NASCC 1

1
Wind Design Considerations for Steel Joists and
Joist Girders
Perry S. Green, PhD, Technical Director Steel
Joist Institute Myrtle Beach, SC Tim Holtermann,
PE, Corporate Engineering Manager, Canam Steel
Corp. Washington, MO Mark Perry, PE, General
Manager, Quincy Joist Company Quincy, FL Joe
Pote, PE, Director Engineering, Research and
Development, CMC Joist Deck Hope, AR
2
Introduction

• Commercial manufacture of open web steel joists
  began in 1923
• The Steel Joist Institute was formed in 1928
• The use of steel joists has continued to grow
  year after year for both floors and roofs.
• Millions of steel joists and Joist Girders are
  put in service each year.

3
General Nature of Wind Loads

• Typical Steel Joist and Joist Girder Buildings
• Windstorms
• Building type commercial, industrial
• Building shape low rise, rectangular
• Roofing systems

4
Windstorm Damage to Roof in Texas 05 March 2004
5
Windstorm Damage to Roof in Texas 05 March 2004
6
Hurricane Charley Category 4 Storm Across Florida
13-14 August 2004
7
Hurricane Charley Category 4 Storm Across Florida
13-14 August 2004
8
EF-2 Tornado Damage Near Ft. Worth, TX 24 April
2008
9
EF-2 Tornado Damage Near Ft. Worth, TX 24 April
2008
10
EF-2 Tornado Damage Near Ft. Worth, TX 24 April
2008
11
EF-2 Tornado Damage Near Ft. Worth, TX 24 April
2008
12
Population Trends in Hurricane-Prone Regions of
the U.S.

• Southeast and Gulf of Mexico Most rapid coastal
  growth in recent decades and will continue to
  grow.
• Southeast 8 million (1960) ? 23 million
  projected (2015)
• Gulf of Mexico 8 million (1960) ? 22 million
  projected (2015)

13
Top 10 Deadliest Hurricanes to Strike the US
1851-2005
Footnotes Could be as high as 12,000. Could
be as high as 3,000. Midpoint of 1,000 2,000
range. AP total as of Dec. 11, 2005.
Midpoint of 1,100-1,400 range. Sources
NOAA Insurance Information Institute.
Hurricane Katrina was the deadliest hurricane to
strike the US since 1928
14
Roof Design to Resist Wind Loads

• Codes and Standards
• 2005 SJI Standard Specifications and Code of
  Standard Practice
• Provisions from 2006 International Building Code
• Provisions from ASCE/SEI 7-05
• Design for Lateral Wind Loads
• Design of Joist Bearing Seats for Wind Uplift
• Design Example - Placement of Joist Bridging
• Summary and Conclusions

15
Standards and Codes

• 2005 SJI Standard Specifications and Code of
  Standard Practice
• Provisions from 2006 International Building Code
• Provisions from ASCE/SEI 7-05

16
Roof Design to Resist Uplift Loads

• The nominal loads and load combinations shall be
  as stipulated by the applicable code under which
  the structure is designed, and as shown by the
  Specifying Professional in the contract
  documents. In the absence of a specified
  building code such as the International Building
  Code (IBC 2006), the ASCE/SEI 7-05 (ASCE 2005)
  Minimum Design Loads for Buildings and Other
  Structures shall be used as the basis for the
  loads and load combinations.

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42nd Edition SJI Catalog 2005

• K-Series Standard Specifications
• K-Series Load Tables
• KCS Joists
• LH- and DLH-Series Standard Specifications
• LH- and DLH-Series Load Tables
• Joist Girders Standard Specifications
• Joist Girder Weight Tables

18
2005 SJI Standard Specification for Open Web
Steel Joists, K-Series

• 5.11 UPLIFT
• Where uplift forces due to wind are a design
  requirement, these forces must be indicated on
  the contract drawings in terms of NET uplift in
  pounds per square foot (Pascals). The contract
  documents shall indicate if the net uplift is
  based upon LRFD or ASD. When these forces are
  specified, they must be considered in the design
  of joists and/or bridging. A single line of
  bottom chord bridging must be provided near the
  first bottom chord panel points whenever uplift
  due to wind forces is a design consideration.

19
2005 SJI Standard Specification for Longspan
Steel Joists, LH-SeriesDeep Longspan Steel
Joists, DLH-Series

• 104.12 UPLIFT
• Where uplift forces due to wind are a design
  requirement, these forces must be indicated on
  the contract drawings in terms of NET uplift in
  pounds per square foot (Pascals). The contract
  documents shall indicate if the net uplift is
  based upon LRFD or ASD. When these forces are
  specified, they must be considered in the design
  of joists and/or bridging. A single line of
  bottom chord bridging must be provided near the
  first bottom chord panel points whenever uplift
  due to wind forces is a design consideration.

20
2005 SJI Standard Specification for Joist Girders

• 1004.9 UPLIFT
• Where uplift forces due to wind are a design
  requirement, these forces must be indicated on
  the contract drawings in terms of NET uplift in
  pounds per square foot (Pascals). The contract
  drawings must indicate if the net uplift is based
  on ASD or LRFD. When these forces are specified,
  they must be considered in the design of Joist
  Girders and/or bracing. If the ends of the
  bottom chord are not strutted, bracing must be
  provided near the first bottom chord panel points
  whenever uplift due to wind forces is a design
  consideration.

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2005 SJI Code of Standard Practice

• 1.4 DESIGN
• In the absence of ordinances or specifications to
  the contrary, all designs prepared by the
  specifying professional shall be in accordance
  with the Steel Joist Institute Standard
  Specifications Load Tables Weight Tables of
  latest adoption.

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2005 SJI Code of Standard Practice

• 6.1 PLANS FURNISHED BY BUYER
• (a) Loads
• The Steel Joist Institute does not presume to
  establish the loading requirements for which
  structures are designed.
•  
• The Steel Joist Institute Load Tables are based
  on uniform loading conditions and are valid for
  use in selecting joist sizes for gravity loads
  that can be expressed in terms of "pounds per
  linear foot" (kiloNewtons per Meter) of joist.
  The Steel Joist Institute Joist Girder Weight
  Tables are based on uniformly spaced panel point
  loading conditions and are valid for use in
  selecting Joist Girder sizes for gravity
  conditions that can be expressed in kips
  (kiloNewtons) per panel point on the Joist Girder.

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2005 SJI Code of Standard Practice

• 6.1 PLANS FURNISHED BY BUYER
• (a) Loads (contd)
• The specifying professional shall provide the
  nominal loads and load combinations as stipulated
  by the applicable code under which the structure
  is designed and shall provide the design basis
  (ASD or LRFD).
• The specifying professional shall calculate and
  provide the magnitude and location of ALL JOIST
  and JOIST GIRDER LOADS. This includes all
  special loads (drift loads, mechanical units, net
  uplift, axial loads, moments, structural bracing
  loads, or other applied loads) which are to be
  incorporated into the joist or Joist Girder
  design. For Joist Girders, reactions from
  supported members shall be clearly denoted as
  point loads on the Joist Girder. When necessary
  to clearly convey the information, a Load Diagram
  or Load Schedule shall be provided.

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2005 SJI Code of Standard Practice

• 6.1 PLANS FURNISHED BY BUYER
• (a) Loads (contd)
• The specifying professional shall give due
  consideration to the following loads and load
  effects
• 1. Ponded rain water.
• 2. Accumulation of snow in the vicinity of
  obstructions such as penthouses, signs,
  parapets, adjacent buildings, etc.
• 3. Wind.
• 4. Type and magnitude of end moments and/or
  axial forces at the joist and Joist Girder end
  supports shall be shown on the structural
  drawings. For moment resisting joists or Joist
  Girders framing near the end of a column, due
  consideration shall be given to extend the
  column length to allow a plate type connection
  between the top of the joist or Joist Girder
  top chord and the column.

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Specifying Design Loads and Load Combinations for
Joists Used as Part of the Lateral Load Resisting
System

• For clear definition of loads for joists used as
  part of the lateral load resisting system, the
  following guidelines should be followed
• All externally applied loads should be defined by
  Load Category (Live, Dead, Snow, Wind,
  Earthquake, Collateral, etc.).
• a. Avoid use of pre-combined load callouts such
  as Total Load, Factored Load, or Net Uplift
  Load, as these cannot be readily separated into
  their various load components, for correct
  assembly of load combinations with appropriate
  multipliers.
• b. For Dead Loads, if Net Uplift is a design
  consideration, be sure to include both a maximum
  dead load for inclusion with gravity loads, and
  a minimum dead load for inclusion with upward
  acting loads.  One convenient method of
  managing this is to specify the minimum dead as
  Dead Load (D) and specify the difference between
  minimum dead and maximum dead as Collateral Load
  (C).

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Specifying Design Loads and Load Combinations for
Joists Used as Part of the Lateral Load Resisting
System

• System internal forces which behave linearly, and
  may be algebraically summed, such as strut forces
  from a deck diaphragm, braced frame, shear wall,
  etc. should also be defined by Load Category,
  just the same as the externally applied loads.
• All potentially controlling design load
  combinations must be specified to the joist
  manufacturer, for investigation during the design
  of the joists and girders.  Along with the
  required joist design load combinations, the
  Specifying Professional must also indicate
  whether the design procedure is to be ASD or
  LRFD.  Either method may be specified, but it is
  important for the load combinations and design
  methodology to be properly aligned.
• 4. System internal forces which behave
  non-linearly, such as joist end moments and axial
  loads determined via a second-order frame
  analysis must be specified for each individual
  load combination.  These second-order analysis
  system internal forces do not behave linearly,
  and therefore cannot be algebraically summed by
  the joist manufacturer.

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Standards and Codes

• 2005 SJI Standard Specifications and Code of
  Standard Practice
• Provisions from 2006 International Building Code
• Provisions from ASCE/SEI 7-05

28
2006 International Building Code

• SECTION 2206 STEEL JOISTS
• 2206.1 General
• 2206.2 Design
• The registered design professional shall indicate
  on the construction documents the steel joist
  and/or steel joist girder designations from the
  specifications listed in Section 2206.1 and shall
  indicate the requirements for joist and joist
  girder design, layout, end supports, anchorage,
  non-SJI standard bridging, bridging termination
  connections and bearing connection design to
  resist uplift and lateral loads.
• 2206.3 Calculations
• 2206.4 Steel joist drawings
• 2206.5 Certification

29
2006 International Building Code

• SECTION 1605 LOAD COMBINATIONS
• 1605.2 Load combinations using strength design or
  load and resistance factor design
• 1605.2.1 Basic load combinations

1.4D (Eqn 16-1) 1.2D 1.6L 0.5(Lr or
S or R) (Eqn 16-2) 1.2D 1.6(Lr or S or R)
(f1L or 0.8W) (Eqn 16-3) 1.2D 1.6W f1L
0.5(Lr or S or R) (Eqn 16-4) 1.2D 1.0E
f1L f2S (Eqn 16-5) 0.9D 1.6W
(Eqn 16-6) 0.9D 1.0E (Eqn 16-7)
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2006 International Building Code

• SECTION 1605 LOAD COMBINATIONS
• 1605.3 Load combinations using allowable stress
  design
• 1605.3.1 Basic load combinations

D (Eqn 16-8) D L (Eqn 16-9) D
(Lr or S or R) (Eqn 16-10) D 0.75L
0.75(Lr or S or R) (Eqn 16-11) D (W or
0.7E) (Eqn 16-12) D 0.75((W or 0.7E)
L (Lr or S or R)) (Eqn 16-13) 0.6D W
(Eqn 16-14) 0.6D 0.7E (Eqn 16-15)
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Standards and Codes

• 2005 SJI Standard Specifications and Code of
  Standard Practice
• Provisions from 2006 International Building Code
• Provisions from ASCE/SEI 7-05

32
ASCE 7-05 Specified Wind Loads

• Basic parameters
• Wind speed, importance, exposure
• Significance / importance of exposure category
• Exposure C is default, while charts are based
  on B
• The difference is often 30 to 40 percent

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ASCE 7-05 Basic Wind Speed Map
34
ASCE 7-05 Specified Wind Loads

• It all looks simple when the building structure
  appears to be a simple rectangle made up of large
  monolithic elements as described in Figure 6-3.
• The reality is when the building shape is more
  complex comprised of numerous elements then it is
  not as easy to determine the loadings on joists
  in corners and Joist Girders that pass through
  both edge and corner zones.

35
ASCE 7-05 Specified Wind Loads
36
ASCE 7-05 Specified Wind Loads
37
Chapter 6 Wind Loads

• When wind uplift is a design consideration, it
  should be specified as net uplift on the steel
  joists and Joist Girders.
• The Specifying Professional knows the design dead
  load and if there are collateral dead loads that
  should not be deducted from the gross uplift.
• Steel joists are considered components and
  cladding (CC).
• The joist tributary width need not be less than
  one-third the joist span.

38
Chapter 6 Wind Loads

• Joist Girders can be considered part of the main
  wind force-resisting system (MWFRS). Most often,
  separate MWFRS pressure values are not provided
  for the Joist Girders, and the joist supplier
  applies the end reaction (net) uplift forces from
  the joists (CC) to the Joist Girders.
• Is this conservative?
• Joist Girder tension webs must be designed to
  resist, in compression, 25 percent of their axial
  force.
• Hence, uplift loads on a Joist Girder of less
  than 25 percent of the gravity loads have minimal
  or no effect on the girder design.

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Positive Wind Pressure Considerations

• The total joist load for the purposes of
  selecting a joist designation should represent
  the maximum result of the load combinations of
  IBC Section 1604, which may include a downward
  (positive) wind force in the controlling load
  case.
• Per ASCE 7-05 Section 6.1.4.2, the design wind
  pressure for components and cladding shall not be
  less than 10 psf.
• The applicable load cases from IBC 2006 are
• For LRFD
• 1.2D 1.6(Lr or S or R) (f1L or 0.8W)
  (Eqn 16-3)
• 1.2D 1.6W f1L 0.5(Lr or S or R) (Eqn
  16-4)
• For ASD
• D (W or 0.7E) (Eqn 16-12)
• D 0.75((W or 0.7E) L (Lr or S or R))
  (Eqn 16-13)

40
Positive Wind Pressure Considerations

• Example (ASD)
• D 20 psf
• Lr 20 psf
• W 10 psf
• TL D Lr 40 psf
• Or
• TL D 0.75 (W Lr) 42.5 psf ? Governs
• Joist Span 46 0
• Joist Spacing 6 0
• Total Uniform Load 42.5 psf x 6 0 255 plf
• Live Load 20 psf x 6 0 120 plf

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Positive Wind Pressure Considerations

• Example (ASD) Contd
• Choose a 28K9 from the SJI ASD Economy Load Table

42
Positive Wind Pressure Considerations

• Example (ASD) Contd
• Choose a 28K9 from the SJI ASD Economy Load Table
• Note If the positive wind pressure of 10 psf
  was not considered in the joist selection
  process, a 28K8 would have been selected. Also,
  one could have selected a 24K10 as it does not
  require erection stability bridging.
• Conclusion
• It is not appropriate to select a joist
  designation based on D Lr only and then provide
  a positive wind pressure expecting the joist
  manufacturer to check the designation.

43
ASCE 7-05 Specified Wind Loads

• Maximum Dead Load (for gravity loading)
• Minimum Dead Load (for wind uplift)
• Or
• DLmin DLmax - Collateral Load (also for wind
  uplift)

44
ASCE 7-05 Specified Wind Loads

• What Constitutes Net Uplift?
• For ASD,
• the uplift load combination is 0.6D W
• For LRFD,
• the uplift load combination is 0.9D 1.6W
• The EOR may need to differentiate between
  minimum and maximum dead load.

45
ASCE 7-05 Specified Wind Loads

• What constitutes Net Uplift? Lets look at the
  ASD Basic Load Combinations
• Amplified DL resistance by 1.65 for uplift is not
  desirable! So instead,

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Wind Loads Net Uplift Zone Diagram
47
Properly Applying Wind Loads to Steel Joists and
Joist Girders

• Many steel joist structures will qualify for the
  Method 1 Simplified Procedure in ASCE7-05.
• Conditions required for use of the Simplified
  Procedure include
• Roof height less than 60 feet
• Enclosed structure
• Regular-shaped building
• Roof is not steeply sloped

48
Properly Applying Wind Loads to Steel Joists and
Joist Girders

• Note that even if the Main Wind Force Resisting
  System (MWFRS) does not qualify for the
  Simplified Procedure, the Components and Cladding
  may.
• Net pressure vs. net uplift

49
Properly Applying Wind Loads to Steel Joists and
Joist Girders

• Clarifications and Interpretations
• ASCE simplified method described in Section
  6.4.2.2 provides a formula for net design wind
  pressure. This is NOT the same as SJI section
  5.11 NET uplift.
• ASCE net is the sum of internal and external
  pressures.
• SJI net, is the final resultant pressure, less
  appropriate dead load result of the load
  combination

50
Properly Applying Wind Loads to Steel Joists and
Joist Girders

• The chart on the following slide is a typical
  components and cladding roof wind pressures chart
  provided on the contract documents.
• Roof pressure needs to be converted to NET
  uplift, or more correctly the result of the
  appropriate load combination for wind forces
  acting upward.

51
Properly Applying Wind Loads to Steel Joists and
Joist Girders
52
Properly Applying Wind Loads to Steel Joists and
Joist Girders

• Per ASCE definition of Effective Width, take span
  times an effective width that is not less than
  one third the span.
• Note This is specifically referenced for the
  ASCE Method 2 charts, but it should also apply to
  ASCE Method 1 (simplified).

53
Properly Applying Wind Loads to Steel Joists and
Joist Girders

• So for steel joists, a simple rule is that for
  all joist spans of 18 foot or greater, use the
  100 square foot values, i.e. 18 x 6 106 gt 100
  ft.2
• So if a project does not have any spans less than
  18 feet, there is no need for a detailed chart
  with values by square foot.
• The light weight of joists under 18 foot spans
  often allows for a conservative uplift value to
  be used rather than a detailed interpolation for
  the exact square footage.

54
Properly Applying Wind Loads to Steel Joists and
Joist Girders

• For spans of at least 13 feet (1313/3 56.33
  ft.2), just use the 50 square foot value, or if
  no values are listed for 50 sq. ft., use the
  average of 10 and 100 sq. ft. values.
• For joist spans less than 13 feet, the 10 sq. ft.
  value could conservatively be used.

55
Wind Design Considerations for Steel Joists and
Joist Girders

• Other considerations
• Overhangs have significant uplift
• TCXs automatically have same capacity as
  downward gravity.
• But uplift on overhangs can easily exceed
  gravity, particularly in coastal areas or
  hurricane prone regions.

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Properly Applying Lateral Wind Loads to Steel
Joists and Joist Girders

• Lateral Load Resisting Systems
• Diaphragm and Shear Walls
• Braced Frames
• Rigid Frames
• Local Wind Bracing (Kickers)
• Roof Wind Screens

57
Lateral Load Resisting Systems Diaphragm and
Shear Walls
58
Lateral Load Resisting Systems Joist Seat
Rollover
Note Force V should be given on the structural
drawings as a joist design requirement.
59
Lateral Load Resisting Systems Deck Support
Angle
60
Deck Support Angle and Joist Seat Rollover
61
Shear Collector with K-Series Joist
Lateral Load Resisting Systems Alternate Detail
to Joist Seat Rollover
62
Shear Collector with LH-Series Joist
Lateral Load Resisting Systems Alternate Detail
to Joist Seat Rollover
63
Lateral Load Resisting Systems Braced Frames
Steel joists may be used as diaphragm chord
elements, or as collector elements in frame lines.
64
Chord Forces - Axial

• Chord Forces are carried as additional axial
  loads by the top chords of joists and/or Joist
  Girders.
• Chord Forces may vary from one end of the chord
  to the other. The additional axial load for each
  joist and/or Joist Girder must be indicated.
• Connections to transfer additional axial loads
  from one joist to another or from joist to
  supporting structure must be indicated.
• Type and magnitude of axial forces at the joist
  and Joist Girder end supports shall be shown on
  the structural drawings.
• Avoid resolving joist or Joist Girder axial
  forces through the bearing seat connection.

65
Diaphragm Chord
F
F
66
Properly Applying Lateral Loads to Steel Joists
and Joist Girders
All top chord axial loads and end moments are
transmitted directly via the tie plates or tie
angles. The eccentricity of horizontal forces
transferred through the bearing seats is avoided.
M
F
e
F
67
Joist Tie Plate
68
Joist Tie Angles
69
Lateral Load Resisting Systems Rigid Frames
70
Rigid Frames End Moments

• The Specifying Professional is responsible for
  the rigid frame design. Also, that unless
  specially designed and detailed as wind only
  flexible connections, rigid frame action will
  induce live load moments, which need to be
  specified.
• Type and magnitude of end moments at the joist
  and Joist Girder end supports shall be shown on
  the structural drawings.
• Avoid resolving joist or Joist Girder end moments
  through the bearing seat connection.
• The top and bottom chord moment connection
  details shall be designed by the Specifying
  Professional. The joist designer shall furnish
  the Specifying Professional with the joist detail
  information if requested.

71
Rigid Frames Axial and End Moments
72
Wind Bracing Kickers
73
Wind Bracing Kickers

• Specifying Professional to provide horizontal and
  vertical components of wind bracing forces being
  transmitted to joists.
• Specifying Professional to provide location of
  attachment to joists, or if possible add note to
  structural drawings to allow flexibility to
  require attachment to nearest panel point.

74
Roof Screens

• This Detail is Not Recommended

75
Roof Screens Perpendicular to Joist
76
Roof Screens Parallel to Joist
77
Design of Bearing Seats to Resist Uplift Loads

• Research
• 2005 SJI Standard Specifications
• Recommended Design Procedure

78
Typical Roof Framing using K-Series Open Web
Steel Joists
79
End Bearing Seat Connections
80
Profile of SJI Standard K-Series Open Web Steel
Joists
81
Components of Uplift Resistance for Test Program

• Anchorage
• Weld
• Strength
• Ductility
• Seat Angle
• Strength
• Ductility

82
Joist Seat Test Program Parameters

• Vary seat angle size (leg and thickness)
• S1 L 1 x 1 x 7/64
• S2 L 1-1/2 x 1-1/2 x 1/8
• S3 L 2 x 2 x 3/16
• S4 L 2 x 2 x 1/4
• Vary seat length
• 4, 6, 8 nominal
• Vary anchorage weld length
• 1, 3, 5 nominal
• Specimen Nomenclature SAS-SL-FWS-WL

83
Typical Test Specimen Configuration

PULL PLATE
JOIST SEAT
ANGLES
3/4" BASEPLATE
9/16" DIA. HOLE FOR ¾ A325N BOLT ( 4 PLACES)
84
Experimental Test Setup


85
End View During and After TestTest Specimen
S3-4-1/8-3
0.30 in. Vertical Displacement at 6.5 kips
Applied Load
Failure Mechanism
86
Typical Load-Deformation BehaviorTest Specimen
S3-4-1/8-3
87
Profile and End View After TestTest Specimen
S1-6-1/8-1
 
88
Yield Line PatternsShort and Long Anchorage Welds
89
Yield Line Analysis Model for Prediction of
Uplift Capacity

90
Yield Line Analysis Using Virtual Work
We (Pu / 2) D We External Work Pu
Predicted ultimate uplift load D Distance
which the load moves thru Wi Mp q (Lyl) Wi
Internal Work Mp Plastic moment capacity of
plate, per unit length Fy Z q Angle
through which YL rotates Lyl Length of yield
line, the lesser of Lw pa and Ls
91
Yield Line Analysis Using Virtual Work
Wi We 0 (Pu / 2) D - Mp q (Lyl) 0 But
since tan q q for small angles, q D /
a Solving for Pu gives Pu 2 Mp Lyl /
a Assumption of a 2.3 t provides
reasonably good prediction of ultimate uplift
strength of joist bearing seat
92
Research Programs Recommendations

• The flexural resistance of K-Series joist bearing
  seats can be predicted using a yield line
  approach.
• The yield line model is based on principles of
  basic mechanics, not on empirical curve fitting.
• A 5/32 fillet weld is adequate to develop the
  flexural strength of the yield line.

93
2005 SJI Standard Specification for Open Web
Steel Joists, K-Series

• 5.6 END ANCHORAGE
• (b) Steel
• Ends of K-Series Joists resting on steel
  supports shall be attached thereto with a minimum
  of two 1/8 inch (3 millmeters) fillet welds 1
  inch (25 millmeters) long, or with two 1/2 inch
  (13 millimeters) ASTM A307 bolts, or the
  equivalent. When K-Series Joists are used to
  provide lateral stability to the supporting
  member, the final connection shall be made by
  welding or as designated by the specifying
  professional.
• (c) Uplift
• Where uplift forces are a design consideration,
  roof joists shall be anchored to resist such
  forces (Refer to Section 5.11 Uplift).

94
2005 SJI Standard Specification for Longspan
Steel Joists, LH-SeriesDeep Longspan Steel
Joists, DLH-Series

• 104.7 END ANCHORAGE
• (b) Steel
• Ends of LH- and DLH-Series Joists resting on
  steel supports shall be attached thereto with a
  minimum of two 1/4 inch (6 millmeters) fillet
  welds 2 inches (51 millmeters) long, or with two
  3/4 inch (19 millimeters) ASTM A307 bolts, or
  the equivalent. When LH/DLH-Series Joists are
  used to provide lateral stability to the
  supporting member, the final connection shall be
  made by welding or as designated by the
  specifying professional.
• (c) Uplift
• Where uplift forces are a design consideration,
  roof joists shall be anchored to resist such
  forces (Refer to Section 104.12 Uplift).

95
2005 SJI Standard Specification for Joist Girders

• 1004.6 END ANCHORAGE
• (b) Steel
• Ends of Joist Girders resting on steel supports
  shall be attached thereto with a minimum of two
  1/4 inch (6 millmeters) fillet welds 2 inches (51
  millmeters) long, or with two 3/4 inch (19
  millimeters) ASTM A307 bolts, or the
  equivalent. In steel frames, bearing seats for
  Joist Girders shall be fabricated to allow for
  field bolting.
• (c) Uplift
• Where uplift forces are a design consideration,
  roof Joist Girders shall be anchored to resist
  such forces (Refer to Section 1004.9 Uplift).

96
ASD Design Procedure

• Pn 2 Mp Lyl / a
• Where
• Pn Nominal uplift capacity
• Mp Plastic moment capacity of plate per
  unit length
• Fy Z
• Z t2 / 4
• Lyl Length of yield line
• a 2.3 t
• W 1.67 (AISC-ASD safety factor for
  bending)
• Pn/W Allowable uplift strength

97
LRFD Design Procedure

• Pn 2 Mp Lyl / a
• Where
• Pn Nominal uplift capacity
• Mp Plastic moment capacity of plate per
  unit length
• Fy Z
• Z t2 / 4
• Lyl Length of yield line
• a 2.3 t
• f 0.90 (AISC-LRFD resistance factor for
  bending)
• fPn Design uplift strength

98
ASD and LRFD Design Example
Seat Angles L 1-1/2 x 1-1/2 x 1/8 Ls 4 Lw
2-1/2 Fy 50 ksi Allowable and Design
Uplift Strengths Z 0.125 2 / 4 0.00391
in.3 / in. a 2.3 (0.125) 0.28750 in. Lyl
2.50 p (0.2875) 3.403 in. lt Ls Mp
50 (0.00391) 0.1953 in.-kip / in. Pn
2 (0.1953)(3.403) / 0.2875 4.62 kips Pn/W
4.62 / 1.67 2.77 kips fPn 0.9
(4.62) 4.16 kips
99
Recommended Bearing Seat Design to Resist Uplift
Loads
100
Recommended Bearing Seat Design to Resist Uplift
Loads
101
Recommended Bearing Seat Design to Resist Uplift
Loads

• The Pweld strength given in the preceeding tables
  does not account for the transverse loading of
  the weld due to uplift and thus could be
  multiplied by 1.5.
• Where a joist seat has been detailed for a bolted
  connection, and for any reason the bolt is not
  utilized, the empty slot in the bearing seat leg
  severely diminishes uplift capacity. In such a
  condition, if a weld and no bolt is to be used on
  a slotted bearing seat, then the weld should be
  applied within the empty slot.

102
Typical Bolted Joist Connection
103
2005 SJI Standard Specification for Open Web
Steel Joists, K-Series

• 5.11 UPLIFT
• Where uplift forces due to wind are a design
  requirement, these forces must be indicated on
  the contract drawings in terms of NET uplift in
  pounds per square foot (Pascals). The contract
  documents shall indicate if the net uplift is
  based upon LRFD or ASD. When these forces are
  specified, they must be considered in the design
  of joists and/or bridging. A single line of
  bottom chord bridging must be provided near the
  first bottom chord panel points whenever uplift
  due to wind forces is a design consideration.

104
2005 SJI Standard Specification for Longspan
Steel Joists, LH-SeriesDeep Longspan Steel
Joists, DLH-Series

• 104.12 UPLIFT
• Where uplift forces due to wind are a design
  requirement, these forces must be indicated on
  the contract drawings in terms of NET uplift in
  pounds per square foot (Pascals). The contract
  documents shall indicate if the net uplift is
  based upon LRFD or ASD. When these forces are
  specified, they must be considered in the design
  of joists and/or bridging. A single line of
  bottom chord bridging must be provided near the
  first bottom chord panel points whenever uplift
  due to wind forces is a design consideration.

105
2005 SJI Standard Specification for Joist Girders

• 1004.9 UPLIFT
• Where uplift forces due to wind are a design
  requirement, these forces must be indicated on
  the contract drawings in terms of NET uplift in
  pounds per square foot (Pascals). The contract
  drawings must indicate if the net uplift is based
  on ASD or LRFD. When these forces are specified,
  they must be considered in the design of Joist
  Girders and/or bracing. If the ends of the
  bottom chord are not strutted, bracing must be
  provided near the first bottom chord panel points
  whenever uplift due to wind forces is a design
  consideration.

106
Design Example

• Building Location
• Near Orlando, FL in open terrain minimum slope
  ¼ / ft.
• Topography Homogenous
• Exposure Category C (Sections 6.5.6.2 and
  6.5.6.3)
• Building Framing and Layout
• Flat roof system consisting of steel joists,
  Joist Girders, and structural roof deck. CMU
  walls on all four sides with debris-resistant
  windows and door infill. Building has a parapet
  height of less than 3-0 and is considered a
  closed building.
• Building Classification Building Category II
• Importance Factor 1.0 (Table 6-1)

107
Design Example

• Dimensions
• Length, l 121-4
• Width, w 80-0
• Height, h 20-0 above the ground
• Roof slope is less than or equal to 5 degrees
• Roof live load deflection is based on L/240
• Design Roof Loads
• Dead Load, D 15.0 psf
• Roof Live Load, Lr 20.0 psf
• Total Load 35.0 psf
• Note 10 psf positive wind load is not used in
  this example

108
ASCE 7-05 Basic Wind Speed Map
ORLANDO
109
Design Example

• Basic wind speed, from Figure 6-1b for Orlando,
  Florida area V 110 mph.
• Design approach is based on the Simplified
  Procedure (Method 1) for both Components and
  Cladding and Main Wind Force System since the
  following conditions exist
• Simple diaphragm building (Section C6.2).
• Building shape is basis and has a symmetrical
  cross section in both directions and a flat roof.
• There is no expansion joints in the building.
• Its a low-rise building with a mean roof height,
  h less than 60 ft. and does not exceed the least
  horizontal dimension (Section 6.2).

110
Design Example

• Since the building has debris-resistant glazing
  and has no dominant opening in any wall it can be
  classified as a closed building. (Section
  6.5.9.3).
• Building has a regular shape.
• Rigid building, where height/width,
• w 20 ft./80 ft. 0.25 lt 4 (Section C6.2).
• The building is not subjected to the topographic
  effects of Section 6.5.7
• No torsional effects meets Note 5 of Figure
  6-10.

111
Steel Joist and Joist Girder Layout
112
Wind Zone Definitions
113
Steel Joist Design
20K6 Rod Web _at_ 40-0 Considering no uplift -
2-0
18 _at_ 2-0
2-0
3-0
3-0
17 _at_ 2-0
Bottom Chord 2 angles 1.5 x 1.5 x 0.137, A
0.784 in.2 End Web 5/8 in. dia. round bar , A
0.307 in.2
114
Steel Joist Design
20K6 Rod Web _at_ 40-0 With (net) uplift -
108 plf
84 plf
8-0
32-0
Bottom Chord 2 angles 1.5 x 1.5 x 0.155, A
0.882 in.2 End Web 7/8 in. dia. round bar , A
0.601 in.2
115
Steel Joist Design
Design Data End Web, left end l 37.49 in.
Reduce to 90 for eccentricity at bearing seat
116
Steel Joist Design
Design Data Bottom Chord, Pc 10.62 kips
4 rows (40)(12)/(41) 96 in.
117
Joist Bridging
Bolted Diagonal Erection Bridging
Horizontal Bridging (typ.)
Bridging Needed for Uplift typical at both ends
(if necessary)
118
Placement of Bridging to Resist Uplift Loads
20K6 Bridging Configuration Option 1
5 _at_ 8-0
Erection Stability Bridging
Uplift Bridging
A Common Alternative (not for this case)
4 Rows Equally Spaced
4 Rows Equally Spaced Between Uplift Bridging
119
Placement of Bridging to Resist Uplift Loads
20K6 Bridging Configuration Option 2
2 _at_ 8-9
3 _at_ 7-6
7-0
7-6
7-0
7-6
5-0
Design Data Bottom Chord, 2 angles 1.5 x 1.5 x
0.137, A 0.784 in.2 Pc 10.62 kips
120
Placement of Bridging to Resist Uplift Loads
At midspan of the joist
For compression, 7-6 space controls Pc 10.54
kips
121
Placement of Bridging to Resist Uplift Loads
With revised bridging locations at the TC, check
spacing
Note The 51 ft. is the maximum spacing for four
rows of bridging and a No. 6 section number from
TABLE 5.4-1.
122
Placement of Bridging to Resist Uplift Loads
20K6 Bridging Configuration Option 3
9.8 ft.
3 _at_ 6.8 ft.
9.8 ft.
5 Equal Spaces Between First BC Panel Points
5 _at_ 6.8 ft.
End TC space 9.8 ft.
123
Joist Girder Bracing Placement
40G8N7K Middle Girder _at_ 40-0 Designed based on
2.62k/PP Uplift Loading (vs. approximately
3.4k/PP Uplift Loading from components and
cladding joist reactions)
No uplift design, i.e. net uplift load 0 1 knee
brace _at_ midspan to limit L/ryy lt 240
With 2.62k/PP uplift load, same Bottom Chord
angles 3 x 3 x 1/4, but now knee brace _at_ 15 ft.
and 25 ft. required.
With 3.4k/PP uplift load, same Bottom Chord
angles 3 x 3 x 1/4, but now 3 knee braces _at_ 10
ft., 20 ft., and 30 ft. required.
124
Summary and Conclusions

• Wind forces can have a significant impact on the
  design and use of steel joists and Joist Girders
• The SJI Standard Specifications pertaining to
  wind and uplift have been reviewed the wind load
  requirements from ASCE 7-05 have been discussed
  and the appropriate 2006 IBC load combinations
  containing Wind have been presented.
• Particular attention needs to be paid to
• Code specified and calculated wind forces
• Seat anchorage welds
• Joist bridging and Joist Girder bracing placement

125
New Resource Soon Available

• SJI Technical Digest No. 6
• Structural Design of Steel Joist Roofs to Resist
  Uplift Loads

126
Any Questions?
SJI Website http//www.steeljoist.org