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.

Hollow-core span and capacity table

The following table provides approximate capacities for standard hollow-core plank sections based on a 4 ft width, normal-weight concrete (f'c = 5 ksi prestressed), and 40 psf superimposed live load. Values are for preliminary sizing only and must be confirmed with the precast manufacturer.

Notes: Service load capacity includes superimposed dead + live load (excludes plank self-weight and topping). Values assume a 2" composite topping. N/A entries indicate the depth cannot span 30 ft; use a deeper section. All values are approximate and vary by manufacturer.

Double-T span and capacity table

Double-T planks provide longer spans and higher capacities than hollow-core at the cost of greater depth and weight. The table below shows typical capacities for prestressed double-T sections in parking structure and commercial applications.

Notes: Weights include 2" flange thickness. Max spans are for untopped DTs under 40 psf superimposed live load with simple supports. Topped systems will have shorter maximum spans due to topping dead load but higher service load capacity. Confirm all values with the precast manufacturer.

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.

Bearing pad selection table

Bearing pads distribute plank reactions uniformly and accommodate beam rotation and plank camber. Selecting the correct pad type is essential for long-term performance.

Notes: Bearing stress is computed as plank reaction divided by the pad contact area. For hollow-core planks, the bearing width is the full plank width; for double-T, the bearing width is the stem width (typically 4"-6" per stem). Laminated pads (internal steel shims bonded to rubber) provide higher capacity and resist pad "walking" under repeated loading. All pads should extend at least 1" beyond the plank edge in the bearing direction.

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-comparison 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).

Composite beam comparison table

The table below demonstrates the advantages of composite design using specific beam comparisons. All beams span 30 ft with 3" normal-weight concrete topping (f'c = 4 ksi) on metal deck or precast planks. Studs are 3/4" diameter.

Notes: W16x26 composite achieves comparable capacity to W18x35 non-composite (300 vs 249 kip-ft) while saving 26% on steel weight. The effective moment of inertia (I_eff) for composite sections is computed per AISC 360 Chapter I using the transformed section method. LL deflection assumes 100 psf live load. Stud cost estimated at $3.00 per stud installed. The net cost impact accounts for the difference in steel cost versus stud installation cost.

Shear stud capacity table

Headed shear stud capacities vary by diameter and concrete strength. The table below gives nominal stud shear strengths (Qn) per AISC 360 Eq. I8-1 for studs in solid normal-weight concrete slabs (no deck ribs). Minimum stud height after welding is measured from the top of the beam flange.

Notes: Stud count for W18 assumes a W18x35 beam (Fy = 50 ksi) with full composite action and a 3" topping slab. Studs in deck ribs have reduced capacity per AISC 360 Table I3-3 (reduction factors Rg and Rp). The 3/4" stud is the most commonly used diameter in precast composite construction. Studs must extend at least 1/2" above the top of the deck into the concrete topping. Minimum stud spacing is 6x stud diameter longitudinally and 4x stud diameter transversely per AISC 360 Section I8.2a.

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.

Diaphragm connection details table

Diaphragm connections transfer lateral forces from the floor diaphragm into the steel frame. The following table summarizes common connection types used in precast-steel composite floor systems.

Notes: Force capacities are typical values for 4 ksi concrete topping. Pour strip width varies with beam flange width; minimum 10" to allow concrete placement and consolidation. Grouted keyway reinforcement must be coordinated with the precast manufacturer to ensure keyway dimensions allow bar placement. Weld plate and clip angle connections are preferred for high-seismic regions (SDC C and above) where positive connection is required.

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.

Worked example: office building with 12" hollow-core

Given: 12" hollow-core planks spanning 28 ft between W18x40 steel beams. Beams span 32 ft and are spaced 6 ft on center. A 2" normal-weight concrete topping (f'c = 4 ksi) is placed over the planks. Shear studs are 3/4" diameter. Live load = 80 psf (office).

Step 1 — Plank capacity check: From the hollow-core table, a 12" plank at 28 ft span carries approximately 120 psf superimposed load. Plank self-weight = 78 psf, topping = 24 psf, total dead = 102 psf. Remaining capacity for live load = 120 - 24 = 96 psf > 80 psf required. OK.

Step 2 — Beam loading: Tributary width = 6 ft. Dead load = (78 + 24) x 6 = 612 plf. Live load = 80 x 6 = 480 plf. Factored load = 1.2(612) + 1.6(480) = 1502 plf.

Step 3 — Non-composite beam capacity: W18x40: phiMn = 294 kip-ft. Mu = wL^2/8 = 1.502(32)^2/8 = 192 kip-ft < 294 kip-ft. OK without composite action. However, deflection will control.

Step 4 — Deflection check (non-composite): I_x = 612 in^4. Delta_LL = 5wL^4/(384EI) = 5(480/12)(384)^4/(384(29000)(612)) = 1.05 in = L/366. Marginal for L/360 limit. Composite action recommended.

Step 5 — Composite design: With 3/4" studs (Qn = 21.1 kips/stud), full composite requires V' = min(0.85 x 4 x 72, 50 x 11.8) = min(245, 590) = 245 kips. Studs per side = 245/(0.9 x 21.1) = 13. Total studs = 26. Effective width = min(32/4, 6 x 12) = 96 in. Transformed I_eff approx 1,600 in^4. Delta_LL = 5(480/12)(384)^4/(384(29000)(1600)) = 0.40 in = L/960. Excellent.

Worked example: parking garage with 10" hollow-core

Given: A two-level parking garage floor using 10" hollow-core planks on W16x36 steel beams. Plank span = 8 ft (planks span between beams). Beam span = 30 ft. A 2" composite topping (f'c = 4 ksi) is placed over the planks with 3/4" headed shear studs. Live load = 50 psf (parking, per ASCE 7 Table 4.3-1).

Step 1 — Plank capacity check: From the hollow-core table, a 10" plank at 8 ft span easily supports the applied loads. The short plank span (8 ft between beams) is well within the 32 ft maximum simple span. 10" plank self-weight = 65 psf, topping = 24 psf, total dead = 89 psf. Live load = 50 psf. Total = 139 psf. The 10" plank can carry approximately 75 psf superimposed load at 30 ft span, so at only 8 ft the capacity far exceeds demand. OK.

Step 2 — Beam loading: The planks span 8 ft onto the W16x36 beams. Tributary width per beam = 8 ft (planks bear on both sides, so each beam carries 4 ft from each side = 8 ft). Dead load = (65 + 24) x 8 = 712 plf. Live load = 50 x 8 = 400 plf. Factored load = 1.2(712) + 1.6(400) = 1,494 plf.

Step 3 — Factored moment: Mu = wL^2/8 = 1.494(30)^2/8 = 168 kip-ft.

Step 4 — Composite beam capacity: W16x36: As = 10.6 in^2, Fy = 50 ksi, d = 15.9 in, I_x = 448 in^4. Effective width beff = min(L/4, spacing) = min(30/4 x 12, 8 x 12) = min(90, 96) = 90 in. Concrete area in effective width: Ac = 90 x 2 = 180 in^2. V' = min(0.85 x 4 x 180, 50 x 10.6) = min(612, 530) = 530 kips. Composite phiMn (full composite, a = V'/(0.85 x f'c x beff) = 530/(0.85 x 4 x 90) = 1.73 in): phiMn = phi x [As x Fy x (d/2 + t - a/2)] = 0.90 x [10.6 x 50 x (15.9/2 + 2 - 1.73/2)] = 0.90 x [530 x (7.95 + 2 - 0.87)] = 0.90 x [530 x 9.08] = 0.90 x 4,812 = 4,331 kip-in = 361 kip-ft. phiMn = 361 kip-ft > Mu = 168 kip-ft. OK by a wide margin. Beam has significant reserve capacity.

Step 5 — Stud count: Qn = 21.1 kips per 3/4" stud in 4 ksi concrete. Studs required (each side of max moment): n = V'/(phi x Qn) = 530/(0.9 x 21.1) = 28 studs per side. Total studs = 56 studs for full composite action. Consider 50% composite (adequate for this loading): n = 28 studs total. phiMn(50%) approx 280 kip-ft > 168 kip-ft. OK. Use 28 studs total (14 per half-span) for 50% composite action. Stud spacing = 30 x 12 / (2 x 14) = 12.9 in spacing.

Step 6 — Bearing check: Each 10" plank (4 ft wide) reaction at the beam = (139 x 8/2 x 4) = 2,224 lb per plank end. Beam reaction (total) = 1,494 x 30/2 = 22,410 lb. Plank bearing on beam: Bearing length = 3" (with neoprene pad). Bearing stress = 2,224 / (3 x 48) = 15.4 psi. Very low. OK. Beam web local yielding: Per AISC 360 J10.2, check concentrated force at plank bearing location. The plank reactions are distributed along the beam, so the maximum local force is small. No web yielding or crippling concern.

Step 7 — Deflection check: LL deflection with composite I_eff (approximately 1,200 in^4 for 50% composite): Delta_LL = 5 x (400/12) x (360)^4 / (384 x 29,000 x 1,200) = 5 x 33.3 x 1.68 x 10^10 / (1.337 x 10^10) = 0.63 in = L/571. This satisfies L/360 for live load deflection. OK.

Summary: The W16x36 beam with 50% composite action (28 studs total) easily supports the parking garage loading. The 10" hollow-core planks at 8 ft span have substantial reserve capacity. Bearing stresses are low and well within pad and steel limits.

Multi-code composite design comparison

Composite beam design principles are consistent across major international standards, but specific formulas, factors, and limits vary. The table below compares key parameters for designing headed stud shear connections in precast composite floor systems under four major design codes.

Notes: d = stud diameter, Asa = stud cross-sectional area, fu = stud tensile strength, f'c = concrete compressive strength, Ec = concrete modulus of elasticity. The AISC and CSA approaches are very similar (both derived from the same underlying research). EN 1994 uses a partial factor approach (gamma factors) rather than phi factors. Australian standard AS 2327 uses similar principles but with locally calibrated coefficients. For precast composite floors specifically, all codes require that studs project into the cast-in-place topping and that the topping concrete is consolidated around the studs.

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.
6. Underestimating stud count. Using the bare beam capacity to reduce studs without checking the reduced phiMn for partial composite action. The actual composite phiMn must exceed the factored demand at the partial composite level selected.
7. Bearing pad creep. Neoprene pads creep under sustained load. Use the appropriate instantaneous and long-term compressive strains from the pad manufacturer to ensure bearing length is maintained over the structure's service life.

FAQ

What is precast composite construction?

Precast composite construction combines precast concrete floor planks (hollow-core or double-T) with a structural steel frame and a cast-in-place concrete topping to create a unified floor system. The steel beams carry gravity loads, the precast planks span between beams and form the floor surface, and the topping slab provides a level surface, develops composite action with the steel beams through shear studs, and creates a diaphragm for lateral force resistance. This system is widely used in parking garages, office buildings, and industrial facilities because it combines the speed of precast erection with the strength and ductility of steel framing.

How do shear studs work with precast planks?

Shear studs (headed steel studs welded to the top flange of steel beams) transfer horizontal shear between the concrete topping slab and the steel beam. In precast composite systems, the studs must project through gaps in the precast planks (at keyways or pour strips) and extend into the cast-in-place topping concrete. The studs do not connect to the precast planks themselves. When the topping concrete cures and bonds to the studs, the steel beam and concrete topping act as a single composite section with significantly greater moment capacity and stiffness than the steel beam alone. This composite action reduces beam deflections and can allow smaller, lighter steel beams.

What bearing length do I need for hollow-core planks on steel beams?

PCI MNL-120 recommends a minimum 2" bearing length for hollow-core planks on steel, with 3" preferred. However, after accounting for steel erection tolerances (+/- 1/4" beam position), precast manufacturing tolerances (+/- 1/4" plank length), and construction adjustments, the actual bearing can be 1/2" to 1" less than detailed. For this reason, detail 3" minimum bearing with a neoprene bearing pad, and ensure the beam top flange is wide enough to provide the full bearing length plus clearance for shear studs and plank end gaps. Double-T stems require 3" minimum, 4" preferred bearing.

How do I account for camber in precast planks?

Prestressed precast planks camber upward due to the eccentric prestressing force. Camber can reach L/300 (e.g., 1" for a 25 ft span). Camber affects design in three ways: (1) the topping slab will be thicker in the camber valley near the supports and thinner at midspan, so dead load estimates should include 0.5"-1.0" of additional topping thickness; (2) the finished floor elevation at midspan will be higher than the support elevation by the camber amount; and (3) differential camber between adjacent planks creates unevenness in the topping, requiring the specifier to call out a minimum topping thickness (e.g., "2" minimum over the highest point of the plank"). The precast manufacturer typically provides predicted camber values at erection and at 60-day intervals.

What is a pour strip and when is it needed?

A pour strip is a gap (typically 10"-14" wide) left between adjacent precast planks along the beam line that is filled with cast-in-place concrete after the planks are erected. Pour strips serve three purposes: (1) they provide access for welding shear studs to the beam top flange and consolidating concrete around the studs; (2) they allow the precast planks to undergo initial shrinkage and creep before the strip is poured, reducing restraint cracking in the topping; and (3) they create a continuous concrete diaphragm at the beam line by connecting the topping on both sides of the beam. Pour strips are essential when composite action or diaphragm continuity is required and should be detailed in the structural drawings with reinforcing steel continuity requirements.

When should I use hollow-core versus double-T planks?

Use hollow-core planks for spans up to approximately 35-40 ft with moderate loads (office, residential, light commercial). Hollow-core is more economical per square foot, lighter (reducing beam and column loads), and provides a flat soffit for direct ceiling application. Use double-T planks for spans over 40 ft, heavy loads (parking garages, industrial), or when the structure benefits from the deeper section for mechanical plenum space. Double-Ts can span up to 80 ft, but they create a ribbed soffit that may require a suspended ceiling for finished spaces. In parking garages, the exposed double-T stems are often architecturally acceptable and eliminate the need for fire-rated ceilings.

Can I achieve composite action without a concrete topping?

No. True composite action between the steel beam and the precast planks alone is generally not achievable because the precast concrete cannot develop the horizontal shear transfer mechanism required by AISC 360 Chapter I. The precast plank surface is smooth (no deck ribs), and grouted keyways between planks do not provide a reliable shear transfer path to the steel beam flange. A cast-in-place topping slab with headed shear studs is the standard method. Some proprietary systems use special connectors embedded in the precast planks, but these require manufacturer-specific design data and are not covered by the general AISC provisions.

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Related references

• Composite Beam Design
• Floor Systems
• Steel Deck Types
• Diaphragm Design
• Beam Design Guide
• Column Base Plate Design
• Beam Formulas
• Deflection Control
• 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.

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