CTC 422 Design of Steel Structures

1. CTC 422Design of Steel Structures Steel Roof and Floor Deck

2. Steel Deck • Why start with steel deck? • Beams and joists cannot be designed until their spacing is determined • Spacing is dependent on deck • Advantages of steel deck • Installation is relatively quick and not overly dependent on weather • Provides a working platform • Braces top flanges of beams and top chords of joists • Acts as a horizontal diaphragm • Student Objectives • Use manufacturer’s data to select and specify steel roof and floor deck

3. Roof Deck Material Cold-formed steel with minimum yield strength, Fy = 33 ksi Usually galvanized Profiles – See p. 7 of USD catalog 1-1/2 inch and 3 inch depths Also “deep deck” – 4-1/2 to 7-1/2 inches deep Thickness – 22 ga (.0295”) to 16 ga (.0598”) Deck types for 1-1/2” deck Type B (Wide rib), Type F (Intermediate rib) and Type A (Narrow rib) Also BA, etc (acoustical), BI, etc (interlocking) Most common deck – 1-1/2”, 20 ga, Type B Some designers use 22 ga. Provides highest load capacity per pound of steel Requires minimum 1 inch rigid roof insulation

4. Roof Deck Design Design in accordance with Steel Deck Institute (SDI) specifications and manufacturer’s catalogs LRFD Load Tables – p. 6 of USD Catalog First value in table gives Uniform Total Factored Load the deck can support based on stress Second value gives Service Load (Live load) that would produce a deflection of span / 240. Values are tabulated for a range of deck gages and spans for 1, 2 and 3 span conditions Assume a 3 span condition unless framing will not allow. Verify span condition on shop drawings

5. Roof Deck Design Roof Deck Data Base – p. 7 of USD Catalog Lists section properties, allowable end reactions and allowable shear forces for various deck types and gages Lists maximum allowable spans based on SDI criteria (construction load). Max. 1 span – Maximum single span Max. 2 span – Maximum multiple span Max. Cant – Maximum cantilever span Also lists maximum allowable span for Factory Mutual Class 1 construction, FMS Span Depends partly on owner’s insurance When in doubt assume this applies Maximum allowable spans in this table apply regardless of total load and live load applied to deck For economy, space framing as far apart as possible using “standard” deck

6. Roof Deck Installation Deck shipped in standard widths to (typically 36” ) in lengths up to 40 feet Deck is shipped with square ends Cutting for width, openings or angular fit is done in the field Deck is fastened to structure with welds or mechanical fasteners Minimum 5/8” diameter puddle weld Welding washers required for 22 ga. and thinner Typical spacing of attachment All side laps and 18” maximum on interior ribs Closer spacing may be required for deck use as a diaphragm to distribute lateral loads to structure Provide framing for all openings ≥ 12” square

7. Roof Deck Design Procedure Choose a trial deck section 1-1/2”, 20 ga., Type B ? Choose joist / beam spacing such that deck span ≤ maximum span from Roof Deck Data Base Maximum 1 span, or maximum 2 span as applicable FMS span as applicable Then, check load capacity in Load Table Maximum total factored load Service load that causes Δ = l / 240 (Service Live Load) If load capacity is inadequate Reduce span Change deck profile

8. Floor Deck Form deck Deck acts only as a form for the concrete Reinforcement is required for bending Composite deck Deck acts as a form and the deck is the positive reinforcement Positive moment => tension on bottom Material - cold-formed steel, minimum yield strength, Fy = 33 ksi May be galvanized or painted (paint on exposed side only) Profiles – See p. 22 – 41 of USD catalog 1-1/2 inch and 2 inch depths Thickness – 22 ga (.0295”) to 16 ga (.0598”) Profiles vary by manufacturer Most common deck – 1-1/2” or 2” deep, 20 ga. Depth chosen depends on span 3” depth sometimes used for longer spans

9. Floor Deck Design Design in accordance with Steel Deck Institute (SDI) specifications and manufacturer’s catalogs Fire rating of composite floor deck Floor deck / slab combination often required to serve as rated fire separation between floors. Type of concrete (NW or LW) and thickness of concrete often governed by required rating Most economical => no spray-on fireproofing on deck Fire rating tables – p. 47 “Concrete Cover” in table = thickness above top of deck NW = 145 pcf, LW = 115 pcf Reduction in live load capacity may be required Check U.L. Fire Resistance directory for details of construction

10. Floor Deck Design Deck properties and load tables – p. 22 – 41 in catalog Allowable span typically limited by Maximum Unshored Span Avoid shoring deck if possible => expensive Maximum unshored span listed for 1, 2, and 3 span conditions Assume 3 span condition unless framing will not allow Properties tables also list required area of WWF for temperature reinforcing Load Tables – Right-hand page Loads listed are allowable uniform live (and superimposed dead) service loads LRFD Design is used, but tables list service loads Loads shown superimposed loads Weight of deck and slab has already been accounted for. Tables for 1 shear stud per foot and for no studs

11. Floor Deck Installation Deck shipped in standard widths to (typically 36” ) in lengths up to 40 feet Deck is shipped with square ends Cutting for width, openings or angular fit is done in the field Joints at panel ends usually butted, not overlapped Deck is fastened to structure with welds or mechanical fasteners Minimum 5/8” diameter puddle weld Welding washers required for 22 ga. and thinner Typical spacing of attachment All edge ribs and 12” average spacing overall Closer spacing may be required for deck use as a diaphragm to distribute lateral loads to structure Shear connectors may be welded to supports through decking units Provide framing for all openings ≥ 12” square

12. Floor Deck Design Procedure Choose concrete type and thickness to provide required fire rating Choose a deck type and span which will allow unshored construction Then, check load capacity in Load Table In most cases, chosen slab and deck will be more than adequate for load For economy, space framing as far apart as possible Several combinations may have to be investigated to find most economical For cantilever spans, see pages 44 - 45 In direction of ribs, top (negative) reinforcing required Parallel to ribs, check pour stop