By Cathy Thomas – NZCE (Civil), BEng Tech (Civil), MIPENZ, CPEng, Chartered Structural Engineer and Structural Engineering Manager
Many Landlords have the perception that a Seismic Upgrade involves a major rebuild from the footings up. If a Building has substandard or inadequate foundations and vertical structures, they would need to be addressed, but that is not always the case.
Provincial building owners all over the country are faced with seismic strengthening of buildings to meet local authority compliance requirements. To meet seismic requirements may be difficult to justify as economically feasible in many cases. However, targeted strengthening as well as utilisation of all the buildings inherent strength can converge with a positive outcome as the following example demonstrates.
A Prendos Client , the owner of a two storey Commercial Building in a NZ Provincial Centre, with a 52%NBS rating explored the possibility of increasing the rating to >67%NBS. This was to satisfy a tenant’s demand. A large, recognisable, engineering consultancy were engaged to carry out a seismic assessment and consider the feasibility of strengthening the building to a minimum of 67%NBS. The concept proposed involved major structural work, with structural steel at ceiling level and extensive foundation work. The Ground Floor retail operators would be required to vacate the space for 4-6 months to complete the upgrade. Estimated costs were $200k to $250K. Upgrade expenditures at that level represented in excess of 25% of the replacement value of the Building, and such an upgrade would have destroyed the economic viability of the Building. It is noted that in the NZ Provincial areas seismic upgrades do not increase the value or rents of a building.
Dissatisfied with this result, the building owner engaged Prendos structural engineering for an alternative proposal. The concept provided utilised the existing timber floor over steel trusses as a structural diaphragm. A new GIB ceiling diaphragm was proposed as well as GIB brace wall panels to existing walls enhanced with steel strap braces. This design effectively connected all the upper floor elements and stiffened the structure. Therefore, the strengthening could be carried out from the upper level, with minimal disruption to the ground floor tenant, at a fraction of the cost of the initial concept proposed.
This approach could be used in this case as the building had full height, well reinforced concrete columns. Although the existing floor and walls would have some bracing capacity, a quantifiable value could be obtained by upgrading the wall and ceiling linings and improving the floor anchorage to the reinforced concrete perimeter beams. The effect of the stiffened upper level was a reduction in the cantilever action of the columns. The action of cantilevered columns is from the base. Therefore the upgrade of the foundations was not required because of the reduced cantilevered column action.
Providing good load paths and providing robust connection of building elements also improves the behaviour of buildings, particularly as they increase in height. The tendency is for forces to be amplified at the top of the building. This effect is exacerbated by poor connections. There is a section in AS/NZS 1170.5, Seismic Actions, called “Parts & Portions’. This section deals exclusively with the amplified effects of ‘parts’ of buildings, fixed to, but not part of, the building superstructure.
The second solution utilising the “diaphragms”, being the floors and ceilings with braced walls was to be carried out on the First Floor only, leaving the retail activities on the Ground Floor undisturbed or inconvenienced, and the office space on the first floor required to vacate for 4-6 weeks. The contract cost is $57k including reinstatement of the Offices. The Plans and Specifications of the Seismic Upgrade have been approved by a Peer Review and a Building Consent issued.
Post the Canterbury earthquakes there has been a significant focus on the assessment and enhancement of existing timber floors as structural diaphragms. Testing has been carried out on typical examples of tongue and groove flooring. An appropriately connected diaphragm interacts with URM (Unreinforced Masonry) in such a way as to provide restraint without being too stiff. Elements that are too stiff are likely to cause localised failure. This approach may be applied to URM buildings or to older reinforced concrete structures.
Provincial New Zealand is inundated with two storey seismically challenged buildings held by Investors and Trusts as quasi superannuation schemes whose focus is on responsible investments driving greater income streams, rather than creating higher capital values. Extensive capital expenditures are not attractive, and targeted seismic upgrade solutions utilising existing structural systems such as floor/ceiling diaphragms could be an attractive alternative in some cases.
Although a practical solution for two storey URM or reinforced concrete buildings, similar solutions are also used in larger, several storey buildings e.g. The Auckland Art Gallery, Whitcouls Corner. These both utilise timber floor diaphragms.