Hollow-core floor performance in earthquake
New guidelines for design, assessment and retrofit
A Pre-cast Concrete Floors Overview Group (PCFOG) has been formed to look at structural issues for all types of pre-cast floors and to compile guidance information to help designers, territorial authorities and owners. The group comprises members of the New Zealand Society for Earthquake Engineering Society (SESOC) and the Department.
At its first meeting the group decided its first priority should be to develop guidelines to help designers and territorial authorities with the design, assessment and retrofit of hollow-core floors. An outline of a guideline document has been developed and work is proceeding on describing the mechanisms of floor performance and setting limits of acceptability on various aspects. A section on retrofit techniques is well under way. The group aims to have a draft document ready for wider review by early next year.
Concerns were raised when the University of Canterbury ran a full-scale simulated earthquake test on a hollow-core floor assembly in October 2001. Units were 300 mm deep, configured to span past an intermediate column. A Technical Advisory Group investigated and, in March 2004, revisions were made to design details for hollow-core floors in an amendment to NZS 3101. One three-dimensional test was subsequently made at the University of Canterbury for each of the two proposed design details. Results indicated that the new design details would tolerate earthquake-generated movements larger than those expected from overall building designs complying with NZS 4203: 1992 and AS/NZS 1170: 2004.
The Department investigated the implications of the original test and in March 2007 issued a Hollow-core Floor Overview Report summarising the outcome. Overall it was concluded that for some building/slab configurations, the performance of hollow-core floors may not meet safety requirements when subject to strong earthquake shaking, comparable to the design level required for new buildings (usually a one-in-500-year earthquake).
The Report recommended that some, mostly flexible, buildings with hollow-core floors should be checked for earthquake performance. (Stiff buildings are generally of less concern, but could be affected.) Territorial authorities were also advised to request a report on hollow-core floor performance when significant alterations were made to a building with this type of floor.
There is a need for a better understanding of the behaviour of hollow-core (and other pre-cast) floor assemblies in strong earthquake shaking. Complex interactions occur between the units and the surrounding structure. Guidance information on the assessment of existing floors and the design of new ones is also needed.
In developing the guidelines the group is focusing on the following issues, many of which apply to other types of pre-cast floor as well.
Potential failure mechanisms
1. Support details
- Loss of support due to insufficient allowance for construction tolerances, minimum bearing, likely beam elongation, relative rotation between beam and hollow-core unit and spalling of cover concrete
2. Bending moment effects
- Positive moment failure close to the face of the support, including dowel/prying action of the concrete plug cast into the ends of the units
- Negative moment failure at the end or close to the end of starter bars in the topping concrete or reinforcement in filled webs if these are present
3. Shear/torsion effects
- Torsional failure due to twist applied to hollow-core units caused by displacement of supporting or adjacent structure
- Web splitting due to vertical shear transfer or differential displacements
- Diagonal tension failure of unreinforced webs in negative moment zones
4. Diaphragm action and column tie-back
- Failure of the connection between the floor and the structure due to differential elongation, vertical displacements and shear transfer
- Loss of diaphragm capability due to failure/lack of ductility of topping reinforcement
- Insufficient tie-backs to columns to achieve stability
5. Cut-outs and on-site modifications
- Premature failure of the floor or units due to cut-outs or modifications made during manufacture or on site.
Vertical seismic loading
This loading, up or down, can be significant in relation to dead and live loads on the floor and needs to be considered when examining potential failure mechanisms.
Beam strength enhancement
The contribution of pre-stressed floor units and topping can substantially increase beam strength. This effect could change the frame failure mode from a ductile beam-sway mechanism to a non-ductile column-sway mechanism.
Depth of units
As the depth and span of units increase, there is an increasing departure from the integrity of monolithic floors. Behaviour or tests for units at one depth or configuration may not be the same for other depths, particularly for larger depths and different configurations.
Proposed retrofit solutions of existing floors have included the addition of vertical steel or fibre-reinforced plastic (FRP) rods to secure the bottom section of the hollow-core units and to add shear resistance. Incorporation of web reinforcement in new units, particularly those of 300 mm depth or greater, is being considered.
Many floor systems are supplied under a performance specification, requiring the supplier to carry out detailed design as a subcontractor to the main building contractor. Overall responsibility for the structural performance of the floor system in place should rest with the structural engineer for the building. However, it is most important that the building designer, the floor unit supplier and the main building contractor each has a clear understanding of the design parameters and factors that could influence the structural performance of the floor system - for earthquake and other effects, not just gravity effects. All parties must work together to define their respective tasks and responsibilities to avoid gaps and to achieve a satisfactory as-built result.
Until the guidelines are available, the Department recommends that for each project, all aspects of structural performance of pre-cast floors, and hollow-core floors in particular, are investigated on a 'first principles' basis, using the latest research findings.
Please note that the Department's Practice Advisory 5 Allow for movement - Precast hollow-core assemblies, issued in June 2005, highlights the need to consider the potential failure mechanisms and the structure displacements that could initiate them.