II - 1

CE479: An Introduction to the Design

of Steel Joists and Joist Girders

Part II

Based on slides from:

Perry S. Green, Ph.D.,

Technical Director, SJI

Fall 2012 – J. Liu

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Roof Loading

Serviceability

Framing Considerations

Connections

Roof Design Considerations

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Roof Loading

Dead Loads

Live Loads

Snow Loads– Including Drift

Rain Loads

Wind Loads

Seismic Loads

Owner Established Loads

• Collateral Loads: Roof Top Units

(RTU’s), Concentrated Loads, Sprinkler

Mains, Folding Partitions, Basketball

Goals, etc.

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Snow Loads and Snow Drift

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Rain Loads

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Rain Loads

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Rain Loads

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Wind Loads: Net Uplift

Specify Loads and Zones

Might specify different “zones” based on anticipated dead

loads in those zones.

50 psf

30 psf

15 psf

85'

50'

10'

12'

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Wind Loads and Seismic Loads

Provide chord forces for wind load and

seismic load combinations.

Specify whether the force is a wind load

or seismic load.

Specify whether the chord force is

applied to the top or bottom chord.

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Owner Established Loads

Collateral Loads:

• RTU’s

• Concentrated Loads

• Sprinkler Mains

• Folding Partitions

• Basketball Goals

http://aarcweb.com/

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Collateral Loads- Roof Top Units

Header System for Roof Top Unit

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Structural Support Design for RTU’s

Structural Support for RTU’s

RTUs Do Not Have Uniform Density

(Support reactions are not all equal!)

Weight of Curb and Support Frame

Joist Selection

Special Joists

KCS Joists

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Roof Top Zone

24KCS3

24KCS3

24KCS3

24KCS3

40'

40' +1.5K

+1.5K

+1.5K

+1.5K

+1.5K

+1.5K

Additional

Girder Loads

(ASD)

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RTU’s – Other Considerations

Snow Accumulation

Wind

Reinforcement for Chord Bending

• Special Diagonals

Roof Screens

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Concentrated Load Reinforcement

for Chord Bending due to RTU

P P

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Accessories and Details: Added Members

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Sprinkler Mains

What size pipe is included

in uniform design loads?

Does the contractor have

the option to relocate or

resize mains?

Can pipe hangers be

located at joist panel

points?

http://www.swirnowstructures.com/mechanical.html

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Folding Partitions

Be aware of deflections

and shear related

problems.

Consider both a uniform

and a “stacked” condition

(folding partitions).

A one inch deflection is

normally a reasonable

and acceptable limit for

the partition only.

http://www.cesolutions.co.uk/

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Roof Design Considerations

Roof Loading

Serviceability

Expansion and contraction

Deflection

Vibration

Framing Considerations

Connections

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Membrane Roofs

Structural

Element Deformation Recommended Loading

Roofing

Expansion Joints

Horizontal

Movement

150 ft. to 200 ft.

Maximum

spacing

Thermal

Metal Deck

(Two Span)

Vertical

Deflection

L / 200

Maximum

300 lb

Centerline Load

Metal Deck Vertical

Deflection

L / 240

Maximum LL

Metal Deck Vertical

Deflection

L / 240

Maximum

200 lb

Centerline Load

Metal Deck - Deck Span

SDI Specification

Maintenance &

Construction

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Membrane Roofs (Cont’d)

Structural Element Deformation Recommended Loading

Steel Joists* Vertical Deflection L / 240

Maximum LL

Joist Girders* Vertical Deflection L / 240

Maximum LL

Roofs Slope

Minimum

1/4 in. per foot

Minimum Drainage

* SJI Criteria

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Vibration Considerations – RTU’s

Supporting Joists should have a natural

frequency 50% greater than, or 50% less

than the Operating Frequency of the

Roof Top Unit.

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Joist Frequency

Conc. Load @ Midspan:

Uniform Load:

f = Natural frequency of the joist,

(cycles/minute)

D = Joist deflection at midspan (inches)

f = 188 / D0.5

f = 213 / D0.5

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Roof Design Considerations

Roof Loading

Serviceability

Framing Considerations

Connections

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Roof Framing Considerations

Bay Size/Shape

Square most economical

Direction of Joist Span

If rectangular bay, joists in long direction best

Joist Spacing

As far apart as practical for deck system

Joist and Joist Girder Depth

K series joists and deepest joist girders

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Roof Design Considerations

Roof Loading

Serviceability

Framing Considerations

Connections

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The Basic Connection

Strong Axis of Column

D O N O T W E L D

( 2 ) - 3 / 4 " D I A . BOLTS

3 / 4 " S T A B I L I Z E R P L A T E

J O I S T B O L T E D T O G I R D E R

7 . 5 "

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D O N O T W E L D

( 2 ) - 3 / 4 " D I A . BOLTS

3 / 4 " S T A B I L I Z E R P L A T E

J O I S T B O L T E D T O G I R D E R

The Basic Connection

Weak Axis of Column

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The Basic Connection

THE SPECIFYING PROFESSIONAL IS

RESPONSIBLE FOR THE DESIGN OF THE

JOIST AND JOIST GIRDER [BASIC]

CONNECTION WHEN IT IS SUBJECT TO

ANY LOADS OTHER THAN SIMPLE SPAN

GRAVITY LOADING INCLUDING UPLIFT

AND LATERAL LOADS.

SJI Code of Standard Practice

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Floor Design Considerations

Floor Loading

Dead and Live loads

Owner established loads (sprinklers,

etc.)

Serviceability

Framing Considerations

Connections

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Floor Design Considerations

Floor Loading

Serviceability

Vibration control (SJI Technical Digest

no. 5)

Deflection limit – span/360

recommended by SJI

Framing Considerations

Connections

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1 10 100 FREQUENCY, CPS

0.0001

0.001

0.01

0.1

1

DIS

PL

AC

EM

EN

T A

MP

LIT

UD

E, IN

CH

ES

Floor Vibrations

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Floor Design Considerations

Floor Loading

Serviceability

Framing Considerations

Connections

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Floor Framing Considerations

Bay Size

30 ft square bays economical

Joist Span Direction

Cheaper to span joists in the long

direction

Joist Spacing

Widest spacing allowed by deck

Seat Depths

Note that joists at wider spacing may

require 5” seat depths

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Traditional Joist Spacing in Buildings

40'

24'

H H

H H

Joists @ 3 ft. spacing

Joist Girder Joist Girder

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New Joist Spacing in Buildings

40'

24'

H H

H H

Joists @ 6 ft. spacing

Joist Girder Joist Girder

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Advantages of Wider Floor Joist

Spacings

Fewer joists to manufacture

Fewer joists to erect

Less bridging to erect with larger, stiffer chords

Easier for other trades to “string” pipes, conduit, etc. through joists

Better vibration performance due to an increased tributary slab area for damping

Reduced costs for spray-applied fire-proofing

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Floor Design Considerations

Floor Loading

Serviceability

Framing Considerations

Connections

Typically brackets

Framing into wide-flange column webs

Be careful of width of joist girder chord!