Flaxmere Waterworld, originally built as an outdoor pool for Hastings in 1977, was enclosed in 1990. When cracks appeared in the central timber portal, which formed the main support for the pool hall, Hastings District Council asked local engineering consultants LHT Design to conduct a seismic assessment of the building.
LHT found that the building was below 33% of the National Building Standard and was, therefore, earthquake prone.
LHT Design engineer Royce Finlayson and lead engineer on the project Toby Mason (now director of engineering consultants Create Ltd), came up with the concept of using an external tri-chord truss from which the existing roof could be hung.
“The primary reason for this was to remove the loads being transferred by the defective timber portal and provide an alternative, dependable load path,” Mr Mason says.
Mr Finlayson says with the concept for strengthening the main pool enclosure defined, LHT needed to introduce a degree of separation from the adjacent amenities building.
“Seismic separation was required to prevent pounding between the reinforced concrete columns and the slab and block-work walls of the first floor of the amenities building,” Mr Finlayson says.
“To reinstate the loads paths removed by the separation, we designed an arrangement of CBFs and local brackets. The gravity loads of the purlins and cladding are transferred through the existing glulam rafters, which span simply supported between reinforced concrete columns.
“The new Circular Hollow Section droppers, in line with the apex of the truss, allow the gravity load, via portal action, to be transferred to the external foundations at either end of the building.”
Patton Engineering project manager Mike Baylis says the tri-chord pipe truss was 45m long, 4.6m high and 4.7m at its widest point.
“So we worked in two halves. Most of the pipe welding was full penetration and subjected to independent inspection and stringent testing,” he says.
“Position welding added to the complexity. After sandblasting and painting, the two halves were then joined and transported to the site as a single 22-tonne section. This enabled us to avoid site welding and to keep erection time to a minimum.
“The connections to the existing building consisted of 26 clevis plate assemblies supported along the length of the truss by droppers in pairs. As we were working with 6mm clearance for each dropper, considerable surveying, accurate fabrication and stringent quality control were critical.
“The truss was finished with a high-build, three-coat paint system and the droppers, which penetrated through the roofing and inside the pool hall, were given an aluminium metal arc spray for increased corrosion protection.”
From an engineer’s perspective, Mr Finlayson says in the longitudinal direction, the seismic mass of the lightweight roof is transferred to the ground in two ways.
“On both sides of the building, the roof is supported by the RC (reinforced concrete) columns, which provide bending stiffness in the plane of the wall. Along the roof centre-line, the rafters are picked up by the series of CHS (Circular Hollow Section) droppers, which cantilever down from the tri-chord portal. The portal action of the truss, in turn, provides lateral sway stability to the building,” he says.
“In the transverse direction, the in-plane forces from the roof are transferred via the rafters to the RC columns. The PC (precast concrete) columns are now separated from the adjacent amenities building. This transfers its moment down to its footing which is, in turn, resisted through over turning. The same is true on both sides of the building.
“Along the centre of the building, the new steel dropped from the tri-chord truss picks up load from the existing rafters and transfers this load via bending into the truss.
“The truss then acts as a lateral beam to transfer this load to the braced gable ends. Here, large diagonal struts act in both tension and compression to transfer the lateral forces to the ground. Large screw anchor piles are required to counter the significant uplift forces.”
Gemco Construction project manager Hayden Earl says the erection went smoothly, and took only six hours for Patton Engineering to complete all of the connections.
Some members of Hastings District Council were on hand to witness the event. This was earthquake strengthening by suspending a tri-chord truss over an existing building without disturbing it.
Flaxmere Waterworld was closed for the day, but open to the public the day after.
