Hollowcore - anchors for services restraints and bracing

Hollowcore floor systems: structural characteristics relevant to anchorage

Hollowcore slabs are precast, prestressed concrete elements manufactured by either extrusion or slip-forming. The process creates a series of longitudinal voids running the full length of the unit, separated by webs of concrete. The top and bottom flanges are thin relative to the overall depth, typically in the range of 30 to 50 mm, while the webs carry the majority of shear load between supports.

This geometry creates specific constraints for anchor installation. The flange sections offer limited concrete volume for anchor embedment, and the voids limit the available base material directly below a drilling location. Prestressing strands run longitudinally through the lower portion of the unit, and drilling that intersects a strand can compromise the structural integrity of the element in ways that are difficult to inspect or remedy after the fact.

Concrete compressive strength in hollowcore units is typically higher than in cast-in-place slabs, often in the range of 40 to 65 MPa, which is favourable for anchor capacity. However, the thin sections and void geometry mean that the failure modes governing anchor performance in hollowcore differ substantially from those in solid concrete, and strength alone cannot be used to extrapolate published solid-concrete data to hollowcore applications.

How anchors fail in hollowcore under seismic loading

In solid concrete, the dominant anchor failure modes under tension are concrete cone breakout, splitting, and pullout. In hollowcore, these modes are modified by the section geometry in ways that reduce capacity relative to what the same anchor would achieve in an equivalent solid section.

Flange breakout occurs when the flange section is too thin to develop a full cone failure. The available concrete above the anchor is limited, and breakout can occur at lower loads than the standard cone breakout formula would predict. Web splitting is governed by the limited width of the web between adjacent voids. An anchor installed in the web may induce splitting perpendicular to the web axis under tensile or shear load.

Pull-through can occur in thin-flanged sections where the anchor bearing surface is close to or at the bottom of the available concrete. Cyclic seismic loading introduces an additional factor: each load cycle can progressively damage the concrete around the anchor hole, reducing residual capacity below the value measured in monotonic pull-out testing. This is particularly relevant for services restraint applications, where the connection may be subjected to repeated low-level ground motion events over the building's life.

These characteristics mean that static test results from solid concrete cannot be applied to hollowcore, even with a conservative reduction factor applied informally.

ICC evaluation criteria and testing-based design

ICC-ES (International Code Council Evaluation Service) publishes acceptance criteria for post-installed anchors. AC01 covers expansion anchors and AC308 covers post-installed adhesive anchors. Both criteria include testing protocols specific to hollowcore substrates, requiring anchors to be tested in actual hollowcore units rather than in representative concrete blocks.

An ICC-ES Evaluation Report (ESR) for a product tested in hollowcore will specify the product, the installation method, the hollowcore units in which testing was conducted, and the characteristic capacities measured in tension and shear. This information allows the designer to select anchor capacities that are directly supported by test data for the actual substrate condition.

The key point for designers is that an ESR based on solid concrete testing does not cover hollowcore, even if the same product is being used. To rely on published capacity values, the ESR must include hollowcore-specific test data. Where no hollowcore ESR exists for a product, the designer must either use a product for which such data is available or commission project-specific testing.

Reading the capacity tables in an ESR requires attention to the conditions under which values apply: embedment depth, edge distance, spacing, and whether the tested hollowcore unit matches the geometry being used on the project.

Applying seismic demands from AS 1170.4 to services restraints

AS 1170.4 defines the seismic design demands on non-structural components through the part acceleration coefficient Cph and the component ductility factor. The lateral seismic force on a non-structural component is calculated as a function of the site hazard factor, the part response factor, and the height of the component within the building. Services restraints for mechanical and electrical equipment attached to hollowcore floors are non-structural components for the purposes of this calculation.

The part response factor accounts for the amplification of ground motion through the structure. Components at higher levels in a building experience greater acceleration demands than components at ground level, which AS 1170.4 captures through the height interpolation in the part acceleration coefficient expression.

Once the design force on the restrained component is determined, that force is distributed to the anchor connections as a tension or shear demand depending on the restraint geometry. This demand is then compared against the anchor capacities from the product ESR, applying the appropriate capacity reduction factors from AS 3600 or the ESR itself.

Documentation for an engineered services restraint installation should include the design force calculation, the selected anchor product and its ESR reference, the installation specification (embedment depth, edge distance, torque), and confirmation that the installation location falls within the scope of the tested hollowcore geometry. A proprietary restraint system may be appropriate where standard geometry fits the application, but a custom-engineered solution is required where load demands or geometric constraints fall outside the system's validated range.