Transcript 2009 IRC Bracing Provisions

2009 IRC Bracing
Provisions
2009 bracing
provisions are
significantly different
in organization,
scope and detail
from the previous
version of the code.
Quick overview
• Check lists
• Examples
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.
Determine climatic and geographic design criteria
none
85
none
D2
none
Exposure C
assumed
none
V.H.
CEC
none
YES
none
CEC
Very
Heavy
Page 43 CRC
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.
5
Adapted from IRC
Figure R301.2(2).
SLO County
Engineered Design per CBC OK
12
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.
Pg. 220 CRC
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
252-253
254-255
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 steelframed 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)).
Pg 218 of CRC
R301.2.2.1.2 Alternative determination of
Seismic Design Category E.
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.
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.
Page 218 CRC
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.
See added
handout pages
114-116
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.
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.