portion to be counted towards the minimum length required for one wall. The angled corner is limited to 8 feet in length, thus minimizing its impact on the structural rectangularity of the building.
(R301.2.2.2.5) IRREGULAR means: 7. When stories above-grade partially or completely braced by wood wall framing in accordance with Section R602 or steel wall framing in accordance with Section R603 include masonry or concrete construction. Exception: Fireplaces, chimneys and masonry veneer as permitted by this code. When this irregularity applies, the entire story shall be designed in accordance with accepted engineering practice.
No cripple walls today We will cover this in another lesson. Please disregard Pages 203-210 in your handout. It would be a good idea to get a notebook for the handouts I will give you. Eventually it will amount to a book.
Today’s handout 203-210 - Cripple walls - later 252-253 - 2009 IRC Code Reference Index 254-255 - 2009 IRC Bracing Methods Overview 228-231 - Example 11.2: Two-story house in SDC D2 243 - Bracing in high seismic regions 244-245 - BRACING T- AND L-SHAPED BUILDINGS
Wall Bracing Tables Chapter 6 TABLES R602.10.1.2(2)are the results of a multi-year project undertaken by a team of building officials, academics, design professionals, builders, structural engineers, ICC staff, product manufacturers and others. Each table includes adjustment factors. These adjustment factors – modifications to the amount of bracing based on variations in the structural geometry – are provided in footnotes and are different for each table. It is important to note that neglecting an adjustment factor can result in insufficient bracing for a specific application. For example, the wind bracing tables are based on a roof eave-to-ridge height of 10 feet. If the roof height of a given single-story structure is 15 feet and Footnote c of the wind bracing table is ignored, the wall bracing will be insufficient by 30 percent. For this reason, all table footnotes must be considered carefully.
Seismic Design Categories D0, D1 and D2 Height limitations (IRC Section R301.2.2.3.1) This section defines the maximum number of stories permitted for light wood- and steel- framed buildings in SDC D0, D1 and D2. Light wood-framed buildings are permitted to be a maximum of three stories, but only two stories in D2 (this limit is met by reference to the seismic bracing table, IRC Table R602.10.1.2(2)).
2. Buildings located in Seismic Design Category E that conform to the following additional restrictions are permitted to be constructed in accordance with the provisions for Seismic Design Category D2 of this code: 2.1. All exterior shear wall lines or braced wall panels are in one plane vertically from the foundation to the uppermost story. 2.2. Floors shall not cantilever past the exterior walls. 2.3. The building meets all of the requirements of Section R301.2.2.2.5 for being considered as regular. R301.2.2.1.2 Alternative determination of Seismic Design Category E.
How much bracing is needed for seismic? For the 2009 IRC, the seismic bracing table has undergone a major formatting revision with minor changes to the actual content. The table has been reworked to provide the amount of required bracing in total feet of bracing length instead of a percentage of braced wall line length. This revision was accomplished by basing the table on the length of the braced wall line and fixing the braced wall line spacing at 25 feet.
As illustrated, the information in the seismic bracing table is based on: Soil site classification D 10-foot wall height 10 psf floor dead load 15 psf roof and ceiling dead load Braced wall line spacing of 25 feet or less
Adjustments to the table The footnotes to the seismic bracing table include adjustment factors to accommodate variations from these assumptions for the wide range of residential structures covered by the IRC. These adjustment factors are included in IRC Table R602.10.1.2(3), While not all of these adjustments are new to the IRC, many have been taken from other sections and collected into a common location in the 2009 edition.
Example 11.2: Two-story house in SDC D2 Example 11.2 highlights: your pages 252-254 The amount of bracing required is based on the greater amount required by the ≤85 mph wind speed category and SDC D2. For a single-family dwelling located in SDC D0, D1 and D2, braced wall panels on the second floor cannot be placed over an opening on the first floor except in certain cases (IRC Section R301.2.2.2.5, Item 3, Exceptions). In SDC D0, D1 and D2, braced wall panels are not permitted to be located away from the ends of braced wall lines except in certain cases and only with Method WSP (IRC Section R602.10.1.4.1, Exception). WSP= Wood Structural Panels For structures in SDC D0, D1 and D2, braced wall line spacing is limited to 25 feet (IRC Section R602.10.1.5). Describes how to compare the amount of bracing required to the amount of bracing provided.
We might want to prepare a SLO Co. B.D. policy handout asking for the designer to follow this format when they use the CRC without engineering. And require braced wall lines to be indexed on a grid as in this example.
This and the following could also be incorporated into the CONVENTIONAL CONSTRUCTION per the CRC handout and used as a checklist for us and the designer to determine whether it meets the CRC requirements. See added handout pages 114-116
Bracing in high seismic regions Braced wall line spacing must not exceed 25 feet, except that braced wall line spacing can be up to a maximum of 35 feet with the exceptions defined in IRC Section R602.10.1.5. Braced wall panels must be located at ends of braced wall lines with an exception for Method WSP (wood structural panel) bracing, as defined in IRC Section R602.10.1.4.1. Adhesive attachment of bracing panels is not permitted, as discussed in IRC Section R602.10.2.2. Minimum 3-inch x 3-inch x 0.229-inch (9-gage) steel plate washers are required on all anchor bolts for all braced wall line sill plates, as described in IRC Section R602.11.1. Additional attachment and foundation requirements apply, as discussed in IRC Section R602.11. Houses cannot have irregularities without meeting additional requirements, as discussed in IRC Section R301.2.2.2.5.