Senior Structural Engineer, CPG New Zealand Ltd

QUAY PARK ONE, MAHUHU CRESCENT AUCKLAND

John McCurran

ABSTRACT

This paper provides an insight into some of the design considerations and background reasons for selection

of structural systems used in Quay Park One, an office building in the CBD of Auckland.

INTRODUCTION

Quay Plaza One was one of the last properties to be developed in the Quay Park area, which was originally part of the Auckland central rail yards. The building was part of the East on Quay development, but was bought by our client, Manson TCLM Ltd. The Architects, JCY, re-designed the building within the restraints of the resource consent granted for the East on Quay development and then the urban design panel modified this again in approving the changes to the original outline building plans. The brief from the client was to provide the structural design and site observation of the main structural components for an office building in the Quay Park area of Auckland. The building comprises 3 levels of excavated basement carpark, mixed use ground floor and four upper floors of office space. The building straddles the underground urban railway exiting from the Britomart railway station. The client required the building to have a 5 star green rating. The key to this building was the use of a two way stressed slab which minimised floor to floor height, and lessened restrictions for services. The design team comprised of; Architect Jensen Chambers Young Structural Engineers Bruce Wallace Partners Mechanical Engineers BECA Fire Engineers BECA Services Engineers BECA Noise Engineers Norman Disney Young Bruce Wallace Partners were bought out by Duffill Watts Consulting Group, who have recently

rebranded and are now called CPG New Zealand Ltd.

SITE

The site is on old reclaimed land on the waterfront. It was the original shunting, workshop and handling yards for the Auckland station sited between the old railway station and the wharves, close to what is now the Vector Arena. Although this area was “cleaned up” when subdivided all excavated materials were tested for contamination and disposed of accordingly. The site comprises hardfill over clay fill excavated from adjacent ridges when the original waterfront reclamation was carried out in the late 1800’s. This was placed over a sand layer which overlies the soft marine deposits on top of the weathered East Coast Bays Formation and un-weathered ECBF. The depth of the weathered ECBF was expected to be close to or just above the final basement level. The original ground level and street levels were approximately RL 4m in terms of Lands and Survey Datum, or just above high water sea level. The excavations extended well below mean sea level. The southern two thirds of the site were fully excavated, up to the Britomart rail tunnel and trench which runs through the north side of the site. The tunnel is a concrete box section supported on piles taken down into the ECBF rock. This tunnel limited the extent of the basement excavations.

DESIGN FEATURES

There are several design features used on this project which make it different from the typical office building. (a) The temporary sheet pile walls are used as

permanent basement walls.

(b) The use of two way post – tensioned slabs stack cast and lifted into place, to give minimum ceiling to floor height and adequate ceiling space for services.

(c) The ductile shear walls (µ=3) using narrow panels in the base, but stitched together at higher levels to stiffen the building and thus reduce lateral drift.

(d) The columns “pinned” top to bottom for simplicity of construction.

(e) The construction methodology being an integral part of the design process.

LIFT SLAB CONSTRUCTION This type of construction has not been used in Auckland very often but has been around for some years. The slabs were constructed using a two way post tensioned floor system. The design of the slab was undertaken by VSL in Australia with Jacking Systems doing the lifting. The sequence of construction was that each slab was constructed on the previously poured slab. As soon as the lower slab under was poured and suitable to work on, any pods that were required were laid out and a polythene or similar bond break laid down. The lower beam or bands of reinforcing were placed, and about this time the underlying slab was post–tensioned and lifted. Once at the right level and blocked off, the reinforcement and pre-stressing tendons continued to be placed, along with edge formers and handrail system. The columns under were also reinforced, boxed and poured in these few days. The precast shear walls used for construction stability were erected, the edge and infill strips were also completed at this time. There were a number of activities happening at the same time to get the slabs completed as quickly as possible. The final step was then to pour the slab. The whole sequence then started again for the next slab.

The floor slab being reinforced around the columns and lifting jacks.

BASEMENT

The basement was constructed using driven sheet piling on all four sides of the three level basement. The sheet piling was driven to below excavation level and into bedrock sufficient to retain the toe. As the excavation extended to below ground water level, a method to minimise ingress of water was required. We were advised that driven sheet piling was a suitable solution as the interlocking grips on the edges would be such as to restrict flows to a low seepage rate of less than 1m3/day, and that long term seepage was expected to be even less. With the sheet piles and rock subgrade to act as cut-off for the groundwater, there was minimal ingress so a subfloor drainage system was all that was deemed necessary and installed to prevent the lowest slab from lifting due to hydrostatic pressure. Excavation was carried out to the level just above the uppermost basement level and the rock anchors and horizontal walers placed. All the reinforced concrete piles for this area of the building were also drilled and cast from this level and the piles concrete filled to foundation beam level. Excavation then proceeded to the required depth. Once the steel jacking piles and lowest slab were in place the next slab was constructed and lifted into place as described above. In the basement the infills extended out to the sheet piling and acted as props to the sheet piling. Once the second basement floor was in place and infills completed, the anchors were released and walers removed.

With the ground floor in place the construction of the upper floors commenced.

The first part of the lowest floor slab being poured, showing the anchored pile wall. The finishing of the basement was debated with the architect and client, and in the end it was decided to leave the sheet piling exposed on the inside as the permanent wall rather than a secondary concrete wall. With the sheet piling propped at each floor level the stresses in the sheet piling were low even after allowing for corrosion allowance on the external face of the sheet piling. Several small leaks occurred in the grips of the sheet piling, so these were welded locally and a light false wall constructed inside to conceal this work and the perimeter pick up drains at each floor level which were installed to pick up any future leaks.

FLOOR SLABS

The basement slabs are flat slabs of 200mm thickness and post tensioned both directions. The loadings are for cars with restricted head room so that there are no heavy loads to take account of. The ground floor slab over the basement is 250mm thick to account for some localised heavier loads and steps in the floor slab.

In order not to load the rail tunnel a foundation beam was constructed on the north side of the tunnel and the area over the tunnel bridged using 350 T units. The depth was restricted as the slab was required to be on a constant level across the entire floor.

The clear and open ceiling spaces provided by the underside of the post tensioned slab

The grids were not of a uniform pattern, but generally 7.65 m centres along the site and 7.0 to 9.55 m across the site. The close grids in some areas allowed a one way slab system, but with larger grids and irregular perimeter, the majority of the slab required a two way slab system.

The upper office floors were constructed in two halves. The southern half was constructed first. The floor system for the upper slabs was two way 2400mm wide by 350mm deep beams with a 180 mm thick two way spanning slab between.

All four levels were constructed on the south side before the north side commenced. When the second half was lifted into position the slabs were joined by an infill strip.

On most floors there is a small cantilever along the front and rear in the order of 2.5 and 3.5m. The top floor was designed with a 5m cantilever along Tapora St which posed falsework problems if it was constructed as part of the post tensioned slab with insitu concrete. The Architect had shown slender columns along the front face of the building. These columns spanned from ground floor level to the fourth level and we proposed that these be load bearing to support the front edge of the fourth level slab. This was agreed so we designed a steel frame to provide support and used a thin Traydec slab to form the outside deck. This allowed the Traydec to span from the post tensioned slab to the columns and forming the falsework so that the slab could be constructed with a minimum of scaffold falsework.

SHEAR WALL SELECTION

The shear walls were located on available walls, plus others that could be made shear walls due to vertical similarity between floors.

The obvious wall was the rear stair well wall which ties into the slab on one side. This wall resists horizontal loads in the longitudinal direction. There are no other walls in the core which tie into the slab on one or both sides.

We ascertained that the services and toilet block area was common to all floors on the ground floor and above, and fitted in with the car park layout in the basement floors. This allowed us to place a wall on each side to resist horizontal loads in the lateral direction.

We also identified the two side and rear lift shaft walls as potential shear walls although not connected to the slab on either side. It was decided to utilise these walls by incorporating drag beams to transfer the loads into and out of the shear walls. These drag beams were designed using the same design criteria as for columns.

All loads into and out of the shear walls were shown on the drawings so that the floor slab designers could incorporate them into their design of the slab.

My preference was to take the shear induced moment out by a back shear couple over the depth of the basement. This reduces the requirement for larger gravity or tension piles in areas of lightly loaded walls. If all walls work on the same resistance system then there is no requirement to assess size of load attraction due to relative stiffness and displacements. The couple resisting the seismic moment at the ground floor and basement are taken out by the perimeter wall. In this building the steps in the sheet piling transfer the loads to the ground by shear in the side walls and also end wall passive pressure. Normally the perimeter walls would transfer the load into the base slab, and then the wall friction and lower wall passive pressure using a similar mechanism.

Having taken the direct shear in the core, we checked the torsion and found that it exceeded the code limits.

We noted that the external perimeter columns were infilled with a louvre or contrasting cladding systems. Of all these walls we identified four that were founded on ground or the perimeter of the building. The remainder were either not connected to the building until level 4 or on the beam across the rail tunnel, hence did not have a rigid base connection.

The H panels are infilled with louvres. Similar panels around the building are architectural only.

The four walls provided sufficient torsional restraint, but needed to be relatively soft so as not to attract significant direct shear.

SHEAR WALL DESIGN

The shear walls were designed with a ductility µ=3. In order to keep the hinge relatively short the longer walls were divided into shorter sections. In order to avoid having joints in the ductile hinge area of the precast walls the hinge length was set as being from the ground to first floor level. The panel joints were then placed immediately under the ground floor and above the first floor.

Dividing the walls into short walls reduced the stiffness, the deflections became significant and the torsional walls attracted significant load. To overcome this the walls were stitched together up to ground level and above the second level. The perimeter torsional panels were short wide walls coupled together. As these walls are on the perimeter the seismic shear moment is resisted by pile action.

Due to the construction methodology, only walls on the perimeter of the floor plate could be precast. The two walls on the sides of the toilet-services core were cast insitu and constructed after the floor was raised. To maintain continuity of reinforcing and wall vertically, and the slab horizontally, slots were left in the slab to place the reinforcing and concrete.

This was considered acceptable as these walls were brought up one or two floors below finished slab level above and there was sufficient shear resistance capability in the precast walls for construction period requirements.

Openings left in the slab for shear wall reinforcement to pass through and place the concrete.

SHEAR WALL CONSTRUCTION REQUIREMENTS

The lifting of the floor slab required lateral resistance for accidental lateral loads and general guidance. The jacks provided a lifting force only.

This requirement was achieved in the basement by using UC sections as a rail to spread the lateral brace runner loads into the sheet piling.

Above ground level the perimeter end panels and the lift core panels were used as a lateral resistance. This required these members to be placed and grouted so that they could develop sufficient cantilever load resistance to resist the inline slab forces and any accidental or natural (wind etc.) lateral loads.

The south end ductile panels in place to support the raised floors

The perimeter panels were sufficiently wide that the weak axis bending was adequate to provide resistance to wind face loads while cantilevered. The rear stair wall was 250 thick and also had sufficient resistance to cantilever for one floor above finished level.

The lift shaft panels were not tied into the slabs so required a secondary support structural form to resist face loads.

As the lift core was for three lifts and lift rail support frames were required between lifts, these frames were integrated into the core structure to provided lateral restraint at third points for the rear wall.

The two end walls are supported by the rear wall and front columns. All tied into the slab at floor levels, while the long rear wall has lateral restraint provided by the two end walls and the two internal frames.

All these walls have adequate capacity to span horizontally between the vertical support lines. The lift rail support frames are steel SHS frames and bolted to the rear walls at close centres, and fixed to a PFC which is bolted to the slab edge full length. This PFC was also required by the lift suppliers to fix the lift door structure.

COLUMN DESIGN

The columns were designed to be pinned at the floor levels. This was done to make construction easier and to reduce moments in the column.

This was achieved by making the external cage full height between floor slabs and providing a small cage through a hole in the slab. The hole was large enough for the four large bars required plus enough room to place and vibrate the concrete.

Having a smaller hole also provided a direct seating around the edge of the slab. The hole was boxed using a reinforcing lathe mesh so that there was a good shear friction bond.

A standard column size was used throughout so all columns could be boxed and poured using standard formers in three days at the most.

CONSTRUCTION REQUIREMENT DIFFERENCES

Construction equipment requirements need to be known when considering different forms of building structure. In typical beam-column construction, the amount of falsework required depends on the

amount of construction load that can be supported by the beams and floor system.

In this project there was no requirement for falsework except for finishing the exterior perimeter where there are finishes fixed to the exposed concrete columns and coupling beams. This could be eliminated if a fully glazed facade was used.

The south half is full height, the north half is coming up. Again the H panels can be seen.

For a typical beam-column building a site crane is used to lift all the structural members into place.

In this project the site crane was just a small jib crane as the heaviest lift was reinforcing.

A large mobile was used for the short time it took in the lifting of the precast panels into place. It was more economical to use this combination than to have a larger tower crane on site for the whole construction period.

GREENSTAR RATING

This building has achieved a 5 star rating for design in terms of the Green star Building Council and is currently being assessed for a 5 star rating for as-built. Points were obtained for using recycled reinforcing