Metal Decking and Composite Floor Systems

Introduction to Metal Decking

Metal decking (or steel deck) is a structural element made of corrugated steel sheet used as permanent formwork, roof diaphragm, or floor reinforcement. In steel-framed buildings, it provides the immediate working surface for other trades and contributes to the lateral stability of the structure.

Roles of Decking

Decking serves three distinct functions depending on the application:

6.Permanent formwork: It supports the weight of fresh concrete during curing, without shoring (or with temporary shoring depending on the span).

7.Tensile reinforcement: In a composite system, the deck ribs act as positive reinforcement for the cast-in-place concrete.

8.Structural diaphragm: It transmits lateral loads (wind, seismic) to the vertical bracing systems, provided the fastenings are properly designed and installed.

Standard Profiles

The most common profiles in Canada are designated by their rib height in inches:

Profile	Height (mm)	Coverage Width (mm)	Typical Use
B-36	38	914	Roof, light floor
B-42	38	914	Composite floor
P-2436	76	914	Heavy floor, long spans
P-3615	76	610	Very heavy floor, concentrated loads
N-75	75	914	Roof, long span

The designation "B" indicates a 38 mm (1.5 in) profile, "P" a 76 mm (3 in) profile. The last two digits indicate the coverage width in inches.

Mechanical Properties of Deck Steel

Steel Grades

Decking is manufactured according to CSA G40.20/G40.21 (Structural Quality Steel) or ASTM A653 (Galvanized Steel Sheet). Common grades:

Structural quality steel: yield strength from 230 MPa (33 ksi) to 550 MPa (80 ksi).

High-yield steel: 345 MPa (50 ksi) for P profiles, 550 MPa for B profiles.

Protective Coatings

Decking is typically hot-dip galvanized (Z275 or Z350 depending on zinc thickness in g/m²). The coating protects against corrosion throughout the building's service life. For highly corrosive environments, a thicker coating or a paint system may be specified.

Nominal Thicknesses

Sheet thickness ranges from 0.76 mm (22 ga) to 1.52 mm (16 ga). The minimum thickness for floor decking is generally 0.91 mm (20 ga). Thickness calculations must account for the rolling tolerance (typically ±10%) and the thickness loss due to galvanizing (approximately 0.04 mm per face for Z275).

Deck Installation

Site Preparation

Before installation, verify:

25.Beam alignment: supports must be level, with a maximum deviation of 6 mm over 3 m.

26.Surface cleanliness: top flanges of beams must be free of debris, ice, or snow.

27.Shop drawing availability: layout drawings indicate sheet placement, overlaps, and fastener quantities.

Sheet Placement

Sheets are typically installed perpendicular to the beams for simple spans. Sheets are placed from left to right, with each sheet interlocking into the adjacent rib via a side lap.

Placement procedure:

31.Lifting: use wide-belt slings or a magnetic lifting system to avoid deforming the sheets. Decking must never be lifted by the ribs.

32.Alignment: the first sheet is aligned with the beam support axis. A maximum deviation of 12 mm is tolerated over the total width.

33.Interlocking: each successive sheet is slid into the rib of the previous sheet. The interlock must be complete over the entire length.

34.Temporary fastening: immediately fasten the sheet with screws or pins to prevent movement from wind or vibration.

35.Permanent fastening: after alignment verification, install all fasteners according to the plan.

Fasteners

Deck fasteners are of three types:

Type	Use	Advantages	Disadvantages
Welded studs	Composite floors	High shear resistance	Requires a welding gun
Self-drilling screws	Roofs, non-composite floors	Fast, no special equipment	Less shear resistance
Crimping pins	Roofs, light loads	Economical	Not suitable for high loads

Fastener spacing:

Support beams: 300 mm (12 in) on center, at every rib or every valley.

Intermediate beams: 600 mm (24 in) on center.

Edge supports: 150 mm (6 in) on center.

Overlaps

Longitudinal overlaps (between sheet ends) must be at least 50 mm on supports. Transverse overlaps (side laps) are fastened at maximum intervals of 300 mm.

Composite Floor Systems

Composite Principle

A composite floor combines metal decking, cast-in-place concrete, and steel beams to form a single structural system. The bond between concrete and steel is provided by headed studs welded through the decking onto the top flange of the beam.

Headed Studs

Studs are high-strength steel shear connectors, conforming to CSA W59 (Welded Steel Construction). Common dimensions:

Diameter: 19 mm (3/4 in) or 22 mm (7/8 in)

Length: 75 to 150 mm (3 to 6 in)

Tensile strength: 415 MPa minimum

Installation requirements:

54.The stud must penetrate the decking and be welded directly to the beam flange.

55.The stud length after welding must extend at least 38 mm above the deck rib.

56.The hole diameter in the decking must be 6 mm larger than the stud diameter.

57.The stud must be positioned at the center of the rib, with a minimum clearance of 25 mm from the rib edges.

Shear Resistance Calculation

The shear resistance of a stud is calculated according to the Canadian Highway Bridge Design Code (CSA S6) or the Canadian Steel Design Code (CSA S16). The basic formula:

qᵣ = 0.5 × Aₛ × √(f'c × E꜀) ≤ Aₛ × Fᵤ

Where:

Aₛ = cross-sectional area of the stud (mm²)

f'c = concrete strength at 28 days (MPa)

E꜀ = modulus of elasticity of concrete (MPa)

Fᵤ = ultimate strength of the stud (MPa)

Effective Slab Width

The effective width of the composite slab is limited to:

1/8 of the beam span on each side of the beam axis

Half the distance between adjacent beams, on each side

The distance to the free edge of the slab

The minimum value of these three criteria is used.

Concrete for Composite Floors

Strength and Composition

Concrete for composite floors must have a minimum strength of 25 MPa at 28 days, in accordance with CSA A23.1 (Concrete: Constituents and Execution of Work). Common requirements:

Parameter	Typical Value
Minimum strength (f'c)	25 MPa
Slump	75 to 100 mm
Aggregates	20 mm maximum
Air content	5 to 8% (freeze-thaw exposure)
Water-cement ratio	0.45 maximum

Slab Thickness

The minimum slab thickness above the ribs is 50 mm for interior floors and 65 mm for floors exposed to weather. Total thickness (including rib height) ranges from 100 to 200 mm depending on loads.

Concrete Placement

Pre-pour precautions:

80.Verify that the decking is clean and free of debris, oil, or ice.

81.Wet the decking if the temperature exceeds 30 °C to prevent rapid evaporation of concrete water.

82.Verify that temporary shoring is in place if the span exceeds the unshored capacity of the decking.

83.Install safety barriers around openings and slab edges.

During placement:

Deposit concrete in regular piles, never at a single central point.

Use a vibrating screed or needle vibrator to ensure proper filling of the ribs.

Avoid walking on freshly placed ribs.

Check slump at every delivery.

After placement:

Apply a curing compound or protective membrane within 2 hours of placement.

Protect the surface from freezing if the temperature drops below 5 °C.

Wait a minimum of 7 days before removing shoring (or according to strength tests).

Floor and Roof Diaphragm

Diaphragm Function

Decking acts as a rigid diaphragm that transmits lateral loads (wind, seismic) to shear walls or bracing. Diaphragm capacity depends on:

96.Sheet thickness and rib geometry

97.Spacing and type of fasteners on supports

98.Sheet continuity (properly fastened overlaps)

99.Connection to bracing elements (angles, plates, clips)

Diaphragm Design Rules

According to the National Building Code of Canada (NBCC), metal deck diaphragms must be designed according to structural engineering principles. Checks include:

Shear resistance of the decking

Fastener resistance (shear and pull-out)

Free edge stability (stiffeners required if the free length exceeds 50 times the thickness)

Openings in the diaphragm (reinforcement required for openings larger than 300 mm)

Diaphragm Fasteners

Diaphragm fasteners must be capable of transmitting the calculated shear forces. Fastener resistance values are provided in manufacturer's tables and verified by tests conforming to CSA S136 (Cold-Formed Steel Structural Members).

Practical Calculations for Installation

Deck Quantity Calculation

Number of sheets = (Building width) ÷ (Coverage width)

Example: 18 m wide building, B-36 profile (914 mm coverage width):

18 m ÷ 0.914 m = 19.7 → 20 sheets per row

Number of rows = (Building length) ÷ (Sheet length)

Example: 60 m long building, 6 m sheets:

60 m ÷ 6 m = 10 rows

Total = 20 × 10 = 200 sheets

Add 5 to 10% for waste due to cutting and overlaps.

Stud Calculation

Number of studs = (Effective slab width) × (Beam length) ÷ (Stud spacing)

The maximum stud spacing is 600 mm along the beam. The minimum spacing is 4 diameters in the longitudinal direction and 2 diameters in the transverse direction.

Deck Deflection Check

The maximum deck deflection under the weight of fresh concrete is limited to L/180 or 20 mm, whichever is more restrictive. For spans greater than 3 m, temporary shoring is generally required.

Applicable Codes and Standards

Canadian Reference Standards

Standard	Title	Application
CSA S16	Design of Steel Structures	Design of beams, studs, connections
CSA S136	Cold-Formed Steel Structural Members	Deck design
CSA W59	Welded Steel Construction (Arc Welding)	Stud welding
CSA A23.1	Concrete: Constituents and Execution of Work	Composite floor concrete
CSA G40.20/G40.21	Structural Quality Steel	Deck steel grades
NBCC	National Building Code of Canada	Loads, general requirements
CSA S6	Canadian Highway Bridge Design Code	Stud design (reference)

Safety Requirements per NBCC

Fall protection: guardrails mandatory around openings and slab edges, in accordance with occupational health and safety regulations.

Shoring: shoring must be designed to support the weight of fresh concrete plus a surcharge of 2.4 kPa.

Access: ladders or stairs must be installed before pouring begins.

Quality Control and Inspection

Pre-Pour Inspections

132.Fastener verification: the number and spacing of fasteners must match the plans.

133.Stud verification: studs must be straight, properly welded, without cracks or porosity.

134.Overlap verification: overlaps must be complete and fastened.

135.Cleanliness verification: no debris accumulation in the ribs.

Stud Weld Testing

Welded studs are verified by:

Bend test: one stud out of 100 is bent 30° from its axis. If it detaches, the weld is defective.

Visual test: the weld must show a uniform circular fillet around the base of the stud.

Tensile test: one stud out of 500 is subjected to tension until failure.

Installation Tolerances

Parameter	Tolerance
Sheet alignment	± 12 mm over the width
Fastener spacing	± 25 mm
Stud position	± 12 mm from the beam axis
Stud height above rib	38 mm minimum
Slab thickness	± 6 mm

Pitfalls to Avoid

Common Exam Errors

145.Confusing B and P profiles: B profiles have a height of 38 mm, P profiles 76 mm. Always check the rib height in the questions.

146.Forgetting the rolling tolerance: the actual sheet thickness is less than the nominal thickness. Strength calculations must use the minimum thickness.

147.Neglecting the effective width: the effective width of the composite slab is limited by three criteria; the minimum value is used.

148.Confusing deflection limits: deflection under service load is limited to L/360 for floors, but deflection under fresh concrete weight is limited to L/180.

149.Forgetting shoring: for spans greater than 3 m, the decking alone is not sufficient to support the weight of fresh concrete.

150.Ignoring stud welding requirements: CSA W59 requires systematic bend tests and visual inspections.

151.Not checking fastener compatibility: self-drilling screws are not suitable for decking thicker than 1.2 mm; use welded studs or self-tapping screws.

152.Confusing units: deck dimensions are often given in inches (B-36 profile = 36 in coverage width). Convert correctly to millimeters.

On-Site Installation Pitfalls

Installing in the wrong direction: sheets must be placed so that the ribs are perpendicular to the beams, unless otherwise indicated on the plans.

Incomplete interlocking: partial interlocking reduces the shear resistance of the diaphragm.

Insufficient edge fasteners: free edges of the decking require closer fastener spacing.

Welding studs through overlaps: studs must never be welded through a sheet overlap; they must be positioned in the rib, away from overlaps.

Exam Tips

Response Strategy

160.Read the question carefully: identify the deck profile, thickness, span, and loads before starting calculations.

161.Check units: convert all dimensions to millimeters or meters before calculating.

162.Use the tables: exam questions often provide resistance tables; learn to read them correctly.

163.Check limits: after a calculation, always verify that the result respects regulatory limits (deflection, spacing, minimum thickness).

Formulas to Memorize

Stud shear resistance: qᵣ = 0.5 × Aₛ × √(f'c × E꜀)

Effective width: bₑ = min (L/8, distance between beams/2, distance to free edge)

Maximum deflection: Δ_max = L/180 (fresh concrete) or L/360 (service load)

Number of sheets: N = Width ÷ Coverage width

Summary

Metal decking and composite floor systems are essential components of steel-framed construction. Key points to remember:

171.Profiles: B (38 mm) for roofs and light floors, P (76 mm) for heavy floors.

172.Fasteners: welded studs for composite floors, screws or pins for roofs.

173.Studs: 19 or 22 mm diameter, extending 38 mm above the rib, welded per CSA W59.

174.Concrete: minimum strength 25 MPa, minimum thickness 50 mm above the ribs.

175.Diaphragm: decking transmits lateral loads; fasteners must be designed for shear.

176.Shoring: required for spans greater than 3 m under fresh concrete weight.

177.Quality control: stud bend tests (1 in 100), visual weld inspection, tolerance verification.

178.Standards: CSA S16, CSA S136, CSA W59, CSA A23.1, NBCC.

The candidate must master the basic calculations (quantities, deflections, stud resistance) and know the installation and quality control requirements to succeed on questions covering this topic.

Pitfalls to Avoid

Pitfall	Consequence	Solution
Confusing B and P profiles	Incorrect strength calculations	Check rib height (38 vs 76 mm)
Forgetting rolling tolerance	Overestimated strength	Use minimum thickness (nominal - 10%)
Neglecting effective width	Overestimated composite strength	Take the minimum value of the three criteria
Confusing L/180 and L/360	Excessive deflection under fresh concrete	L/180 for fresh concrete, L/360 for service loads
Ignoring stud testing	Connection failure	Bend test 1/100, tensile test 1/500
Insufficient edge fasteners	Diaphragm failure	150 mm spacing on edge supports
Welding studs on overlaps	Poor steel-concrete bond	Position studs away from overlaps
Forgetting inch/mm conversion	Calculation errors	1 in = 25.4 mm; check profile dimensions

Mastery of these concepts and knowledge of the applicable Canadian standards will allow the candidate to confidently approach Red Seal exam questions on metal decking and composite floor systems.

Metal Decking and Composite Floor Systems