CTC 422Design 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

• 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

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

• 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

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