CTC 422 Design of Steel Structures

1
CTC 422Design of Steel Structures

• Steel Joists

2
Steel Joists

• Student Objectives
• Use manufacturers data to select and specify
  steel joists

3
Steel Joists

• Standardized prefabricated trusses
• Often used instead of steel beams, particularly
  in roof framing
• Lighter and more economical than beams for a
  given span
• Simpler quicker to install. Attached to top of
  beams.
• Typical components
• Top and bottom chords 2 angles
• Some bottom chords are 2 round bars
• Webs Single or double angles or round bar
• Some manufacturers use non-standard sections for
  components

4
Steel Joists - Types

• Open web steel joists - K-Series Joists
• Depths 8 30
• Spans up to 60
• Chords Fy 50 ksi, Web Fy 36 ksi or 50 ksi
• Longspan Joists - LH-Series
• Depths 18 48
• Spans up to 96
• Deep Longspan Joists - DLH-Series Joists
• Depths 52 72
• Spans up to 144
• Joist Designations
• Example 18K5
• 18 Depth, K K-Series, 5 Chord Size

5
Steel Joists - Types

• Open web steel joists - KCS-Series Joists
• Depths 8 30
• Constant shear and moment capacity throughout
• Joist substitutes
• 2-1/2 total depth
• Used for short spans such as hallways and skewed
  bays
• Joist Girders
• Long span primary members
• Support equally spaced concentrated loads at
  panel points
• Standard depths 20 -72 (96 for some mfrs)
• Spans to 60 (100 for some mfrs)
• Designation such as 48G8N8.8K
• 48 depth, G Girder, 8 of joist spaces,
  8.8 load at each panel point

6
Steel Joists

• Joist seats Depth varies depending on joist
  type
• K-Series 2-1/2 deep standard, 5 deep optional
• Beams supporting joists are typically 2-1/2 to
  5 below the top of joists, depending on joist
  seat depth
• LH and DLH-Series, 5 deep
• Joist girders 6 or 7-1/2 deep
• Joist extensions cantilevered ends
• K-Series
• S Type Simple, R Type Reinforced
• Allowable loads tabulated in catalog
• Ceiling extensions for light loads
• LH and DLH-Series
• Show length of extension and load on drawings

7
Design and Installation

• Minimum joist depth span / 24
• Minimum end bearing
• 2-1/2 on steel
• 4 on bearing plates on concrete or masonry
• End anchorage welded or bolted to support
• Joist bridging
• Top and bottom chord bridging, either horizontal
  or diagonal, is required
• Type of bridging and number of rows required is
  dependent on joist span and chord size
• See Tables on p. XI and p. 19
• Much more information in catalog

8
Joist Design (Selection)

• Design Assumptions K, LH, DLH
• Simply supported
• Top chord braced
• Uniformly loaded
• Maximum slope ½ per foot
• Parallel chords
• If these conditions dont apply
• Special Joists
• Show profile and load diagram on drawings
• KCS Joists
• Specified to support uniform, non-uniform and
  concentrated loads
• Have a constant shear and moment capacity
  throughout length

9
Joist Design (Selection)

• Standard Load Tables
• For a given joist designation and span, table
  lists
• Total factored load capacity (based on strength)
• Service load that produces a deflection of ? l
  / 240 (Live Load)
• Multiply load in table by 360/240 1.5 to get
  load that produces a deflection of ? l / 360
  (Live Load)
• Economy Load Tables Appendix C
• Joists are arranged by weight (economy)
• Starting from left, first joist to satisfy both
  load criteria is the most economical
• Bridging
• See Tables on p. XI and p. 19 for required
  bridging