CTC 422 Design of Steel Structures

1. CTC 422Design of Steel Structures Steel Joists

2. Steel Joists • Student Objectives • Use manufacturer’s 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 manufacturer’s 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 mfr’s) Spans to 60’ (100’ for some mfr’s) 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 don’t 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 Δ = l / 240 (Live Load) Multiply load in table by 360/240 = 1.5 to get load that produces a deflection of Δ = 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