Stratum’s affordable and low damage dwellings in Wellington

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To meet the rising demand for residential property in Wellington, Stratum Management Ltd has built a number of multi-storey buildings.

The 15-level Elevate Apartments on Taranaki Street features a seismically structured, low-damage building that is new to Stratum.

While gathering our material, we noticed that Stratum tends to engage the same team of subcontractors for large projects, quite consistently — Architecture + is the preferred architect, Aurecon is the preferred engineer, MJH Engineering is the preferred structural steel fabricator and erector, and the project manager is invariably Stratum’s own Craig Lyford.

The Canterbury earthquakes in Christchurch have highlighted the need for building structures that not only protect the lives of occupants but also offer resilience against the building’s loss of function.

In the Elevate building, Stratum Management has demonstrated a readiness to implement modern, low-damage design systems.

Its rocking concentrically-braced frames (CBFs) with Ringfeder springs, and Moment Resisting Frames (MRFs) with Sliding Hinge Joints (SHJs) are, however, not new to Wellington.

They had been used by Aurecon as the seismic resisting structural systems in the Victoria University of Wellington Te Puni accommodation project.

Although Te Puni had different architects (Architectus), the same team that successfully completed Te Puni was working together again on Elevate but directed by Stratum as the developer, with Architecture + as the architects.

Structural overview

The structure consists of seven tension-limited CBFs in the E-W (transverse) direction — three are located at the southern end of the building, three through the centre of the building and one on the northern end. A total of seven MRFs with SHJs are spaced approximately evenly across the building in the N-S (longitudinal) direction.

Transverse bracing

The base hinge consists of pre-stressed Ringfeder friction springs and a vertically orientated sliding friction connection. This system allows controlled holding down of the CBFs, which limits the lateral loads to be resisted by the structure. Thus the base connection to the CBF is the strength limiting element, and this prevents damage from occurring in the primary structural elements.

By comparison, a traditional CBF relies on yielding of the braces to impart ductility into the structure, which results in inelastic deformation of the structural system. Post earthquake, significant repairs are required.

The Ringfeder springs enable the connection to be preloaded so as to set the performance criteria at which “lift off” occurs. The friction sliding connection has two functions:

It provides additional resistance to uplift, which reduces the size of the Ringfeder, and

It provides resistance during the downward motion of the column, reducing the impact loads from the column.

Under a design-level seismic event, the tension columns of the CBFs are designed to uplift. Uplift will occur once the spring’s pre-stress, sliding friction connection and gravity loads are overcome.

As the column begins to uplift, the Ringfeder spring is compressed between the base plate and a cover plate.

This system limits seismic forces in the primary structure as well as in the foundation, and prevents damage occurring in structural elements and connections. The benefit is that post-earthquake, the building can be occupied quickly and safely.

Longitudinal bracing

The longitudinal bracing system consists of Moment Resisting Frames and uses Sliding Hinge Joints. One of the main advantages of this form of construction is that stiffness of the beam can be de-coupled from its strength.

Larger beam sections can be chosen to limit seismic drifts. The efficiencies achieved result in savings in the foundation sizes required.

The Sliding Hinge Joint is essentially a semi-rigid beam-column connection that provides a rotational pin on the top flange and a sliding detail on the bottom flange. It works when the moment demand from seismic actions induces beam flange forces that exceed the sliding resistance of the bottom flange and web-plate bolts, at which point the joint will slide, allowing rotation to occur about the top flange.

Once the imposed moment reduces, the sliding stops and the joint becomes rigid. By positioning the pin at the top flange, any undesirable floor slab participation can be minimised.

The design ensures that at the design based earthquake, inelastic rotation occurs within the slotted holes, equating to only minimum joint degradation and minor slab cracking.

Complex geometry

Because of Stratum’s intention to provide maximum residential layouts, the angles of the beam lines and the locations of the columns resulted in the building having a complex geometry.

 Even more complex is the car park helix, which twists and rotates through the first five levels.

There was considerable interaction between main contractor Stratum Management and steelwork fabricator MJH Engineering, who also created detailed workshop drawings for the structural steel.

Working from Revit software, Aurecon and MJH produced an accurate 3D model of the building in Tekla software, which enabled structural clashes to be avoided.

Gravity system

Composite steel tray floor systems span in the transverse direction between the MRFs. Because the MRFs are closely spaced, very few gravity-only beams are required to resist gravity loads. The gravity load is transferred to the MRF and CBF columns and into the foundation.

One advantage of using the SHJ in the MRFs is that the concrete floor can be poured prior to the SHJs being tightened. This results in a beam that is effectively simply supported.

The SHJs can be tightened once the floor is poured. This enables a simplification of the SHJs and reduces the connection size. The SHJ can be designed for only seismically-induced moments.

Column to pile connection

The foundation system consists of 40m x 1.2m diameter reinforced concrete bored piles. These are situated below the CBF columns and the NRF columns at a depth of 20m. A 300mm reinforced concrete diaphragm allows the distribution of lateral loads amongst the piles.

The connection of the CBF column to the RC pile needs to be robust enough to resist the ultimate strength of the base connection. A cast-in UC member with shear studs is embedded approximately two metres into the pile, and transfers tension and compression forces into the pile.

Ringfeder and sliding joint CBF base connection

The Ringfeder spring is a compression-only spring. Upon compression, the outer ring is forced outwards and the inner ring is forced inwards. At all instances, the rings remain in the elastic range. Lock-up occurs when the displacement capacity of the rings is exhausted, effectively resulting in a solid pack of steel rings.

The spring is compressed between two plates using a high-strength large diameter turned down bolt. The bottom plate is connected to the CBF column while the top plate is free.

As seismic axial tension forces develop in the CBF column, gravity loads are overcome, and further hold down is provided by the pre-stress of the springs. Once the level of pre-stress is reached, the sliding joint will provide the next tier of resistance until the sliding force is reached.

At this point the column will begin to uplift. As it does, the spring provides additional resistance to uplift. Typically, the Ringfeder spring is pre-stressed to about 50% of its ultimate capacity, so once the spring begins to lift there is approximately another 50% of reserve capacity.

The purpose of the turned down bolt is to act as a fuse. The diameter of the turned down length is chosen so that the bolt yields prior to the lock-up of the spring. This is to prevent damage occurring to the spring and to the building structure.

Three months into the Elevate Apartments project, Stratum added a new project — Nouvo Apartments. How would the team cope?

Stratum project manager Craig Lyford, and his counterpart at MJH, Mark Shirtliff, weren’t fazed.

“As we do with all Stratum projects, we render the engineer drawings in 3D Tekla, because this facilitates co-ordinating the work of the follow-on trades,” Mr Shirtliff says.

“So we simply added the new project into our production schedule, treating it more like an additional package of Elevate rather than a different project.

“But of course it was, and on its own separate site. Nouvo Apartments is topped off at five storeys, and its townhouses have only two. But Stratum was committed to showing that it can meet the affordability needs of a wide range of residents.”

Allan Wright, a director of Architecture +, which had also designed Elevate, explains how the Nouvo concept developed in response to both height and proximity restrictions.

“Situated on the cusp of the city between Rugby and Alfred Streets, Nouvo is a neighbour to two Wellington landmarks — the Basin Reserve and Government House,” Mr Wright says.

“The latter is protected from close observation by proximity and height constraints. Our solution was to align the five-storey apartments in a north-south, double-loaded corridor, while the low-rise townhouses were to face east-west — that is, parallel to the Government House boundary.

“This offers two different styles of accommodation — outside space with “feet-on-the-ground” for the townhouse dwellers, and in the loftier apartments, which have no outside space, the wide frontage along the bedroom areas have windows designed to admit the premium views and aspects.”

To summarise — the project consists of four blocks — A, B, C and D. Blocks A and B are five-storey apartment buildings with 30 and 12 apartments respectively. Blocks C and D are two-storey townhouses with five and six townhouses respectively.

A and B are structural steel-framed buildings, with the top storey being constructed out of timber, using plywood walls for bracing. C and D are timber-framed, with structural steel portals for lateral bracing at level one.

Once again, Stratum subcontracted Aurecon as the consulting engineers on the project. Senior structural engineer Malcolm McGechie explains that the steel-framed blocks A and B share the same access core, with stairs and an elevator.

“There is a seismic gap located in the corridor adjacent to the stairs between the two blocks,” Mr McGechie says.

“The lateral load resisting systems are the same for each block, with Moment Resisting Frames (MRFs) in the longitudinal direction and Eccentrically Braced Frames (EBFs) in the transverse direction.”

Aurecon design engineer Phil Don incorporated Sliding Hinge Joints in the MRFs to provide a low damage solution in the longitudinal direction. In a major seismic event, damage is localised to the beam/column joint, which can be more easily remedied than damage to the main structural members.

Around Wellington, many Stratum Management buildings take their places among the city’s best known landmarks (see pic at left).

From this vantage point we can see Elevate Apartments under construction, but to the left are Monument and Piermont Apartments and to the right Portal Apartments.

For all of these, Stratum has engaged the same architects (Architecture +), the same engineers (Aurecon) and the same structural steel fabricator and erector (MJH Engineering).

Asked why, Stratum development manager Robert Clemens says when you have good subcontractors, you get to understand how they work as they, in turn, appreciate how we work.

“It leads to consistency because we realise that we share the same philosophy and the same approach to achieving objectives. There’s a loyalty that grows stronger with each success, but there’s also a great sense of professional fun.”

How then do you keep your subcontractors competitive against their own rivals in the marketplace?

“We have our own quantity surveyor so we can check what each project should be costing. We can then negotiate a fair deal for our subcontractors, ensuring that they are happy to come to the party,” Mr Clemens says.

“But it’s never just about the price. We know we can count on our team to make sure the project always finishes on time. There’s no point in agreeing on a margin if it gets chewed up by a programme overrun.

“And when it’s time for Wellington City Council to issue compliance certificates, we find that as they too have come to recognise the high standards that our team achieves, so they can feel confident about our being consistent.”

How could Elevate and Nouvo be run concurrently without a loss in efficiency?

“We often run more than one project at the same time. To maintain efficiency we talk to our subcontractors and give them all the information they need to make sure they are geared up.

“Wellington doesn’t have the same stress that you find in Auckland and Christchurch today. So we are looking for more opportunities, and Stratum always strives to improve its performance. We don’t preoccupy ourselves with corporate things like vision statements. We prefer to be hands on, and act with a clear social conscience.

“When the people who live in our buildings say they are satisfied, we take that as confirmation that we, and all of the subcontractors on our team, are contributing to the social fabric of Wellington.”

MJH Engineering managing director Malcolm Hammond says he regards Stratum Management as “ahead of their time, especially in their use of 3D modelling systems that enable accuracy”.

“With Tekla we can plan ahead and know what’s coming while at the same time detailing in advance for the follow-on trades,” Mr Hammond says.

“And Stratum Management knows that design resolution need not always depend on the engineer — it’s often dealt with by allowing the fabricator to suggest details that can be incorporated in the early stages of the engineer’s design.

“Because Stratum Management knows the value of time, they appreciate all those who can help them make the best use of it.”

Note: Thanks to Aurecon structural engineer J D Tait, engineering consultant G K Sidwell and technical director J F Finnegan for their permission to quote and paraphrase from their engineering report on the Elevate Apartments.