I-Joist

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I-Joist

Offset Bearing Walls

Perpendicular to Joist -Opinion Page

Engineered Wood I-Joists have become the building material of choice due to
their performance and design flexibility. It is important to reiterate that design
and specification of a Wood I-Joist framing system differs from conventional
construction provisions addressed in building codes. While these engineered
products are recognized as an alternative building material, the manufacturer
should be consulted for code compliant design assistance. Some areas that require
special design include hole size and placement restrictions, cantilever detailing,
safety / erection bracing, vertical / lateral load transfer, etc.

One application that WIJMA has been asked to clarify is the transfer of loads
through the floor system from load bearing walls above. When using conventional
2x floor joist framing in residential construction, the United States and Canadian
codes allow the placement of offset bearing walls within a specified distance from
supporting girders, walls, or partitions without additional design calculations. This
does not hold true for engineered wood I-joists due to the differences in
cross-section.

The use of engineered wood I-joists with offset or stacking bearing walls is not
covered in the building codes; therefore, each application should be designed in
accordance with manufacturers literature, software, or consultation. Typically,
blocking is specified between joists below stacked bearing walls. The I-Joist design
assumes that this blocking transfers the loads around the I-Joist and directly to the
bearing locations. This cannot occur for offset wall applications; so, the point load
created by the offset wall must be considered in the design of the wood I-joist.

Individual manufacturers should be contacted for assistance, due to the proprietary
nature of their I-Joist design properties.

3/4-inch Plywood or OSB used as Rim Board

Rim boards provide basic closure and critical

structural load transfer in a structure. The transfer

of vertical loads includes the weight of the

structure above the rim board as well as design

and construction loads. Lateral forces resulting

from wind or seismic loads are also transferred

from the floor and roof diaphragm through the rim

board.

The rim board is typically positioned directly under a load bearing wall and
acts as a column, transferring these vertical loads into the supporting wall
below. The vertical capacity of the rim is limited by buckling or bearing
stresses. The transfer of lateral loads is dependent upon effective nailing
from the sheathing to the rim, from the wall sill plate above to the rim's top
edge, and from the rim into the sill plate below. This nailing transfers shear
forces from the floor diaphragm and the shear wall above to the wall or
foundation below. While these forces and design requirements are a prime
design consideration in all coastal high wind and seismic areas, they are
present in every geographic area and for every structure in North America.

The floor joist system cannot be counted on to absorb either the vertical or
lateral forces. In the case of a Prefabricated Wood I-joist system, the design
criteria for the floor system includes evaluation of bearing forces and stresses.
These designs assume that all external loads are transferred around the joist
and not through it. Applying additional loads beyond these assumptions can
result in an over-stressed condition. For this reason, it is imperative that the
rim board utilized with an I-joist system be the same dimensions and of
similar moisture content as the joist system so that dimensional differences
do not occur as the materials dry.

In response to recent high wind and seismic events, a group of industry
experts worked together to develop the ICBO Acceptance Criteria for Rim
Board (AC 124). It was determined that the minimum allowable vertical load
supported by a rim joist should be 2000 pounds per foot and that it should be
capable of laterally transferring a minimum of 180 pounds per lineal foot.
Rim board meeting these loads shall be recognized as being permitted for
use in structures complying with conventional construction requirements.
A 3/4-inch rim board ripped from structural wood panels provides
inadequate vertical transfer and is too narrow to effectively nail and therefore
is not recognized as providing adequate lateral load transfer.

The Wood I-Joist Manufacturer's Association (WIJMA), a technical
association comprised of every major manufacturer of prefabricated
wood I-joists, has investigated the use of 3/4-inch structural wood panels.
WIJMA concludes that this material is suitable for use only for closure and
that it cannot be counted on to provide minimum ICBO lateral structural
load transfer.

Solid Sawn Rim Board Used with Prefabricated
Wood I-Joists

2001 Position
Statement from WIJMA

The design and use of a rim board is to provide for the transfer of loads
applied from the walls, floors and roof loads from above, including lateral
forces from the diaphragm, to the wall or foundation below. Both of these
load transfers must occur in order for the structure to perform properly.

Prefabricated Wood I-Joist (PWIJ) depth criteria includes bearing forces
and stresses. Such stresses often control the selection and resulting design
of a PWIJ system. The design of a PWIJ system assumes that loads from
above are transferred into the rim material and around the PWIJ and that
these loads are not transferred through the PWIJ. The rim depth must be
predictable and stable to avoid loading the PWIJ throughout the service
life of the structure.

For lateral load transfer, the design limitation is often nailing from the
sheathing into the rim. This nailing is required to develop and transfer
shear forces in the diaphragm to the wall or foundation below. A rim board
that is short will significantly affect the lateral load that can be achieved by
nailing between the sheathing and the rim.

For these reasons, it is essential that the rim board specified match the
remainder of the system. I-joists are manufactured to tight tolerances and in
a dry condition. Solid sawn material is manufactured at various moisture
contents, varied depth tolerances depending on the mill and from different
raw materials and a resource base. I-joist and solid sawn material of the same
nominal size often vary in depth and are simply incompatible. The two products
cannot be expected to perform adequately as a system when combined.

The Wood I-Joist Manufacturers Association (WIJMA), a technical group
representing all major manufacturers, recognizes the inherent incompatibility
between a prefabricated I-joist and solid sawn rim material. The position of this
group is that the use of solid sawn material as a rim board is not compatible
with I-joist systems, that the use of such can lead to structural deficiencies.

WOOD I-JOIST

Wood I-joists are well accepted throughout the construction industry. I-joists are a
high strength, cost efficient alternative to conventional framing. They are exceptionally
stiff, lightweight and capable of long spans.

Holes may be easily cut in the web according to manufacturer’s recommendations,
allowing ducts and utilities to be run through the joist. I-joists are dimensionally stable
and uniform in size, with no crown. This keeps floors quieter, reduces field
modifications, and eliminates rejects in the field. I-joists may be field cut to proper
length using conventional methods and tools.

Manufacturing of I-joists utilizes the geometry of the cross-section and high strength
components to maximize the strength and stiffness of the wood fiber. Flanges are
manufactured from solid sawn lumber or structural composite lumber, while webs
typically consist of plywood or oriented strand board. The efficient utilization of raw
materials, along with high quality exterior adhesives and state of the art quality control
procedures, result in an extremely consistent product that maximizes environmental
benefits as well.

Wood I-joists are produced as proprietary products which are covered by code
acceptance reports by one or all of the model building codes. Acceptance reports
and product literature should be consulted for current design information.

Prefabricated wood I-joists are used throughout the world. They are widely used as
a framing material for housing in North America. I-joists are made in different grades
and with various processes and can be utilized in various applications. Proper design
is required to optimize performance and economics.

In addition to use in housing, I-joists find increasing use in commercial and industrial
construction. The high strength, stiffness, wide availability and cost saving attributes
make them a viable alternative in most low-rise construction projects.

Prefabricated wood I-joists are typically used as floor and roof joists in conventional
construction. In addition, I-joists are used as studs where long lengths and high
strengths are required.

Each wood I-joist producer develops its own proprietary design values. The
derivation of these values is reviewed by the applicable building code authorities.
Since materials, manufacturing processes, and product evaluations may differ
between the various producers, selected design values are appropriate only for
the specific product and application.

To generate the design capacity of a given product, the producer of that product
evaluates test data. The design capacity is then determined per ASTM D5055-97.
The latest model building code agency evaluation reports are a reliable source for
wood I-joist design values. These reports list accepted design values for shear,
moment, stiffness, and reaction capacity based on minimum bearing. In addition,
evaluation reports note the limitations on web holes, concentrated loads, and
requirements for web stiffeners.

At end bearing locations, the critical shear is the vertical shear at the ends of the
design span. The practice of neglecting all uniform loads within a distance from the
end support equal to the joist depth, commonly used for other wood materials, is not
applicable to wood I-joists. At locations of continuity, the critical shear location for
several wood I-joist types is located a distance equal to the depth of the joist from
the centerline of bearing (uniform loads only). A cantilevered portion of a wood
I-joist is generally not considered a location of continuity (unless the cantilever length
exceeds the joist depth) and vertical shear at the cantilever bearing is the critical shear.
Individual producers, or the appropriate evaluation reports, should be consulted for
reference to shear design at locations of continuity.

Bearing lengths at supports often control the design capacity of an I-joist. Typically
minimum bearing lengths are used to establish design parameters. In some cases
additional bearing is available and can be verified in an installation. Increased bearing
length means that the joist can support additional loading, up to the value limited by
the shear capacity of the web material and web joint. Both interior and exterior
reactions must be evaluated. Use of web stiffeners may be required and typically
increases the bearing capacity of the joist. Correct installation is required to obtain the
specified capacities. Additional loading from walls above will load the joist in bearing,
further limiting the capacity of the joist if proper end detailing is not followed.

Published moment capacities of I-joists are determined from empirical testing of a
completely assembled joist or by engineering analysis supplemented by tension testing
the flange component. If the flange contains end jointed material, the allowable tension
value is the lesser of the joint capacity or the material capacity.

Because flanges of a wood I-joist can be highly stressed, field notching of the flanges
is not allowed. Similarly, excessive nailing or the use of improper nail sizes can cause
flange splitting that will also reduce capacity. The producer should be contacted when
evaluating a damaged flange.

Wood I-joists, due to their optimized web materials, are susceptible to the effects of
shear deflection. This component of deflection can account for as much as 15% to
30% of the total deflection. For this reason, both bending and shear deflection should
be considered in the deflection design. A typical deflection calculation for simple
span wood I-joists under uniform load is shown below.

Deflection = Bending Component + Shear Component

Individual producers provide equations in a similar format. Values for use in
the preceding equations can be found in the individual producer’s evaluation reports.
For other load and span conditions, consult the supplier/manufacturer.

Since wood I-Joists have the inherent capability to span farther than conventional
lumber, the model building code maximum live load deflection criteria may not be
appropriate for many floor applications. Many wood I-joist producers recommend
using stiffer criteria, such as L/480 for residential floor construction and L/600
for public access commercial applications like office floors. The minimum code
required criteria for storage floors and roof applications is normally adequate.

In addition to strength calculations, deflection must be checked relative to
code-prescribed limits. Additionally most manufacturers publish recommended
deflection limits that are more stringent than code minimums.

The design span used for determining critical shears and moments is defined as t
he clear span between the faces of support plus one-half the minimum required
bearing on each end. For most wood I-joists, the minimum required end bearing
length varies from 1½" to 3½" (adding 2" to the clear span dimension is a good
estimate for most applications). At locations of continuity over intermediate bearings,
the design span is measured from the centerline of the intermediate support to the
face of the bearing at the end support, plus one half the minimum required bearing
length. For interior spans of a continuous joist, the design span extends from centerline
to centerline of the intermediate bearings.

For more information go to:

http://i-joist.org/pdf/Asd_ij.pdf or http://i-joist.org/policies.asp

Manufacturers Statements:

Leading Manufacturers Information about Engineered Product:
http://www.apawood.org/level_c.cfm?content=pub_joi_libmain

SSI I-Joist Manual

3/4 " OSB of Plywood Used as Rim Board:
go to http://www.i-joist.org/OSB%20as%20rim.pdf

Offset Bearing Walls:
http://www.i-joist.org/Offset%20Bearing%20Walls.pdf

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