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