Safe Buildings Strengthening, retro-fitting, future-proofing our buildings to ensure the safety of users

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1 Construction Techniques Group Ltd 6 Neilpark Drive, East Tamaki PO Box Pakuranga Auckland, New Zealand T F pwymer@contech.co.nz Safe Buildings Strengthening, retro-fitting, future-proofing our buildings to ensure the safety of users 30 th & 31 st August 2011, Stamford Plaza, Auckland Day pm: Systems for Strengthening Structures Prior and Post-Earthquake Paul Wymer Managing Director, BBR Contech (Construction Techniques Group) Synopsis This paper will focus on utilising FRP systems for strengthening structures as part of planned structural upgrades and as a result of repairs and strengthening applied to damaged structures. FRP (fibre reinforced polymers) generally comprise of glass fibre fabrics or carbon fibre fabrics and plates. These materials have been is use in NZ since the early 1990s and have been applied to a large number of structures. FRP materials are now commonly used as a repair and strengthening technique because the technology is well-proven and the application can be carried out with a minimum of disruption to existing structure or business operation. The lightweight product can be applied to a wide range of structural elements and is usually easily disguised by thin coatings or wall coverings. This has a very small impact on the original fabric of a building and hence it is ideally suited to older buildings and heritage structures. This presentation is designed to provide an introduction to the application and use of FRP systems and reference this to a number of case studies. The written paper is configured to be complementary to the images and bullet point comments as contained in the PowerPoint presentation. The recent earthquakes in the Canterbury region have provided a real opportunity to examine the performance of FRP applications when subjected to actual seismic events. Although unintended, due to the close timing of successive earthquake events, we are also able to examine the behaviour of some newly repaired and strengthened structures (after September 2010 earthquake) to see how they performed during the significant events of February and June This is only an overview but it does provide some useful insights. There are other strengthening systems used for structural upgrading and this paper also provides an overview of these and the building types they may be applied to. There are also a number of new design configurations that may be applicable to new construction that can better resist earthquakes and minimise damage. The paper will conclude with some recent examples of these new configurations. 1. FRP System Description FRP systems mostly comprise glass fibre and carbon fibre. These systems are usually applied to concrete or masonry (concrete block, brick, stone) and are designed to enhance the flexural strength, shear strength or to better confine the concrete around reinforcing. The particular use of

2 a system will depend on the building type, type of strength enhancement and level of strengthening required. Not all systems are applicable to all building types and in some cases, FRP may not be suitable at all. Glass fibre fabric a stitched or woven fabric comprising strands of E-glass fibre (fibreglass as we commonly refer to it). This material comes in rolls which are about 50m long and 50mm wide. The fabric is approximately 0.5mm thick and weighs less than 100g/m 2. The glass fibre is saturated with epoxy resin and bonded to the concrete or masonry. Carbon fibre can be either in the form of a woven fabric or a factory pre-saturated pultruded carbon fibre plate. Carbon fibre has a higher tensile strength than E-glass and the dry material thickness ranges from 0.7mm-2mm depending on the specification. The fabric comes on rolls which are about 50m long and 500mm wide. The pultruded plate is also available in rolls and the plate has a range of widths (50-150mm) and is generally about mm thick. The advantages of this material for strengthening are: Low unit weight ( g/m 2 ) Low profile thickness (approx mm per layer) Does not impact on detailing or form of historic structures Ease of application due to lightness High E-modulus Excellent fatigue behaviour High strength to weight ratio Corrosion resistant Covering with a variety of thin plaster finishes and coating if required 2. Reasons for strengthening FRP is used as bonded reinforcement for the strengthening of concrete, masonry, stonework, timber and steel as a result of: Enhancement of load carrying capacity due to changed usage Upgrading to satisfy current building codes Seismic strengthening (increased load and ductility) Alteration to intended structural form Rectification of construction mistakes Enhanced durability 3. Selection of the appropriate FRP system The choice on whether to use a sheet or laminate system is based on the application and on the designers preference. The orientation of the main fibres in the FRP is also an important consideration. The applied forces are resisted by the main fibres, which may run in one direction only (uni-directional), or in two directions (bi-directional). Carbon fibre (sheet or laminate) appears to be more economic for use in flexural or shear strengthening. Carbon has better fatigue properties than glass and is preferred when required to carry fluctuating live loads. Glass, because of its lower modulus of elasticity, is more suitable for use in confinement of concrete.

3 Laminates can only be applied to plane surfaces and carbon and glass sheets are best suited to curved surfaces or wrapping situations. Glass fibre can be used for increasing the shear strength of masonry walls and lighter weight material is used where the substrate strengths are low, such as in old historic masonry or brick buildings. 4. Strengthening Applications Columns Axial load enhancement (without increase in vertical stiffness), confinement, increased ductility and if required, increased flexural strength are all advantages offered by FRP. Figure 1shows glass fibre being applied to columns on Princes Wharf to provide additional confinement. The column in Figure 2 is wrapped with multiple glass fibre layers to provide confinement and enhance axial capacity. Figure 1: Princes Wharf Figure 2: Stamford Plaza Beams Beam applications address flexural and shear deficiencies required for structural upgrading or seismic strengthening. Figure 3 utilises carbon fibre sheet and laminate for flexural and shear strengthening. Figures 4 and 5 show carbon fibre laminate being used for flexural strengthening and special carbon fibre L plates being applied for shear enhancement. Figure 3: Civic Centre Car park Figure 4: Grafton Bridge Figure 5: Grafton Bridge Walls Applied to concrete and masonry walls to provide additional resistance to both inplane and out-of-plane forces. Applications include concrete and masonry walls, web elements of concrete box structures, abutment walls and shear walls. Glass fabric is used for the shear wall strengthening at the Christchurch Arts Centre in Figure 6 and carbon fibre laminate provides strengthening around penetrations in Figure 7. Figure 6: Christchurch Arts Centre Figure 7: Queenstown Courthouse

4 Slabs Additional flexural capacity and deflection control. This may be required due to a change of use, introduction of an opening or repair of some structural deficiency. Figure 8 shows carbon fibre sheet being applied onto the slab to strengthen the perimeter of some penetrations that are to be inserted to house some new escalators. Figure 8: SkyCity Casino Durability Enhancement The durability of some concrete elements can sometimes be enhanced by containment with an FRP wrap. Depending on the application, FRP can enhance structural capacity and protect the element from further degradation. The circular columns shown in Figure 9 were originally formed using concrete pipe sections as permanent formwork. The aggressive marine environment had cracked the outer pipe sections and a glass fibre Figure 9: Central Bank of Samoa FRP wrap was able to contain the element and prevent further contamination and cracking. A final coating disguised the repair work. 5. FRP Case studies This section focuses on a representative sample of structural and seismic upgrading and discusses the application and performance of the FRP. Some examples were strengthened prior to the earthquake and others involve repair and strengthening post-earthquake. Images relating to this section can be referenced in the PowerPoint presentation. Christchurch Arts centre Considered the cultural centre of Christchurch, the Christchurch Arts Centre is also one of the city s most significant heritage sites established in 1873 as the University of Canterbury and acknowledged today as one of the best and most extensive examples of Gothic Revival architecture in the country. Now 135 years old, some of its buildings require seismic strengthening. With a Category 1 rating from the New Zealand Historic Places Trust, the Arts Centre s restoration plan requires meticulous attention to detail and compliance with the highest standards of conservation practice. This extends to the earthquake strengthening project at the former Arts School building, which is part of a wider plan that includes re-wiring, upgrades to corridors, toilets and kitchen areas and the installation of air conditioning units. The roof will also be strengthened with plywood and steel before being re-slated. BBR Contech strengthened five of the building s internal walls in 2008 with fibre-reinforced polymer (FRP) technology. The original wall linings were removed prior to strengthening. After some rebuilding of the underlying original volcanic basalt stone walls, BBR Contech applied about 200m 2 of SikaWrap unidirectional glass fibre fabric. The internal linings were then reinstated to return the rooms to near-original condition with no visible sign of the restoration work beneath.

5 This strengthening all took place prior to the September 2010 and February 2011 earthquakes and the strengthened buildings performed very well and to expectations. The buildings with the strengthened walls were amongst the few that suffered minimal damage. The engineering analysis and strengthening philosophy has been thoroughly tested in these earthquakes and gives some confidence that a carefully engineered regime can produce high performing outcomes on historic structures such as these. Shirley Community Centre The Shirley Community Centre in Christchurch is a masonry/brick structure. The structural engineer had identified seven individual wall areas requiring FRP strengthening using a glass fibre wrap. Work was carried out in early Initial preparation was required to the brick surfaces to application tolerances with approximately 60m 2 applied to walls in total. At the completion of the strengthening, internal linings were reinstated over the applied FRP. Access has been restricted to the building following the February 2011 earthquake but the building performed well after September 2010 and appears to have survived the February event as relatively intact. Lichfield St The strengthening of this three-level building in Christchurch is an excellent demonstration of using FRP to confine columns and to increase load-carrying capability and ductility. A glass fibre fabric was applied to the upper and lower 1400mm of rectangular external columns on the ground and first floors, to enhance column confinement. The works included removal of the surface plaster back to original concrete substrate, preparation of the underlying surfaces to meet surface plane tolerances, and application of a single layer of FRP. The FRP wraps were subsequently overlaid by solid plaster to build the column out to the original surface profile before painting providing an invisible seismic upgrade of the building. There were some irregular element shapes and some special FRP detailing was configured to work around these shapes and existing window frames. The building did remain standing after the February 2011 earthquake but it is within the dangerous red zone (close the CTV building) and the current decision on what its future will be is not known. However, it is evident that the strengthening did provide the necessary life protection during these severe earthquake events. BNZ Sydenham This four level building was relatively undamaged in the September 2010 and February 2011 earthquakes. Following an engineering structural assessment, it was decided that all 48 No. circular columns be strengthened with carbon fibre wrap for confinement purposes. This decision was taken to increase the strength of the structure for possible future events and became part of a general refit to provide greater comfort for new tenants. The Level 1 carpark, Level 1 tenancy and Level 2 tenancy areas were fitted out for a tenant during June 2011 with a brief delay following two large aftershocks on 13 June until the site was cleared to continue work. The Ground Level BNZ Sydenham retail operation was subsequently closed for a two week period in August to complete the last of the column strengthening with a total 48 No. circular columns strengthened full height from m high. Lancaster House This three-level building was strengthened in 2007 when a structural assessment identified that the existing 350mmx350mm concrete columns had insufficient confinement steel to meet current codes. A total of 28 columns had a glass fibre wrap applied to the top 700mm of each column and mm to the bottom depending on column location. There was restricted access to the

6 full column perimeter in some cases due to window frames and walls and special detailing was required to ensure the continuity of confinement. The building was inspected after the February 2011 earthquake and the columns looked in good condition although the building did suffer extensive damage elsewhere. Again, the strengthened elements contributed to keeping the building intact but it is understood that it will in fact be demolished due to the location and other damage. University of Canterbury ISS Building Commencing in August 2010 the work involved exposing the concrete substrate on 23 No. circular columns and application of two layers of glass fibre fabric to the top and bottom 600mm of each column. Of particular note is that the FRP strengthening works were completed on 31 August 2010 only 5 days prior to the September 2010 magnitude 7.1 earthquake. Subsequent inspection of the columns appeared to indicate cracks in the column confined behind the fibre as intended. Because the FRP had not been coated at the time of the earthquake, the fracturing beneath was visible and illustrated the success of the FRP in confining the concrete and preventing failure. The columns did not suffer any further damage post-february Orion Substations Approximately 10 Christchurch substations were strengthened with FRP in An engineering and risk assessment had identified that these structures were critical and the earthquake upgrading was designed to add additional shear strength to the concrete block masonry walls. To the best of our knowledge, this strengthening regime was successful in identifying critical infrastructure and providing additional protection for earthquake events. Christchurch retail complex A large retail complex suffered damage following the February 2011 earthquake and many hundreds of rectangular concrete columns suffered distress in the top and bottom regions. The damaged concrete was repaired and a carbon fibre FRP wrap solution was configured over a 1m length of column at the top and bottom. The FRP solution was fully tested after the 13 June 2011 earthquake and the FRP performed to expectations. There was some additional damage to a number of columns and adjacent beams but these repairs were able to be implemented relatively quickly. The complex did experience loadings in excess of its design requirements as a result of the 3 major earthquake events but it did perform to expectations. The strengthening has reinstated the damage and returned the structure to the required design level. 6. Other Strengthening Systems External post-tensioning This involves the addition of high tensile steel elements to enhance the flexural strength or shear capacity of an element. There are many examples of masonry (and concrete) walls being strengthened by running high tensile bars through the centre of the walls and then tensioning these bars to improve the strength. In fact, a similar technique was used in the Stamford Plaza when an additional 10 floors were added to the existing building in Concrete repair Usually, concrete repair is synonymous with any FRP strengthening project. There is usually some damaged concrete to reinstate or a requirement to improve the surface to which FRP is to be applied.

7 Ground anchoring Ground anchors are usually utilised when a building, tower, dam or other structure is exposed to high uplift forces during an earthquake. The structure requires tying down into the ground and ground anchors are commonly used. Grouting Improving the ground conditions around the foundation of a structure can enhance the performance under seismic events or to reinstate ground that has been damaged by an earthquake event. Examples of successfully applying ground anchoring techniques and grouting to enhance seismic performance can be found in the Canterbury region. Over the past 5 years, Transpower has undertaken a detailed regime of strengthening old transmission tower foundations so they are able to perform better when subjected to seismic events. In the Canterbury region, a large number of tower foundations have been strengthened and these performed to a very high level during the recent earthquake events. The towers that were strengthened were previously identified in high risk areas with respect proximity to roads/rail/buildings and where ground conditions were known to be marginal. Some of the strengthened towers were in ground that suffered from liquefaction and they performed admirably. The design for safe buildings applies equally to other structures that have to provide critical infrastructure requirements subsequent to major earthquake events. 7. Other technology for superior earthquake performance (future-proofing) As part of the lesson learned from Christchurch, it is noted that some other forms of technology may warrant some more serious consideration. Although all of the below mentioned technology is currently in use, there is growing interest in using this technology more widely as it did demonstrate some very good performance in the Christchurch region. The considerations and consequences of many months of downtime, loss of occupancy and sometimes irreparable level of damage is now receiving much greater attention from building owners. PRESSS Technology Precast Seismic Structural Systems (PRESSS) is a design and construction technique for producing damage resistant buildings. The concept provides building owners a significantly reduced cost (aggregated over the life of the building), reduced in-building accelerations during earthquake rocking and a fully precast system with advantages of speed, accuracy, safety and quiet site erection. The system can also be used for precast concrete and timber. There are 4 examples of this type of building in New Zealand: Figure 10: Victoria University Alan MacDiarmid Building. Photo courtesy of dunning Thornton

8 Alan MacDiarmid Building, Wellington. This award winning building utilises un-bonded post-tensioning and rocking joints within the structure to ensure the building returns to an upright position without significant structural damage, even after a major seismic event. This was the first multi-storey PRESSS building in NZ and it was completed in Figures 10 and 11 illustrate typical rocking joints with energy dissipaters. Southern Cross Hospital Endoscopy Building, Christchurch. Finished before these earthquake events, this five-storey building performed according to expectations. Based on the design philosophy and observed performance, there is strong support to see more of these buildings constructed. Figure 11: Victoria University Alan MacDiarmid Building. Photo courtesy of Dunning Thornton Nelson Marlborough Institute of Technology Arts and Media Building, Nelson. The NMIT building is a three-storey timber building that utilises the earthquake-resistant PRESSS technology. Like the concrete PRESSS buildings, it has the advantage of withstanding a major earthquake with only minor maintenance and repairs to restore the building before it is restored to use. It was opened in April Figure 12: Massey University CoCA Building Massey University CoCA Building, Palmerston North. This is another three-storey timber PRESSS building as shown in Figure 12 which utilises un-bonded post-tensioning through column and beam elements. The structure is nearing completion. Post-tensioned floors (elevated and on grade) There are some 30 buildings in the Christchurch region that have post-tensioned slabs. These buildings range in size from 1,000-35,000m 2. Subsequent to the earthquakes, the post-tensioned floor slabs have performed very well and not suffered damage. Some of the other elements in the buildings did suffer damage but the floor slabs withstood the significant vertical and horizontal accelerations. These floors are all tied together using post-tensioning (with minimal or no joints) and have a better ability to withstand variable ground conditions and will bridge over local areas of weakness. The minimal joints mean that there are no lines of weakness as in a conventionally reinforced floor slab which may have a large network of sawcuts. An observed added advantage was an undamaged post-tensioned floor had the ability to quickly clear damaged racking and stock and to re-establish new stock once the clean-up was complete. There is a cost premium to install post-tensioned floors for smaller buildings (but not for larger buildings) and there is a growing number of building owners who are exploring replacing damaged floors with post-tensioned configurations to mitigate damage should an earthquake occur. Figure 13 illustrates a 13,000m 2 post-tensioned slab at the K-Mart distribution centre in Auckland. This slab has no movement joints or saw cuts whatsoever.

9 Elevated post-tensioned slabs are also ideally suited minimising the damage that may result from earthquakes. These floors are cast in-situ and don t have some of the potentially troublesome precast connections. These types of slabs have been used in New Zealand for multi-level car parks and low level multi-storey buildings. The new building layouts for Christchurch might favour consideration of this type of construction alongside PRESSS type buildings. Figure13: A 13,000m 2 post-tensioned warehouse floor completely devoid of any movement joints or saw cuts. Photo courtesy of Goodman NZ Ltd. 8. Conclusions Applications, examples and case studies for the use of FRP strengthening systems have been outlined. This provides a general outline and introduction to the use of this technology for seismic strengthening and structural upgrades. The technology is not new to New Zealand and it has been around since the early 1990s. There are estimated to be close to 200 projects in New Zealand that have used this technology in a variety of different applications. The Christchurch earthquakes in 2010 and 2011 have provided a unique insight into how this technology has performed under real-life loading and the severity of the earthquake events has tested it to the extreme in some circumstances. Our knowledge in the design and use of FRP has been extended as a result of the earthquakes and there is ongoing research and development in New Zealand to extend our knowledge even further. The PowerPoint slides prepared for the Safe Building conference are available to delegates and should be referred to together with this paper. The design and application of FRP strengthening systems does require specialist knowledge and competence in this field of activity. There are many details that have not been covered in this presentation but the specialist advice should be sought to ensure appropriate materials and techniques are employed. Some other techniques for strengthening, retro-fitting and future-proofing our buildings were briefly outlined to provide an introduction to other forms of technology that also have a role in protecting our buildings and other essential infrastructure.