Planning For Openings And Services Through Hollowcore Floors

An essential part of the design of hollowcore floors involves planning openings for service pathways. Virtually all modern multi-storey buildings will require some measure of mechanical, electrical, plumbing and other services which need to penetrate the floor structure. The location, size and distribution of these openings all have a potential impact on the design and cost of the floor system, as well as the construction program (ie when to construct the openings). Naturally, all openings are not created equal, and will necessarily vary in size, shape, location, purpose and schedule, but they can all be defined as follows:

A slab opening can be defined as any reduction in cross sectional area along the span length of the member.

By this simple definition, there are 4 main types of openings usually encountered in hollowcore floors:

  • Full depth, full width openings
  • Full depth, partial width openings
  • Partial depth openings
  • Cored penetrations and weep holes

All types of openings have the potential to affect the slab layout, required thickness, strand pattern, cost and schedule. Therefore early consultation between the hollowcore producer and the design / construction team is important.

Full and Partial Depth Openings

These openings are typically large and accommodate the passage of mechanical, HVAC and other services and are executed before grouting and topping of the floor. Gasoline concrete saws and walk-behind ‘street’ saws are sometimes used to create partial width openings, but it is most common to create openings in the plant by removing the fresh concrete after the production machine has passed. Special steel headers are used to support the ends of slabs for full width openings1.

Any opening that extends through the full or partial depth of the slab compromises the flexural and shear capacity at that location and introduces load concentrations and torsional effects which are distributed to the remainder of the slab and adjacent members. This requires careful consultation between the design team, subcontractors, general contractor and the producer to confirm the ultimate opening location, slab design, layout, and fabrication and erection drawings. See Section 3.3 of the PCI Hollowcore Design Manual to learn more.

Depending on the size and location of the opening in the slab, it may be more convenient to field cut the opening on site rather than transport the slab with the opening already cut, as this increases the likelihood of the slab arriving damaged, or worse, a failure during lifting. Moreover, lifting clamps will not work if keyway edges are removed near or at the lifting points. Lifting anchors can be cast into the portion of slab to be removed to assist in shoring and hoisting while the installation crew field cuts the opening.

For openings greater than 50% of the slab width it is generally best practice to share the opening equally between adjacent slabs if the layout allows, to reduce the effect of the opening on the structural capacity of the individual members. Otherwise, full width openings supported by headers are usually considered the only option.

Cored Penetrations and Weep Holes

These openings are typically minor and allow the passage of small pipes and conduits, or drainage of trapped water from the cores. They are generally drilled or cored on site and range in size from ½” up to 4” or even 6”. Weep holes can be drilled at the plant during production using a special jig on the production bed as field-drilling from below can be tedious.

Coring is typically carried out by subcontractors such as plumbers, electricians and others using a wet-coring machine. Cored openings must be located to avoid cutting the strands and weakening the slab. It is therefore imperative that there is guidance from the slab producer and the general contractor in locating these holes before coring work starts.

Coring and drilling can take place before or after topping of the floor.

A Special Note About Openings and Firestopping

Q: What happens after openings are executed and services are run?

A: Besides topping, depending on the class of building, special attention usually needs to be paid to firestopping details at openings after the services have been installed to eliminate the possibility of fire spreading through the building. These details are required to meet building codes for fire safety and are the design responsibility of the Architect, but are ultimately executed by the General Contractor.

In general, any remaining clear space around pipes and conduits is typically sealed between floor levels – cast with concrete and then special intumescent foams, caulks, sleeves or collars may be applied around the pipes and conduits depending on their material specification to ‘seal out’ any flames that could otherwise breach the floor.

For more information on this please see the International Building Code Chapter 7
  • See Section 5.7 of PCI Hollowcore Design Manual for Example Steel Header Details