Precast Concrete on Steel Frames — Bearing, Connections, and Composite Action

Precast concrete planks (hollow-core and double-T) on steel beams are a common floor system for parking garages, commercial buildings, and industrial facilities. The steel frame provides the gravity and lateral system while precast planks span between beams to form the floor. Design requires coordination between the precast manufacturer's standard product capacities (PCI Design Handbook) and the steel frame design (AISC 360-22). Composite action between the topping slab and steel beams can be achieved with headed shear studs per AISC 360 Chapter I.

Precast plank types

Hollow-core planks

Extruded or slip-formed prestressed concrete planks with circular or oval voids to reduce weight. Standard depths: 6", 8", 10", 12", and 16". Typical spans: 20-45 ft depending on depth and loading. Weight: 40-100 psf (self-weight varies with depth and void pattern). Hollow-core is the most economical precast floor system for routine spans and loads.

Double-T (DT) planks

Prestressed flanged sections with two stems. Standard depths: 24", 28", 32", and 34" (total depth including flange). Widths: 8 ft, 10 ft, 12 ft, and 15 ft. Typical spans: 40-80 ft. Used for longer spans and heavier loads than hollow-core, especially in parking structures where the DT stems are exposed below.

Bearing on steel beams

Minimum bearing length

• Hollow-core on steel: 2" minimum, 3" preferred (per PCI MNL-120)
• Double-T on steel: 3" minimum, 4" preferred
• With bearing pad: Neoprene pad (60-70 durometer, 1/4"-3/8" thick) prevents stress concentrations and accommodates rotation

Bearing detail considerations

The bearing detail must accommodate: (1) plank manufacturing tolerances (+/- 1/4" length), (2) steel frame erection tolerances (+/- 1/4" beam position), (3) plank camber (up to L/300 for prestressed members), and (4) differential temperature and shrinkage movement. A total gap allowance of 1" is typical at each bearing end.

The steel beam top flange must be wide enough to support the plank bearing plus clearance for shear stud installation if composite action is desired. A minimum of bf = bearing + stud clearance + 2" overhang = typically 8-10" for composite beams.

Composite action with topping slab

A cast-in-place concrete topping (typically 2"-3" over the plank) can create composite action with the steel beams using headed shear studs welded to the beam top flange:

Qn = 0.5*Asa*sqrt(f'c*Ec) <= Rg*Rp*Asa*Fu    [AISC 360 Eq. I8-1]

For 3/4" studs in normal-weight concrete (f'c = 4 ksi): Qn = 21.1 kips per stud. The number of studs between maximum moment and zero moment is:

n = V'/(phi*Qn)    where V' = min(0.85*f'c*Ac, Fy*As)

Critical detail: Studs must project into the topping slab, not the precast plank. The topping thickness must exceed the stud height by at least 1/2" (e.g., 3" topping for 2.5" stud length after welding). Studs welded through hollow-core keyways are not effective because the hollow-core concrete is precast and cannot develop shear transfer to the stud.

Composite vs. non-composite comparison

For a typical W18x35 spanning 30 ft with a 3" composite topping:

Composite action typically saves 15-25% on steel beam weight, but adds the cost of shear studs ($2-4 per stud installed).

Connection design

Plank-to-beam connections

Precast planks are typically not structurally connected to the steel beams for gravity loads (they bear by friction on pads). However, connections are needed for:

• Diaphragm shear transfer: Welded connections or reinforcing bars grouted into the keyways and anchored to the beam transfer lateral forces. Pour strips at the beam line ensure continuity of the topping slab.
• Restraint of bearing: Clip angles or welded plates prevent the plank from sliding off the beam during seismic events.
• Tying requirements: IBC Section 1604.8.2 requires structural integrity ties connecting floor members to the structure. Minimum tie force: 1500 lb per lineal foot.

Beam-to-column connections

Standard gravity connections (shear tabs, double angles) are used at beam ends. The beam size is governed by the composite section capacity at midspan but the connections see only the beam end reaction. Ensure the beam web can support the concentrated plank reaction at the bearing location -- check web local yielding and crippling per AISC 360 Section J10.

Tolerances and coordination

The topping thickness must be specified as a minimum (e.g., "2" minimum over highest point of plank"). Camber variations mean the average topping will be thicker than the minimum, adding dead load. Account for 0.5"-1.0" additional topping in dead load estimates.

Practical tip: beam size selection for precast floors

Size steel beams to match precast plank depths so the plank bears on top of the beam, not on a ledge or haunch. For an 8" hollow-core floor, use W14 or W16 beams (beam top at the same elevation as the plank top). Avoid shallow beams where the plank would need to notch around the beam flange -- this is expensive and structurally problematic.

Common mistakes

1. Insufficient bearing length. After accounting for all tolerances, the actual bearing may be 1" less than detailed. Provide generous bearing (3-4") to absorb cumulative tolerances.
2. Studs in the wrong location. Shear studs for composite action must project into the cast-in-place topping, not into precast concrete. Precast concrete cannot develop the required shear transfer to headed studs.
3. Not accounting for camber in topping weight. Prestressed planks camber upward. The topping must fill the camber valleys, adding weight that must be included in the dead load calculation.
4. Missing diaphragm connections. Precast planks sitting on steel beams do not automatically form a diaphragm. Pour strips, grouted keyways, and mechanical connections are needed for lateral force transfer.
5. Ignoring temporary conditions. During erection, planks are placed on bare steel beams before the topping is poured. Check the bare beam for unbraced length (Lb = full span if no intermediate bracing) and construction live loads.

Run this calculation

• Beam Capacity Calculator
• Section Properties Database
• Bolted Connections Calculator

Related references

• Floor Systems
• Composite Beam Design
• Steel Deck Types
• Diaphragm Design
• How to Verify Calculations

Disclaimer

This page is for educational and reference use only. It does not constitute professional engineering advice. All design values must be verified against AISC 360-22 Chapter I, PCI Design Handbook, and the governing project specification. The site operator disclaims liability for any loss arising from the use of this information.