Kennedy Building Seismic Strengthening

As a key heritage building in the heart of Wellington, it was important to preserve the character of this historical gem while improving the safety of it.



Location

  • Wellington, New Zealand

Client

  • Mibar Enterprises

Project Value

  • $250,000

Project Status

  • Complete: February - December 2018

The Kennedy Building, a four-storey unreinforced masonry (URM) building located on 33-39 Cuba Street in Wellington, was constructed in 1905 and is a good example of Edwardian Classical style architecture. The building forms an important part of the built landscape of Cuba Street, and its façade contributes to the heritage nature of the area. The building is listed as a Category 2 Historic Place and is scheduled as a Heritage Building in the operative Wellington City District Plan in recognition of its aesthetic, historic and social value with a high level of authenticity. The building was assessed as earthquake prone, and under the Hurunui/Kaikōura Earthquakes Recovery Order 2017 is required to have the façade and parapet secured to a minimum of 34% NBS within a strict deadline. It was important to preserve this heritage while improving life safety concerns.

CLIENT BRIEF/PROJECT CHALLENGES

There were several challenges that the project faced during its duration, the biggest one being the tight timeframe imposed by the Hurunui/Kaikōura Earthquakes Recovery Order 2017. The Order relates to buildings in Wellington City, Hutt City, Marlborough District and Hurunui District Council that have one or more URM parapets or façades that are <34% NBS. These URM elements are at a high risk of falling onto one of the public roads listed in the Order Schedule during a moderate earthquake event, therefore required urgent URM securing works. In 2017, the Kennedy Building was identified by Wellington City Council (WCC) as potentially falling within the parameters of the parapet and facade securing requirements.

WSP were commissioned to assess the façade and parapet in accordance with the requirements of the Order. It was found that, while in-plane and out of plane strength of the Cuba Street façade is relatively high, the diaphragm connection between the façade and the timber floors achieved a rating of 20-30% NBS, and the parapet above roof line achieved a rating of 25% NBS. The façade and parapet securing works were required to be completed with a very tight deadline, with fines up to $200,000 and evacuation of the building and surrounding street being enforceable by WCC if the time limit wasn’t met. The team were required to assist the building owner in engaging contractors, completing the detailed design, and construction to be finished within an 18-month (initially 12 month) time frame. Although it was a reasonably conventional strengthening design, several construction challenges were enounced.

The tight timeframe meant the construction contract was established and set up based on a strengthening concept design. Due to this, a cost reimbursement contract was utilised, and as a result of the complexity of the steel work design, two contractors were engaged. The main contractor was responsible for the enabling works, temporary protection, and site health and safety. The second contractor was engaged as a separate contractor responsible for the measure, supply and installation of the steelwork. Managing expectations of the tenants and the tailoring the methodology of the construction around their needs was required. Tenancies include a tattoo studio, and a photography and modelling studio, both requiring strict control of dust and minimal disruption during working hours.

The team was required to work within the council regulatory requirements which included fortnightly inspections, reporting on progress, and submitting all the necessary documentation prior to the deadline. The seismic strengthening was not to alter the exterior of the building due to its heritage and visual character. Strengthening an existing building also involves working around existing in ceiling services. The strengthening steel on levels 1 and 2 was designed to be within the ceiling void. The designed placement of the steel members clashed with existing sprinkler pipes which required draining and penetrating through the new steelwork before being recommissioned. 

OUR SOLUTION

The project was completed within a compressed design and construction time frame that was imposed. Use of a horizontal truss with bolted steel sections was used to allow for ease of construction and provided additional resilience to the façade. The work was limited to just the front areas of the building that allowed the building tenants to continue to operate. The key objective was to strengthen the façade and parapet within the time limit and in such a way that the remainder of the building can be strengthened in the future. The design required consideration of particular construction details to de-risking the pressures of the programme. It was also important for the exterior of the building to remain unchanged, to preserve its heritage visual features. The securing works are limited to the western (Cuba Street) façade and parapet of the building, and include: •

  • strengthening the connection of the façade to the restraining diaphragm at levels 1, 2, and roof;
  • improving the load path between the façade and return walls at levels 2, 3 and roof;
  • improving behaviour of the parapet.

Under the Hurunui/Kaikōura Earthquakes Recovery Order, the strengthening works are exempt from all consent requirements. The design however, still requires consideration of the heritage values, to the extent that is reasonably practicable in the circumstances, and the implementation of the strengthening should also not adversely affect the current use of the building when finished. A site visit involving the building owner, contractors, and consultants, confirmed the need for intrusive investigations to finalise the detail of the design and confirm contractor methodology. This included access to the ceiling voids and to the roof area to inspect the parapet.

For urgency and safety reasons, a drone survey of the façade, parapet and roof was undertaken. The information gained from the photos, videos and 3D model of the façade and parapet was used to establish the extent of the strengthening and the weather tightness remediation at the connection between the roof and parapet. The data gathered during this exercise was invaluable during the detailed design phase as much of the concept was based off assumptions and dimensions taken from the original drawings.

OUTCOME/CLIENT BENEFITS

The design is elegant in the way that it respects the heritage aspects of the building, with visible strengthening on level 3 incorporated into the existing building fabric through careful use of composition, detailing and applied finish. Existing weathertightness issues between the roof and parapet were addressed with the addition of a false wall added behind the length of the parapet, encapsulating the new strengthening work. The team also utilised modern technology to obtain details of the existing building, collaborated effectively throughout the project, and was agile in its decision making which made for a very successful project.