open-web composite steel joist floor systems
TRANSCRIPT
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This Online Learning Seminar is available through a professional courtesy provided by:
Ecospan Composite Floor System6230 Shiloh Road, Ste. 140
Alpharetta, GA 30005
Tel: (678) 965-6667
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Email: [email protected]
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©2017, 2020 Ecospan Composite Floor System. The material contained in this course was researched, assembled, and produced by Ecospan Composite
Floor System and remains its property. Questions or concerns about the content of this course should be directed to the program instructor. This multimedia
product is the copyright of AEC Daily.
Open-Web Composite Steel Joist Floor Systems
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Open-Web Composite Steel Joist Floor Systems
To ensure the current status of this course, including relevant association approvals, please view the course details here.
The American Institute of Architects
Course No. AEC1055-01
This program qualifies for 1.0 LU/HSW Hour
Course Expiry Date: 12/03/2023
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learning program may be sent to AIA CES ([email protected] or (800) AIA 3837, Option 3).
This learning program is registered with AIA CES for continuing professional education. As such, it does not include content that may be
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members. Certificates of Completion for both AIA members and non-AIA members are available upon completion of the test.
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How to Use This Online Learning Course
To view this course, use the arrows at the bottom of each slide or the up and down arrow keys on your keyboard.
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Purpose and Learning Objectives
Purpose:
Composite construction utilizes dissimilar materials to exploit the benefits of each. While composite construction in
general has been used extensively for several decades, open-web composite joist construction is now becoming a more
popular choice through new and innovative solutions. This course presents the components and benefits of composite
joist systems, addresses connector types and layouts, and offers specification tips and design considerations.
Learning Objectives:
At the end of this program, participants will be able to:
• describe the components and benefits of open-web composite floor systems
• select the appropriate connector and layout for use with open-web composite systems to ensure safety and shear
force distribution
• specify composite joists to adequately account for both construction and service loading scenarios and to meet HVAC
and plumbing coordination, corridor framing, and fire and sound rating requirements, and
• use design considerations to minimize floor vibrations, achieve proper camber, and select bearing and framing
options.
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Contents
Introduction to Composite Construction
Types and Layouts of Connectors
Specifying Composite Joist Systems
Design Considerations
Case Study
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Introduction to Composite Construction
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What Is Composite Construction?
Composite construction is a method of construction that utilizes
dissimilar materials. Oftentimes, the materials are a combination
of steel and concrete to exploit the efficiencies of each, e.g.,
steel in tension and concrete in compression. These dissimilar
materials work together by being connected, usually with a
standoff shear connector.
Composite construction in general has been used extensively for
several decades. Whether as composite wide-flange beams,
composite columns, composite joists, or simply long-span
composite deck, composite construction strives to achieve
efficiencies not available in noncomposite construction.
School
Military barracks
Student housing
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Benefits of Composite Construction
Once dominating the multistory, nonresidential market (through
composite beam and deck), composite construction is now
becoming a more popular choice in the multifamily residential
market through new and innovative composite joist solutions.
Applications now include apartments and condominiums, senior
living facilities, student housing and schools, hotels and resort
buildings, military housing and facilities, medical and office
buildings, and mezzanines.
These systems allow building owners to more easily achieve large
column-free areas, shallower floor-to-floor construction, enhanced
sound and fire ratings, and a more reliable structure all around.
Medical office
Hotel
Senior living facility
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Composite Construction with Steel and Concrete
In this presentation, the composite
“dissimilar materials” being discussed are
steel and concrete. More specifically, the
focus is on where the steel will be the
composite joist floor/roof system and the
concrete will be the overlying concrete
slab spanning between joists.
These materials are configured in such a
way that they are used in the most
efficient manner to exploit the advantages
of their respective material properties.
Steel is very efficient in tension, while
concrete is very efficient in compression.
In a floor application under normal gravity
loading, the materials are loaded in such
a manner to realize these efficiencies.
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Composite Construction Connection Types
To generate composite action, the slab and joist must work as an
integral unit and deflect together. To achieve this, some positive
connection must be provided at the steel/concrete interface to resist
interfacial “slip” between the two materials.
This presentation groups connectors into two distinct types:
• continuous shear connectors, that is, continuous across the
longitudinal axis of the joist or
• discrete shear connectors, that is, shear connectors that are
installed at a finite point along the joist span at either variable
spacing or constant spacing, depending on the type of discrete
connector used.
The interfacial connection is graphically explained on the next four
slides.
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Basic Design Philosophy: Noncomposite Member
This slide shows a typical noncomposite member,
which may be thought of as a joist supporting an
overlying concrete slab. Because there is no shear
connection, there is relative slip between the two
members. This is highlighted in the upper image and
can clearly be seen at the end of the member where
slip is the greatest.
Because the members have no interconnection and
therefore undergo this slip, there is a release of
internal stress at the interface of the joist and
concrete members. In the lower image, each
member acts independently, resulting in two
separate and distinct members independently
resisting the applied load. This is illustrated by the
stress discontinuity at the interfacial boundary. If a
mechanism were provided to resist this interfacial
slip, a continuous stress distribution would be
realized, offering a more efficient section.
Typical Noncomposite Member
Interfacial slip
Stress discontinuity
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Basic Design Philosophy: Noncomposite Member
The joist strength is based on the cross-sectional
area and orientation of the top chord, bottom chord,
and web configuration. Under normal gravity load
cases, the bottom chord resists tension while the top
chord resists compression.
The effective depth of this section is equal to the
distance between the centroidal axes relative to the
top chord angles and bottom chord angles.
Typical Noncomposite Member
Interfacial slip
Stress discontinuity
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Basic Design Philosophy: Composite Member
With a composite member, slip is resisted. Note the
interfacial slip at the boundary does not exist
(theoretical condition) in the upper image.
There is a mechanism present to transfer horizontal
shear between the joist and slab. This allows
members to act in conjunction with one another,
assuming sufficient shear connection has been
provided.
Typical Composite Member
Interfacial slip
Continuous stress distribution
via shear connectors
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Basic Design Philosophy: Composite Member
Steel joists and concrete used in composite
construction act as a unit, creating an assembly that
is stronger than each of the materials acting
independently.
The effective depth of the composite section is larger
than the noncomposite section because the post
composite compressive forces are primarily resisted
by the concrete, not the top chord of the joist,
moving the compressive component of the force
couple further up into the section.
The flexural strength of the assembly is increased
proportionally with the increase of effective depth.
This increase allows for longer spans for the same
total framing depth.
Typical Composite Member
Interfacial slip
Continuous stress distribution
via shear connectors
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Open-Web Composite Floor System Components
What is an open-web composite joist floor system? It consists of the following components:
• Steel open-web bar joists are typically spaced 4′-0″ to 6′-0″ on center with standoff screws, or 8′-0″ to 12′-0″ on
center with welded studs. Spacing can be dependent on fire rating selected, loading, or performance requirements. If
no limit is specified in the fire rating, joists can be spaced as far apart as the deck will span or the joist will allow
structurally.
Open-web bar joist
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Open-Web Composite Floor System Components
• Steel deck: For residential applications, 1″ to 1-5/16″ form deck is typically used. In this application, joists are typically
spaced at 4′-0″ on center to provide support for hat channels used in direct-applied ceiling applications. At a 4′-0″
spacing, 24-gauge material is found to be an economical solution. For commercial or more heavily loaded applications,
more robust composite deck profiles are utilized. Standoff mechanical shear connectors typically limit the deck height
to 1.5″, while welded shear connectors allow for deck depths up to 3″.
Steel deck
Open-web bar joist
• Shear connectors can be mechanically
fastened or welded. Welded attachment
requires a minimum chord width and
chord thickness to accept the stud and
associated weld, while mechanically
fastened connectors may have a
maximum chord thickness requirement
based on drill point capacity.
Shear connector
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Steel deck
Shear connector
Open-web bar joist
Open-Web Composite Floor System Components
• Concrete topping slab: This material is not provided by the joist manufacturer, but by others. The engineer of record
is responsible for specifying slab geometry and properties. However, both normal weight and lightweight concretes
are acceptable in composite joist framing.
Concrete with
reinforcement• Slab reinforcement: Again, this material
is supplied and designed by others. The
Steel Deck Institute has published
recommendations for reinforcement type
and quantities, depending on deck type
specified for the project. In general,
distributed fiber reinforcements are not
suggested on form deck applications
unless they are for temperature and
shrinkage only (supplementing wire or
rebar). However, they may be a suitable
solution when used on composite deck
profiles.
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Open-Web Composite Floor Systems
Samuelson* notes the shear connection
between the joist top chord and the
overlying concrete slab allows the steel joist
and concrete to act together as an integral
unit after the concrete has adequately
cured.
Currently the most commonly used forms of
shear connection between the joist top
chord and concrete slab include specially
rolled cold-formed steel-shaped top chords,
specially embossed back-to-back double-
angle top chords, and discrete shear
connectors welded through the deck or
mechanically fastened to the joist top chord.
*Samuelson, David. “Composite Steel Joists.” Engineering
Journal, third quarter, 2002, pp. 111‒120.
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Basic Design Philosophy: Noncomposite Joist
The upper image depicts a joist loaded
uniformly. This elevation shows the top
chord carrying compression, the bottom
chord carrying tension, and the internal
shear forces resolved by the joist webbing.
This depicts the noncomposite state—as
you see, there is no compressive force in
the slab.
The lower image is a section through the
upper image. In this sense, it is still
noncomposite. The dimension “deff” is what
is referred to as the effective depth of the
joist. Note that it is measured from the
centroidal axis of both chords. In this state,
the joist and slab carry the required
loading individually.
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Basic Design Philosophy: Composite Joist
This image now introduces a discrete, mechanical standoff
shear connector.
Note the increase in effective depth and the compressive
force that is driven into the slab.
Because of the increase in effective depth, the joist acts as if
it were deeper than it truly is, providing greater load-carrying
capacity and stiffness characteristics.
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Composite Joist Advantages
By making the joist composite, these inherent advantages are utilized:
• Smaller chords and wider joist spacing allow for mechanical, electrical, and plumbing services to be routed through or
between the joists.
• The plenum space is better utilized, with no need to route services under the joist as is common in concrete or
structural steel beam structures.
• The composite design allows for reduced floor-to-floor heights (if joist depth is not required for routing of MEP
services).
• The stiffer floor system reduces live load deflections as the effective moment of inertia resists post composite loading.
• With reduced deflections come reduced stresses. This allows the joist to span further or carry higher loading when
designed compositely.
• There is a potential for weight savings.
• Erection is simplified with wider joist spacing.
• Longer spans allow for large column-free areas.
Review Question
What is composite construction?
Composite construction is a method of construction that utilizes dissimilar materials that work together by being
connected. The materials are configured to exploit the advantages of their respective material properties. Steel
is very efficient in tension, while concrete is very efficient in compression. In a floor application under normal
gravity loading, the materials are loaded in such a manner to realize these efficiencies.
Answer
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Types and Layouts of Connectors
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Transfer of Forces: Continuous Shear Connectors
With continuous shear connectors, the top chord is embedded
into the concrete slab and is used to develop the horizontal
transfer force. The top chord is usually deformed in a manner to
engage the concrete more effectively. Continuous shear
connectors interrupt the deck span, as shown in this image.
Generally, single-span sheets of decking/formwork are required
(as shown), likely:
• increasing the required deck gauge
• doubling the attachment pattern, and
• significantly increasing the number of pieces that are
required to be installed.
All of these items slow down deck placement, and single-span
is generally less desirable from a safety/shoring aspect.
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Transfer of Forces: Discrete Shear Connectors
With discrete shear connectors, individual connectors
are attached to the top chord at a spacing required to
develop the horizontal transfer force. Conversely to
continuous connectors, discrete shear connectors allow
the deck to span over the supporting member. The
shear connector is either welded through the deck or
installed via a self-drilling mechanical connection
through the deck into the top chord. This simplifies the
installation and solves all of the issues just mentioned
(bulleted on prior slide).
However, the tradeoff is typically a slightly thicker floor
profile to ensure adequate concrete coverage—
generally no more than the depth of the deck, so this
could be as small as ½″ to 1″—as the floor now sits on
top of the supporting member (instead of dropping the
deck down so the top of the deck is at the same
elevation as the top of the supporting member).
While several examples of discrete shear connectors are
shown here, in practice the stud type (whether welded or
mechanically installed) represents nearly all of the
composite shear connectors seen today.
Flat bar
Stiffened
angle
Channel
Spiral
Studs
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Discrete Connectors: Welded Studs
One type of discrete connector is a welded stud. Design criteria is specified by the American Institute of
Steel Construction (AISC) Steel Construction Manual and the Steel Joist Institute’s (SJI) CJ Series
specification.
Fastener capacity depends on the material thickness, stud diameter, and deck profile. Fasteners are
installed with stud welding guns or fillet welds, typically in a variable pattern.
Shear Stud Diameter Minimum Horizontal Flat Leg Width Minimum Leg Thickness
0.375″ 1.50″ 0.125″
0.500″ 1.75″ 0.167″
0.625″ 2.00″ 0.209″
0.750″ 2.50″ 0.250″
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Discrete Connectors: Welded Studs
Stud placement is often
optimized by utilizing a
variable pattern along the
member with concentrations
of studs where shear
demand is at its highest,
generally at the end of
members and near
concentrated loads.
Layout with chalk line and ferrule placement Installation of welded studs
The left image shows the placement of ferrules, while the right shows the installation of welded studs. Welded studs have
specific requirements for material thickness and width to accept the weld. Typically, welded studs are used in longer
spans (50′ or more) and/or heavily loaded applications. Typical stud diameters are ½″ and ¾″, though several other sizes
are available. Stud installation requires a certified welder and special inspections of the installation to ensure a proper
weld to the base material is achieved.
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Discrete Connectors: Standoff Screws
Standoff screws are an acceptable alternative to welded studs. Fastener capacity depends on material
thickness and deck profile. Instead of minimum thickness and width requirements, standoff screws have
a maximum drill capacity of approximately ⅜″ of base metal plus the deck thickness.
The standoff screw shown has a diameter of ⅜″. Because the diameter is less than typical welded studs,
it has slightly less capacity compared to larger studs, which generally means more connectors. However,
the cost for additional connectors is offset since a common tradesperson can install the screw instead of
a certified welder. Screws are self-drilling and self-tapping and can be installed with a standard drill.
Additionally, no special inspection is required as the stud is not welded—so there is no weld to check.
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Discrete Connectors: Standoff Screws
Because the standoff screws are used as both
the shear connector and deck attachment,
standoff screws are placed uniformly across the
member at a constant spacing, unlike welded
studs, which are installed (typically) in a variable
pattern.
The screw is dual-process heat-treated. The
entire part is through-hardened, while the drill
point and first few lead tapping threads go
through a secondary induction heat-treating
process to ensure consistent installation and
hardness to cut through the deck and chord. The
threaded region embedded into the base material
does not go through this secondary process as
we do not want the heat-treated region in the
shear plane.
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Typical Layout of Stud Connectors: Variable
This image shows a variable connector spacing, which is common for welded studs. The layout maximizes force transfer
at the ends, with closer spacing at the ends of joists or with double connectors.
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Typical Layout of Stud Connectors: Variable
In the deeper composite deck profiles, a stiffening rib exists in the low flute of the deck. This does not allow the discrete
shear connector to be installed in the center of the flute. As such, there exists a phenomenon where the shear capacity
of the connector is affected depending on the side of the stiffening rib the stud is installed into.
As stated by Samuelson, when installing shear studs on composite metal deck with a center stiffening rib, ideally, one
installs the studs all on the strong side of the deck stiffening rib. The strong position is the position realized when the stud
is placed on the far side of the stiffening rib relative to the joist centerline for a uniformly loaded joist.
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Typical Layout of Stud Connectors: Variable
Here a welded stud application is shown. The concentration of studs can be seen, illustrating the variable pattern. Stud
groupings are common near the end of the joist, or near concentrated loads as previously noted.
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Typical Layout of Screw Connectors: Uniform
This image shows a constant, uniform spacing, typical for projects utilizing standoff screws. The equal spacing of
connectors throughout the joist results in less layout and installation time.
Note that in both layouts, the connector alternates chords. This is so the shear force can be evenly distributed between
chord members.
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Typical Layout of Screw Connectors: Uniform
Here a standoff screw application is shown.
The uniform pattern specified is shown in
detail under the picture.
In areas where the pattern was not followed
as specified, the erector may have misplaced
the screw (missed the top chord) or simply
provided an extra screw.
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Layout of Connectors
A chord gap exists between the top chord angles shown here. To ensure an even distribution of shear into the joist, it is
suggested that the shear connectors alternate every other chord angle as the installer places them along the joist.
Field conditions may exist (an opening in the floor slab very close to the joist, say) that require consecutive connectors
be placed along one chord. In such a situation, no more than three connectors should be placed on a given chord
consecutively. Looking at the joist globally, no more than 60% of the connectors shall be installed on one chord.
Joist top chord angles Alternate shear connector placement
Deck
No more than three connectors
shall be placed consecutively on
any one chord angle.
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Continuous Top Chord Connector
Of interest in the continuous shear connectors shown, note the
placement of deck/forming. In such situations, the shear connector
interrupts the decking or formwork, and the deck is installed in a
single-span condition, which is generally not desirable. Further, in
steel deck applications, twice the number of deck attachments are
required as the deck is now installed and fastened on either side of
the continuous shear connector.
Additionally, the continuous connectors generally create a weakened
plane in the slab (due to reduced section at the top chord), where
cracking is likely to occur. Therefore, proper care with regard to
detailing the reinforcement over the joist should take place, especially
if concrete is to be the finished flooring application. The lower image
shows reinforcement draped over the joist top chord.
The benefit of using a continuous shear connector is that the top
chord is embedded into the slab, which allows a reduction in structural
depth, though generally only by ½″ to 1″ in most cases.
Cold-rolled top
chord
Unequal leg top chord
with embossments
Review Question
What type of layout is shown
here?
Variable connector spacing is shown,
common for welded studs. The layout
maximizes force transfer at the ends,
with closer spacing at the ends of
joists or with double connectors.
Answer
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Specifying Composite Joist Systems
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Specifying Composite Joist Systems
Specification is determined by on center joist spacing.
Always specify unfactored loads, and provide a section for
special loading/configurations.
Note that noncomposite (or precomposite) dead load is not
in the designation. This can be determined from the
designation since we know the total load and the other
component loadings. Simply subtracting the composite
dead load and live load from the total load will yield the
noncomposite dead load.
Also of importance is the construction loading. The
noncomposite dead load generally represents the weight of
the composite joist system; however, the construction live
load is a very important factor as this live load will be
applied to the joist where it does not have the benefit of the
slab. It’s important to understand the expected slab finishing
or other construction activities to adequately design the joist
for both construction and service loading scenarios.
Joist Designation
30 CJ 1700 / 840 / 270
Joist Depth
(inches)
Joist Type Total Load
(PLF)
Live Load
(PLF)
Composite Dead
Load (PLF)
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Floor Depth
Shown here is a UL G561 application:
• 2.5″ minimum topping slab over deck
and
• minimum ⅞″ hat channel supporting a
single layer of ⅝″ gypsum.
Note in this listing, Type C gypsum is
specifically required. A deep seat is shown
to drive the joist reaction further over the
supporting element so as not to induce a
moment into the wall panel. In this
application, with the minimum topping slab
and ceiling depths shown, the total framing
depth is equal to the joist depth plus 5″ for
1″ form deck, 5.5″ for 1.5″ deck, and so on.
A ceiling extension (dashed line at the joist
bottom chord) has been provided for
continuous support of the ceiling.
Concrete slab Steel deck
Open-web
steel joist
Gypsum
Ceiling
depth
Joist
depth
Slab
depth
Furring channel
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Joist Span-to-Depth Ratio
These drawings show a scaled representation of the added benefit of composite construction; composite strength
increases the allowable spans.
Noncomposite joists have a span-to-depth ratio up to L/24.
Composite joists have a span-to-depth ratio up to L/30.
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Fire Ratings
There are multiple ANSI/UL 263 fire rated assemblies available:
• 1, 1½, 2, 3, and 4 hours
• direct-applied/suspended gypsum board ceiling
• suspended acoustical ceiling
• spray-applied fire-resistive material (SFRM)
Suspended acoustical application, G229 or similar
Spray-applied application, G710, D902, or D916Direct-applied gypsum application, G561 or similar
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Fire Ratings
Direct-applied
gypsum ceiling, G561
Direct-applied
gypsum ceiling
(G561) during
fire test
Direct-applied
gypsum ceiling
(G561) after
fire test
Dropped acoustical
ceiling, G229Spray-applied application, G710, D902, or D916
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Sound Ratings
For residential construction, IBC requires a minimum 50 decibels for both sound transmission class (STC) and impact
insulation class (IIC). Composite open-web joist configurations typically meet these requirements with most floor and
ceiling assemblies/finishes. UL G561 with a single layer of gypsum achieves approximately 57 decibels for STC. IIC will
require minimal sound attenuating material to reach the desired IIC ratings. It is best to discuss sound rating requirements
with your joist manufacturer to determine capabilities with the system to be specified.
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HVAC Coordination
The table lists approximate duct sizes that can penetrate different web
systems. The images show very large rectangular duct framing through
the joist web system with Vierendeel panels to allow duct passage.
Approximate Duct Opening
Joist Duct Shapes & Allowable Sizes
Joist
Depth
Panel
Distance
Round
(diameter)Square Rectangular
10″ 19″ 6″ 4″ x 4″ 3″ x 7″
12″ 19″ 7″ 5″ x 5″ 4″ x 7″
14″ 19″ 8″ 6″ x 6″ 5″ x 8″
16″ 24″ 9″ 7″ x 7″ 6″ x 10″
18″ 24″ 10″ 8″ x 8″ 7″ x 10″
20″ 24″ 11″ 9″ x 9″ 8″ x 10″
22″ 24″ 12″ 9″ x 9″ 8″ x 11″
24″ 48″ 15″ 12″ x 12″ 9″ x 20″
26″ 48″ 17″ 13″ x 13″ 10″ x 20″
28″ 48″ 18″ 14″ x 14″ 11″ x 20″
30″ 48″ 19″ 15″ x 15″ 12″ x 22″
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HVAC Coordination
These images show examples of
duct-friendly systems; open-web
composite joists are ideal for HVAC
placement within the plenum space.
They are generally constructed of
double-angle top and bottom chords
and webs consisting of rod or angles.
Web configurations are typically
uniform for a specific depth of joist.
Even though uniform webbing is
desired for a common depth joist, as
the span of the joist changes, the
panel points may occur in different
locations. Upon request, the joist
manufacturer can align the panel
points in the joist webbing so the duct
has a straight run down the building.
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Plumbing Coordination
When using composite construction, the typical items
shown here need to be coordinated prior to joist
placement, as penetrations affect the composite
action generated.
Typically, vertical holes are cored after concrete
placement.
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Plumbing Coordination
Cored and cut holes must miss the joist members below. Joists perform better when left in one piece!
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Corridor Framing
Joist suppliers and design professionals may specify a variety of corridor transition framing such as minijoists, embedded
upturned angles, and composite deck (shown), creating a situation that removes framing from the corridor in order to
provide maximum flexibility for MEP subtrades, as there are no joists, beams, angles, etc. to avoid. Obtaining a 1- or 2-
hour rating is simple with composite deck in corridor construction by utilizing either D902 or D916.
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Corridor Framing
Shown here is a corridor section utilizing composite deck. Framing would be similar to what would be required as shown
in the photo on the prior slide.
• Must maintain top of
slab
• Corridor deck is
typically deeper and
supported by Z-
closures
• Corridor deck spans
5′ to 9′ typically (deck
depth 1½″–3″)
• Joists may be parallel
or perpendicular to
corridor
• Fire rating and
diaphragm continuity
must be maintained
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Corridor Framing
The underside of this corridor shows the
wide open space offered when composite
deck is utilized.
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Review Question
Explain the meaning of the joist designation elements
shown here.
Joist Designation
30 CJ 1700 / 840 / 270
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Joist Designation
30 CJ 1700 / 840 / 270
Joist Depth
(inches)
Total
Load
(PLF)
Live
Load
(PLF)Composite
Dead
Load (PLF)
Noncomposite dead load can be determined
by subtracting the composite dead load and
live load from the total load. It’s also
important to understand the expected slab
finishing or other construction activities to
adequately design the joist for both
construction and service loading scenarios.
Answer
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Design Considerations
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Vibration Analysis
Floor vibration is a concern for builders and owners. The perception of
vibration is very subjective, particular to the occupant, and affected by
use (assembly, office, residential, dance space, etc.). Slab thickness,
framing orientation, spans, and partition configuration affect vibration
characteristics.
Evaluating the vibration characteristics of a composite joist requires a
review of the supporting structural system. The design professional
must understand the supporting structure and ancillary framing in
order to determine the dynamic response and damping properties.
This can be completed by utilizing the Steel Joist Institute’s “Technical
Digest No. 5” or AISC’s Design Guide 11. These guides will aid the
design professional in specifying the appropriate design parameters
for the project.
The damping ratio should be determined for each system; full-height
partitions and thicker slabs increase the damping ratio.
Steady-state response of spring-mass-damper
system to a sinusoidal force
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Vibration Analysis
Design parameters usually specified include the level of damping inherent to the structure, live and dead loads present
during the vibration event (note, these loads vary drastically from service loading; less load/mass on the floor during a
vibration event generally generates a conservative response), and performance requirements—whether those are
acceleration, frequency, or velocity limits for the floor system.
As joist manufacturers are essentially suppliers, they typically prefer to receive a required moment of inertia for the joist
to meet the project requirements. However, the design professional may not know the range of possible chord sizes for a
given joist depth. This makes it hard for the design professional to accurately supply a required moment of inertia that
can actually be fabricated.
The point here is that when a specific vibrational performance is required for an elevated floor system using composite
joists, it is always best if the design professional and the engineer for the joist manufacturer discuss the issue
beforehand to work out the details.
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Camber
Composite joists are generally cambered for 100% of the
noncomposite dead load deflection so the joist may receive a
slab of constant thickness and have the camber settle out once
concrete is placed. It is important to note that concrete over a
composite joist is placed to a constant thickness and not to a
specific elevation. The upper image illustrates a scenario
where the joist is cambered prior to concrete placement.
If the concrete installer places the slab to the specified
concrete thickness uniformly across the joist span, the joist will
deflect and will result in a flat condition as shown in the lower
image. This is of greater importance where direct-applied
ceilings are desired and the bottom chord of the joist will be the
substrate for ceiling installation. Proper concrete placement will
allow the installer to avoid shimming the ceiling to achieve
proper ceiling elevations.
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Camber
Camber will vary with noncomposite loading:
• Concrete thickness
• Joist depth and span
• ½″ to 2″ for ordinary spans for standard composite joist
• Adjacent spans of variable length should be addressed
Cambering for 100% of the noncomposite dead load generally results in more camber than what “standard” camber is for
noncomposite joists under the Steel Joist Institute’s specification. As such, special care should be given to composite
joists adjacent to openings, expansion joints, beams, walls, or other hard points in the floor that will influence the joists’
ability to settle out once concrete has been placed.
If something other than 100% noncomposite dead load camber is desired, it needs to be specified by the design
professional so the joist manufacturer can properly fabricate the joist to meet the needs of the project.
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Bearing on CFS Walls
Joists and cold-formed steel (CFS) load-bearing
metal studs present a very efficient framing option
for low- to mid-rise buildings. Several bearing
options are available. This image and section show
a steel hollow structural section (HSS) load-bearing
member.
Note that a shallow bearing seat on an HSS creates
slight eccentricity and induces moment into the wall
panel. Distribution members aid in spanning over
openings and in general allow for the load to be
distributed such that in-line framing may be omitted.
Bearing on steel HSS load distribution member
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Bearing on CFS Walls
These photos show how construction takes advantage of a
monolithic concrete placement onto the load-bearing wall. A
concrete load distribution member is shown in lieu of a
structural steel load distribution member. In this application, the
top track of the wall panel must be sized to carry the joist
reaction due to construction loading (wet weight of concrete
plus construction live load).
Once the beam has cured, the joist is “hung” from the beam
and concrete distributes the load from stud to stud and over
openings. This also presents a convenient detail to resolve
diaphragm collector and chord forces. Continuous rebar can be
easily provided.
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Bearing on CFS Walls
This section shows discrete shear
connectors (standoff screws) into the
top track.
This can be done to make the track
composite with the concrete beam to
use the top track as positive
reinforcement, or the standoff screw
can be used to transfer diaphragm
loads into shear panels as needed.
Bearing on CFS top track with concrete load distribution member (LDM)
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Framing Direction Alternatives
Framing exterior to the corridor wall typically requires
fewer pieces and simplifies the load path and
foundation operations since long, continuous footings
can be realized.
Generally, the demising wall to demising wall orientation
has shorter spans but requires more pieces to erect and
install. In typical hotels, framing onto the demising wall
presents a situation that requires more lineal footage of
load-bearing element and associated foundation.
In taller projects where the load-bearing studs cannot
carry the entire weight of the building in one orientation
or another, the framing can be rotated to alleviate the
load on the bearing elements below. While this requires
exterior, corridor, and demising walls to now be load
bearing, it is a potential option to achieve taller
structures with these efficient, lightweight materials.
Section A
Section A1
Exterior wall to corridor wall
Demising wall to demising wall
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Podium Construction
Open-web composite joists may be used in place of precast or
cast-in-place concrete.
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Case Study
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Case Study: General Building Information
This building is a six-story hotel in
the Upper Midwest. The hotel has
121 rooms, and each floor is
approximately 14,000 SF.
The building is classified as R-1
construction (hotels, transient). A 2-
hour fire rating is required for load-
bearing walls and floors.
The second floor utilized structural
steel for open areas. A composite
floor system was used with cold-
formed steel and prepanelized
walls.
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Case Study: Floor System
• 14″ composite joists at 4′-0″ o.c.
• Typical span: 27′-4″
• 1.0C deck on typical spans
• Concrete: 3½″ total thickness
• 3″ composite deck in corridor
• Joist staggered at demising
bearing walls
• Total load = 468 plf
• Live load = 160 plf
• Dead load = 140 plf
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Case Study
Shown here are typical details utilized at the joist bearing in this project. An alternate
location for rebar placement is simply on the joist base plate extension in lieu of inside
the end rod as shown.
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Case Study: Load-Bearing Wall System
• 6″ CFS prepanelized walls
• Top plate on all walls:
• Deep leg track 600T250-97, 50 ksi material
• Wall studs:
• 1st floor: 600S250-68 at 12″ o.c., 50 ksi
• 2nd floor: 600S200-68 at 16″ o.c., 50 ksi
• 3rd floor: 600S162-68 at 16″ o.c., 50 ksi
• 4th floor: 600S200-54 at 16″ o.c., 50 ksi
• 5th floor: 600S162-54 at 16″ o.c., 50 ksi
• 6th floor: 600S162-54 at 24″ o.c., 50 ksi
• Wall system engineered by supplier
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Case Study
What made this structural system work well?
• Speed of construction
• Ability to get other trades to start earlier
• Reduced risk of construction collapse
• Methods of efficiency during erection
• CFS wall panels delivered with exterior sheathing
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Case Study
Joists are shaken out directly
onto the CFS walls.
Adjacent spans are completed
and screw connectors are
installed.
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Case Study
Support attachments minimized field welding where possible. The left image shows self-drilling screws to the CFS. The
right image shows masonry screws to the CMU.
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Case Study
A uniform screw pattern
was utilized.
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Case Study
Concrete was
placed with minimal
bleed-through.
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Conclusion
©2017, 2020 Ecospan Composite Floor System. The material contained in this
course was researched, assembled, and produced by Ecospan Composite Floor
System and remains its property. Questions or concerns about the content of this
course should be directed to the program instructor. This multimedia product is
the copyright of AEC Daily.
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