ce479: an introduction to the design of steel joists …jliu/courses/ce479/...ii - 1 ce479: an...
TRANSCRIPT
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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!