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

2. Steel Deck Why start with steel deck? Advantages of 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 Profiles – See p. 7 of USD catalog
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 Composite 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
In direction of ribs, top (negative) reinforcing required
Parallel to ribs, check pour stop