CTC 422 Design of Steel Structures Steel Roof

CTC 422 Design of Steel Structures Steel Roof and Floor Deck

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

Roof Deck • Material • Cold-formed steel with minimum yield strength, Fy = 33 ksi Usually galvanized • 1 -1/2 inch and 3 inch depths • • Profiles – See p. 7 of USD catalog • 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

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

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

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

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

Floor Deck • Form deck • Deck acts only as a form for the concrete Reinforcement is required for bending • Deck acts as a form and the deck is the positive reinforcement • • Composite deck • 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

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

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

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

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