open-web composite steel joist floor systems

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Open-Web Composite Steel Joist Floor Systems

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How to Use This Online Learning Course

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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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• 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


• 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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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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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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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






< >

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.






< >

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.






< >

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






< >


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.






< >


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)






< >

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






< >

Podium Construction

Open-web composite joists may be used in place of precast or

cast-in-place concrete.






< >

Case Study






< >

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.






< >

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






< >

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.






< >

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



• Wall system engineered by supplier






< >

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

Please remember the test password STRUCTURAL. You will be

required to enter it in order to proceed with the online test.






< >

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.






< >

Case Study

A uniform screw pattern

was utilized.






< >

Case Study

Concrete was

placed with minimal

bleed-through.






< >

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

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