HOLLOWCORE FLOORS: CHALLENGING INDUSTRY PERCEPTIONS
NICHOLAS BRAZZALE1; DANIEL KENNETT1
1Stahlton Engineered Concrete, a division of Fulton Hogan Ltd SUMMARY Observed damage to precast concrete floors in Christchurch and Wellington following recent
earthquakes has focused the attention of structural engineers (and the public) on the seismic
performance of precast concrete floors, and in particular on the performance of hollowcore
floors. In response to this attention, this paper aims to challenge structural engineers to
consider what should constitute ‘best practice’ in relation to the use of hollowcore floors in new
buildings.
INTRODUCTION
Extruded prestressed hollowcore floor units were introduced into the New Zealand construction
market in the 1970s and became increasingly popular over the following decades due to the
many advantages these units brought to the industry.
Both Stahlton and Stresscrete manufacture Hollowcore floor units in New Zealand. Hollowcore
units are formed by feeding a very stiff, high strength concrete mix into a specialised machine
that extrudes the profile onto a steel bed. Units are cut to the necessary length after casting
and curing. This makes hollowcore very efficient to make and therefore a very cost effective
flooring solution. Previously, a slip-formed profile has also been available but that is currently
not the case.
Over time, details used in the construction of hollowcore floors have evolved and our
understanding of the behaviour of these floors has improved through both research and
observation of their performance in service. More recently, the industry has seen a shift in
attitude toward hollowcore floor units, with the preference swinging away from their use. This
shift has resulted from perceived issues with the performance of hollowcore floors in recent
seismic events in New Zealand coupled with a lack of understanding of the residual capacity
of hollowcore floor units following damage from a seismic event. This has led to further
concerns around the suitability of hollowcore flooring for use in situations where low damage
design is implemented in newly designed buildings, due in large part to a lack of understanding
of economical repair strategies for damaged hollowcore units.
Figure 1 – Hollowcore manufacture process
A summary of the benefits of hollowcore floors to construction is presented, followed by a
summary of the identified issues with hollowcore floors and details developed to address those
issues. Finally, a summary of building types that hollowcore is particularly suited to, and not
suited to, is provided for reference.
BENEFITS OF HOLLOWCORE FLOORS
There are many advantages to Hollowcore, which is why it remains a popular choice of floor
system around the world. When used appropriately, the stiff nature of Hollowcore helps to
control user comfort due to vibration response and the eccentricity of the prestressed strands
means that floor deflections are rarely an issue.
Hollowcore is capable of large clear spans, up to approximately 20m for 400mm deep units
(governed by supply capability). These large spans often do not require propping during
construction, leading to reduced time and cost of construction as well as keeping areas below
free of obstruction.
Due to the hollow cores, the system is lightweight by comparison to other concrete flooring
systems. Hollowcore units can also be spaced, further reducing the weight of a floor system.
Figure 2 – Placing hollowcore floor units
Figure 3 – Spaced hollowcore units in service
Offsite precast manufacturing helps to speed up construction and once placed the units provide
an immediate working platform.
Figure 4 – Clear construction working platform
The hollow cores can be utilised to run services, hiding them within the depth of the floor, which
can save headroom and reduce overall building heights. Large penetrations, where necessary,
may be positioned between units. Due to the high flexural and shear strength of Hollowcore
these can often be accommodated without additional secondary beams.
Adding additional reinforcing in the cores and filling with topping concrete can strengthen
Hollowcore units where heavy loading is required on a floor. Cores only need to be filled to the
length required by the designer and additional cores can be filled incrementally as necessary,
ensuring that no additional weight beyond what is required is added to the overall structure.
ISSUES IDENTIFIED WITH HOLLOWCORE FLOORS
From analysis of structures following past seismic events, such as the Canterbury Earthquake
Sequence and the 1994 Northridge Earthquake, it is recognised that hollowcore floors, when
used inappropriately, can perform poorly. There are three types of failure, which typically result
from lost unit seating:
• Collapse of the hollowcore unit with topping slab;
• Delamination of the unit from the topping slab and
• Collapse of the bottom half of the hollowcore unit only due to the webs splitting
(Mathews et al. 2003).
Seating of a precast element is lost during a seismic event due to elongation of the adjacent
beams, which is due to the formation of plastic hinges in the beams. “Once plastic hinges form
in a beam and the beam undergoes large inelastic rotations, the beam grows in length” –
Mathews et al. 2003.
Hollowcore flooring units are stiff elements in a structure and as such do not elongate with the
adjacent beams. If the beam elongation is such that the length of elongation is greater than
the seating length of the hollowcore unit then it is possible, or even likely that the unit will
collapse in one of the manners described above.
Seating and connection details used in the past, and which are no longer allowed, included for
negative seating arrangements and little positive connection to the structure. The lack of
enough positive seating and connection to the topping and supporting structure, coupled with
the aforementioned beam elongation is a significant factor in the loss of seating and potential
collapse of hollowcore floor units.
The loss of seating can also affect the strut and tie method of diaphragm analysis. As a crack
forms between the hollowcore unit and the supporting and/or adjacent beams the node for the
strut and tie model is lost.
Figure 5 - Hollowcore unit seating loss due to formation of plastic hinges (FIB – Bulletin 78)
As explained in the introduction, hollowcore flooring units are an extruded product and due to
this manufacturing process it is not currently feasible to “cast in” traditional shear reinforcement
such as stirrups or links. While the shear capacity of hollowcore is significant, once reached
the failure tends to be brittle because of the lack of shear reinforcement. This has been
observed in both laboratory testing and events such as Northridge, 1994.
In addition, due to the stiff and brittle nature of hollowcore units, they can be highly susceptible
to torsional loading, and therefore care must be taken when positioning units with consideration
to plastic hinge zones that may deflect in such a way as to induce torsion in a unit. Similar
consideration is required at the edge of a floor where units are placed adjacent to edge beams
that may yield and deflect significantly more during a seismic event that the hollowcore is
designed to.
Finally, the aftermath of the Kaikoura earthquake in November 2016 has highlighted the
susceptibility of hollowcore floors constructed with detailing practises used through the 80’s
and 90’s to damage, and the lack of knowledge within the industry around suitable approaches
or details to remediate such damage.
ADDRESSING THE IDENTIFIED ISSUES
Many of the issues identified are addressed by following the detailing requirements of the
current amendment of NZS3101. Minimum seating is now 75mm and the use of low-friction
bearing strips is required. This goes a long way towards reducing the risk of loss of seating.
Local shear capacity issues can be overcome by casting shear reinforcement in the form of
links into a filled core of the hollowcore units as part of the structural topping. Longitudinal bars
placed in the filled portion of the core and the topping reinforcement anchor this shear
reinforcement.
Details included in the current amendment of NZS3101 are based on a great deal of research
and there is no reason that the industry should not be comfortable that the use of these details
with hollowcore floors leads to a perfectly acceptable and safe solution.
The increased seating requirements that resulted from Amendment 3 of NZS3101 combined
with the detail provided in the commentary (Figure 6) is a robust solution to potential seating
seating related failures noted earlier.
Figure 6 – NZS3101 Figure C18.4 – Hollow-core reinforcing in cells on low friction bearing strips
The use of the alpha unit detail (Figure 7) allows for deformation oncompatibility between the
first hollowcore unit and an adjacent parallel edge beam that may hinge, or for torsion that may
occur on hinging of a perpendicular support beam at internal columns. Note that the code does
not explicitly point out a requirement to use the alpha unit detail or a similar arrangement at
internal columns where hinging may occur, however this is recommended as torsional loading
is not desired as noted earlier.
Figure 7 – NZS3101 Figure C18.6 – In-situ edge slab reinforcement (alpha unit detail)
In addition to all of the above, Stahlton together with The University of Canterbury have
completed a successful series of tests on hollowcore units reinforced with steel fibres. The aim
of these tests is not to increase the shear capacity of hollowocre units, which is already
substantial, but to increase the residual capacity once a shear failure initiates and make the
failure less brittle. Results have shown that adding the fibres does help to control the failure
and increases the residual capacity of the hollowcore units. This is also important while
assessing buildings after a seismic event.
BUILDINGS SUITED TO HOLLOWCORE FLOORS
Like any construction material, hollowcore is well suited to some situations and not so well
suited to others. Due to the numerous benefits that hollowcore floors can bring to the
construction of a building hollowcore should be considered in design. Generally, when well
detailed in accordance with the current standard there is no reason that hollowcore cannot be
a safe solution in any building.
With that said, hollowcore floors are particularly suited for use in stiff structures where the risk
of deformation incompatibility between the floor and the primary structure, and the risk of
inducing torsion in the floor units has either been significantly reduced or eliminated.
Research has shown that well-detailed hollowcore floors are a viable solution up to and beyond
drift levels of 3% however damage at these levels of drift may be substantial. The Matthews
(2004) testing showed that units even with older detailing that is not compliant with the current
standard performed well up to drift levels of ±1%. This testing included less than 50mm seating,
no low-friction bearing strip and no reinforced cores.
Based on this it is safe to say that hollowcore is an appropriate flooring choice for buildings
with expected drift levels less than 1%, as may be expected in low to medium-rise shear wall
buildings, or in low-rise braced frame buildings, without any particular need to consider
detailing requirements beyond the norm.
Lagos et al (2017) noted that Chilean high-rise buildings consisting of relatively dense
reinforced concrete shear walls as a lateral load resisting system have low drift demands
(<0.5%) and have performed well in recent large seismic events, being characterised as
‘almost operational’ under these events of magnitude 8.2 and greater. Under lower level
frequent or occasional events the majority of these buildings are characterised as ‘fully
operational’. This strongly suggests that buildings designed to be stiff and with a low drift
demand are inherently compatible with low damage design philosophies as well as with the
use of hollowcore floors.
Beyond 1% and up to the code limit of 2.5% drift, hollowcore floors can still perform well as
long as detailing is in accordance with the requirements of NZS3101, but the recommendations
of the next section should be considered.
BUILDINGS NOT SUITED TO HOLLOWCORE FLOORS
Hollowcore floors are very stiff and lightweight as mentioned previously, and do not include
traditional shear reinforcement. Because of this hollowcore is susceptible to differential
deflections with primary structural beams and torsion induced by deflections of the primary
structure.
Hollowcore floors would not be the preferred solution in any medium to high-rise highly flexible,
highly ductile structure. Observations from past significant seismic events have shown that
these types of buildings are more likely to see the types of failures associated with hollowcore
floors described earlier in this paper. An example of a precast concrete floor failure resulting
from significant building movements, though not specifically hollowcore, is the Statistics New
Zealand building double tee unit collapse (MBIE, 2017).
CONCLUSION
Hollowcore floors bring a number of benefits to construction, however due to the nature of the
fabrication process they are susceptible to damage under earthquake loading when not well
detailed. By contrast, when thought is put into the type of building that hollowcore is being used
in, particularly in relation to the movement demands that building may exert on the floor during
a seismic event, and good detailing is adopted hollowcore floors are a safe and economical
option. When used sensibly in stiff low to medium-rise (potentially even high-rise) buildings
hollowcore floors are certainly compatible with a low damage design philosophy.
REFERENCES
Anderson, H., Hare J., and Wentz R (2017), Investigation into the performance of Statistics
House in the 14 November 2016 Kaikōura Earthquake, Ministry of Business, Innovation and
Employment, Wellington
Lagos, R., Kupfer, M., Lindenberg, J., Bonelli, P., Saragoni, R., Guendelman, T., Massone, L.,
Boroschek, R., and Yañez, F. (2017), Seismic Performance of Concrete Buildings in Chile,
Conference Proceedings 16th World Conference on Earthquake Engineering, Santiago,
Chile
Matthews J.G., Bull D.K., and Mander J.B. (2003), Background to the testing of a precast
concrete hollowcore floor slab building, Conference Proceedings 2003 Pacific Conference on
Earthquake Engineering, Christchurch
Matthews J.G (2004), Hollow-core Floor Slab Performance Following a Severe Earthquake,
Doctoral Thesis, University of Canterbury, Christchurch
Standards New Zealand (2006) Concrete Structures Standard, NZS 3101, Parts 1 & 2,
Standards New Zealand