_______________________ 1Project Manager, Mainzeal Property and Construction Ltd.

CANTERBURY DISTRICT HEALTH BOARD STAFF CARPARK BUILDING.

Graeme D. Earl1

SUMMARY

In 2002 Mainzeal were selected by the Canterbury District Health Board (CDHB) as one of three tenderers for the design and construction of a three level carpark building for a minimum of 400 cars with the potential to build a future two levels at some later point in time. Provisional structural and architectural drawings and specifications were supplied by the client at the time of tender. The resource consent for this project had already been granted based on these provisional drawings and some colourful external elevations. Based on this we knew that we were pretty well fixed with the external look of the building. The provisional structural drawings were based on a 8.4 x 16.8m module grid spacing and very large raft foundations because of the likely liquefaction of the site and generally soft conditions. To win the contract we knew that we would have to be innovative and look outside the square at possible options to give construction cost savings. The four things that we had to consider were:

Limited construction access.

Most cost effective construction design and materials.

A fast track construction method

Maximise the number of carparks

LIMITED CONSTRUCTION ACCESS The site was bounded on two sides by existing buildings and on the other two sides by two very busy roads namely St Asaph and Antigua St which in turn had power poles/wires located at the edge of the kerb. The cost of installing a tower crane would have been prohibitive so the only option was to utilise mobile cranage and therefore limit the amount of cranage off the roadways.

MOST COST EFFECTIVE CONSTRUCTION DESIGN AND MATERIALS A concrete structure was always going to win the day when it came to selecting a cost effective construction material in Christchurch. How we were to use it would be the question. Traditional precast beams, columns and proprietary flooring were thought to be the best elements to utilise, however how could we improve what is cast as “These Normal Construction Elements”.

Firstly we needed to reduce the proposed foundation design. One way was to reduce the span to 8.6m therefore reducing the loading capacity of the foundations. Piling was not preferable so the use of smaller raft foundations we knew would provide a point of difference. Liquefaction was always a possible “Red Herring” that the local council could raise, however it was decided to run with it anyway. With the reduced span the option to use 200 Dycore in lieu of the previously chosen 400 Dycore would reduce weight and cost. This then gave the option of using a spaced Dycore system again reducing the number of cranage lifts and overall cost of the structure. The façade was to be a clip-on, giving the client (CDHB) the option to install whatever finish they so desired. While we knew that the façade had already been approved as part of the resource consent, we did supply the client with an alternative at the time of tender which was in our opinion more attractive and in keeping with the local area and of course more cost effective. This was however eliminated

as they felt that there was no way that they would have got approval from the local residents for this proposed change. Therefore we proceeded to use precast panels to match the approved elevations.

FAST TRACK CONSTRUCTION METHOD The entire design and construction period was dictated as being eight months from contract award to final completion. Also there were three staged handovers so that areas of the carpark could be used. These coincided with the commencement of the Main Women’s Hospital Construction Contract. In the past the areas where construction has been slowed have been in the traditional forming and pouring of Beam/Column junctions and any in situ shear walls that may be required. We therefore wanted this situation to be modified so as to eliminate both of these lengthy activities. We knew that this would also improve the overall quality of the completed structure, as much would be constructed under a controlled factory environment. Structural steel K-Braces were used in lieu of concrete shear walls.

MAXIMUM CARPARKS We knew that the tender would be evaluated on the total cost of the building being divided by the number of carparks with the lowest cost per carpark being the winner. As this was being designed as a private carparking building we were able to cram in as many carparks as we could. 420 carparks were achieved at a total cost of $3.75 million, that being $8,928 per carpark. Normal allowances being $11-13,000 per carpark, thus making this entire development most cost effective.

SUMMARY All of these points were presented to the design team including Romulus Consulting, The Buchan Group and Stresscrete as the only way that we could secure the contract under the tender conditions being applied. Each concern was addressed and all possible innovations explored. The entire structure was constructed utilising only six skilled tradesmen on site. Both false work and scaffold was minimised due to the beams being completely formed prior to arriving on the site again reducing construction costs.

The final product is testament to the effort that each member of this team put in and it was a credit to all. We are all confident that the innovation that what was developed on this project can and will be carried forward to other similar buildings, not necessarily only carparks.

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² Structural Engineer, Romulus Consulting Group Ltd

CANTERBURY DISTRICT HEALTH BOARD STAFF CARPARK BUILDING

Dr Peter Johnstone ²

THE STRUCTURE THE BUILDING

The parking building is designed to be a (final) height of 4 suspended slabs plus a lightweight roof. These documents describe the construction of a building with two suspended slabs and a ramp going up to the proposed third level in order to accommodate 400+ cars in Stage One. Refer Figs 1 2 and 3. The building has a ground to first (floor to floor) of 4.5 metres to accommodate future offices on the ground floor. The structure is a series of one-way frames of pre-cast beams and columns supporting a Dycore flooring system. Refer Fig 4. The columns are on an 8.15 x 9.60 grid. Each module of double 90° parking was 16.4m wide and was supported on two columns with a beam cantilevering both sides. See figure 5 below.

Figure 5 – Concept Cross Section

These frames alone have no lateral capacity of any consequence; they are designed to resist gravity loads only. A series of ductile structural steel “K” braces have been added to provide lateral load resistance to wind and (predominantly) seismic loads. The additional future floors have been assumed to be steel framed with primary and secondary beams supporting metal deck flooring.

DESIGN PHILOSOPHY This is a structure that was designed initially to be a “shaky” building. In another words, it was a stack of pre-cast beams and columns with no real attempt to give them any lateral strength capacity. This meant that they could be assembled relatively simply, without having to worry about tricky beam/column joints, shear walls, etc.

As a nominal second stage in the design, the structural steel ductile “K” braces were added to provide the lateral capacity. Figures 6 and 7 show sections through the building. The K brace positions in plan are covered in figure 4. As a design concept, the system worked well. It gave identifiable load paths, a simple form of “stacked up” construction, and a very ductile lateral load resisting system. Note that specific attention needs to be paid to the “P ∆” effects during design. . The devil, as usual, is in the detail. The framing plan (figure 4) has beams generally running across the building, but at the ends, to suit the parking configuration, they changed direction by 90°; hence the need to deal with a notional pin connection at the ends of the 90° pre-cast beams. These connected into the joints at the ends of the transverse beams and these are shown in the three-dimensional schemes in figures 8 and 9. These 3Ds proved invaluable for the pre-caster.

The position of the columns within the frames was set so that the cantilever moment at the column equalled the mid-span moment in the beam. Refer to figure 10 below, showing the moment balancing and reinforcing selection.

Figure 10 – Sample Moment Envelope

The cantilevers enabled the foundations to be away from the buildings, to avoid eccentric footings. The weight of the pre-cast panels on the outside of the building were also taken into account for moment balancing.

The columns changed in cross-section at the first floor level so that beams could be effectively "wedged" over the columns themselves. Refer to figure 11 below.

Figure 11

FOUNDATION SYSTEM The foundations are large pad footings tied together with a grid of ground beams. They will be cast directly onto the ground after local compaction and checking for soft spots. The foundations beneath the “K” Braces are designed to rock under an Ultimate Limit State earthquake while not exceeding the allowable bearing pressures.

The ground floor surface is asphaltic concrete over hardfill or directly on top of the large pads as appropriate. Refer fig 12.

LIQUEFACTION POTENTIAL The geotech investigation revealed that the top layers of soil or the crust are unlikely to liquefy. The building has sufficient ductility or “bendability” to accommodate the deformations covered in the geotech report after a damaging earthquake.

FRAMING AND FLOORING The building is constructed from hit and miss Dycore planks with a 75 topping supported on precast concrete beams. These wide beams are a single span with cantilevers at each end, proportioned to equalise the column and mid span moments. Figure 10 shows the wide beams with the slotted hole connections. Figures 13a and 13b below show the reinforcing arrangement in the beams.

Figure 13a

Figure 13b

The beams were precast only to the top of the Dycore planks with shear reinforcement just protruding. They were pre-cambered to allow, in particular, for cantilever droop. The Dycore flooring system is 200 Dycore planks laid with an 800 wide timber infill between planks. It is not stressed to its capacity and was not so flexible as to need a special vibration analysis. The floor itself did not require propping during construction – just the beams at the column positions. The floor topping is 75 concrete at 25 mPa with 663 mesh and saddle bars over the beams. The mesh is heavier than usual to assist in diaphragm action and that the transfer of the loads to the K braces as well as to provide better crack control in the topping. The decks do not have any waterproofing membrane. A sheet of details shown in figure 14 illustrates some of the load transfer mechanisms to the K braces.

The ramps needed a movement joint half way up their length to avoid inadvertently tying the floors to the ground.

FACADES The street facades of the building were a series of precast columns and precast spandrel panels both of which were non structural (apart from holding up their own self weight). At the corners of the buildings were a series of precast (prestressed) wall panels surrounding the lifts and the stairs. These were a designed to rock under earthquake loading, and not attract lateral loads. Refer to figures 1 and 15

THANK YOU To all the participants in the project for working in a team environment, transcending the normal skill boundaries to add real value to the project.

Figure 1 – Birds Eye View

Figure 2 – Ground Floor Plan

Figure 3 – First Floor Plan

Figure 4 - Typical Suspended Slab

Figure 6 - Longitudinal Section

Figure 7 - Cross Sections

Figure 8 - Precast to K Brace Connection

Figure 9 - Beam to 90 degree Beam Connection

Figure 10 - Beam Elevations

Figure 12 - Foundation Plan

Figure 14 – Details

Figure 15 - Street Elevations

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³Stresscrete Christchurch Manager

CANTERBURY DEPARTMENT OF HEALTH BOARD CAR PARK Corner Antigua/St. Asaph St, Christchurch

John Marshall ³

Why Precast? Points of Interest Where did Stresscrete come in?

Identified risks/cost saving opportunities.

Solutions.

Economies of using pre-cast concrete.

Where did we come in? We were invited by Mainzeal Construction to

attend pre-tender design development meetings.

Ensured proposed details were cost effective and practical to manufacture.

Ideas could be designed and detailed at an early stage for full cost benefit.

Identified risks/cost saving opportunities. Beam/column joints.

Small tolerances creates potential time delays and difficulties on site that can be costly.

Complicated formwork involves more skilled labour and remedial work.

Propping & false-work, hinders work-space, time consuming, safety hazard.

Quality of exposed surfaces.

Lack of skilled resources due to buoyant market

Handling site waste

Poor soil conditions

Tight staged hand-over construction program.

Solutions. Beam/column joints

Traditionally beam/column joints create

placing problems with small tolerances. A solution was designed to eliminate them.

Pre-cast concrete elements resisting gravity loads working in conjunction with steel K-brace frames that resist lateral forces.

Propping Use of pre-cast concrete products.

– Unpropped Pre-stressed concrete floor system.

– Pre-cast concrete beams cast with top reinforcement enabling construction load carrying capacity significantly reduces propping.

– Beams underside cast with “precamber” giving perspective of hogging.

– Positive wedge support of beams cast in column.

Quality

Products cast in steel moulds.

Quality assured process.

Eliminate on site remedial work.

Resources Mainzeal assigned only 5 people on site.

Notice how difficult it is to find people in the photographs.

Demands for site concrete and formwork reduced. Especially on a confined site as this one was.

Reduced risk to contractor.

Less site waste

Less on site concrete and boxing reduced

disposal of waste for the contractor.

Pre-caster has opportunity for better utilisation of material by way of other standard products and recycling.

Poor soil conditions

The ground conditions under the structure were poor and the designer, Romulus Consulting was required to make special considerations.

Spaced dycore system reduced the mass of the structure and reduced size of foundations.

Tight construction program

Units can be made ahead of time in a covered

factory avoiding time delays due to poor weather.

Quality of product eliminated time and inconvenience of doing on site remedial work and allowed early hand-over of portions of car park during construction.

Economies of using pre-cast concrete

Repetition increases cost efficiency.

Higher quality standard reduces costly and frustrating remedial work on site.

Reduces on site resources.

Sound processes from conceptual design involvement and advise through to manufacture and delivery.

Reduces construction time with smart planning.

The pre-cast concrete components manufactured and supplied by Stresscrete were:

4430m2 of spaced 200 Dycore,

36 pre-cast columns,

69 pre-cast beams, 800mm wide and up to 17m long weighing 17 tonnes,

14 pre-stressed stairwell wall panels 12m high plus 16 smaller traditionally pre-cast wall panels.

10 pre-cast concrete stair flights,

44 pre-cast concrete spandrel / column façade panels and,

36m of decorative pre-cast concrete panel cappings.