Highrise Residential Building Retrofit: Precast Concrete Wall Panels

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1 B etter buildings Highrise Residential Building Retrofit: Precast Concrete Wall Panels Overview Elizabeth Towers is a highrise apartment building in St. John's, Nfld. In 1984, as Newfoundland and Labrador Housing Corporation prepared to sell it, they found minor leakage in sections of the roof and walls. An initial investigation found serious deficiencies with the precast concrete wall panels. After a detailed preconstruction condition survey of the building envelope, Newfoundland and Labrador Housing renovated the building between 1989 and 1992 at a total construction cost of $4,503,954. The main purpose of the renovation was to comply with the minimum requirements of the National Building Code and the Canadian Standards Association (CSA). The problem The consulting engineering firm that investigated the source of rainwater entering the building found bulging and cracking of the precast concrete wall.the firm also found general concrete spalling and noted inadequately supported precast concrete wall panels.the panels were poorly fastened to the building structure, which presented a serious safety concern. Assessment Inadequate Vertical Support

2 Steel clip angles did not support many wall panels, as called for in the original exterior wall design.two clip angles were to have lain on the edge of the concrete floor slab to support each wall panel.wood shims were used where clip angles were not present. In these instances, the lower panels supported the upper wall panels. For most of the wall panels, only one of the two clip angles was in fact bearing on the floor slab. The second clip angle was floating in mid-air and not providing any support. The 150-mm (5.90-in.) concrete block masonry walls were probably built before installation of the precast concrete panels, giving access to only one side of the precast panel during erection. Steel shims were placed below the clip angles in most cases where access was available. Inadequate Lateral Support A combination of steel straps and plates was used to support the precast concrete wall panels to the structure. Vertical steel dowels anchored the bottom and top panels together. All steel components had rusted.the concrete had also spalled at most of the lateral support locations. Bulging and Deteriorated Concrete Panels The inadequate anchoring to the structure caused bulging and cracking of the panels, while freeze-thaw cycles led to spalling.the concrete cover over the reinforcing bars measured from 13 to 38 mm (1/2 in. to 1.5 in.). For St. John's weather, it should have been around 50 mm (1.97 in.). Compressive strength testing and air content determinations also showed measurements that did not meet CSA A23.1 requirements for structurally-reinforced concrete exposed to freezing and thawing conditions. Restoration Options The serious safety concerns created an urgent need to repair the problems. The retrofit work had to adhere to several criteria.

3 The new exterior wall had to be designed and erected to meet the minimum requirements of the National Building Code of Canada, and relevant CSA standards. The work had to be performed from the exterior of the building to limit disruption to tenants. The existing concrete block masonry wall had to be stiffened, and the tie system had to accommodate variations in the vertical alignment of the building. Selected bricks had to resist cyclic freezing and thawing. Four restoration alternatives were evaluated. 1. Removing, repairing and reinstalling the precast concrete panels.this was considered impractical, since the extent of the work could only be determined after removing the panels, which would have meant an open-ended contract. 2. Removing the existing precast panels and replacing them with new ones.this option kept the original appearance of the building. It appealed to the owner, but was the most expensive. 3. Removing the precast concrete panels and replacing them with new steel cladding.the owners rejected this because they felt that steel cladding was inappropriate for a residential neighbourhood. 4. Removing the precast concrete panels and replacing them with new brick masonry veneer. This option was selected because it offered simpler construction procedures, and the bricks were readily available from a local manufacturer. The brick cladding was considered appropriate for a residential neighbourhood. The Work Construction of the new brick cladding involved removing the precast concrete wall panels and restoring the vertical and lateral support system.the contractor built a system of hollow structural steel columns.they spanned from floor to floor within the existing concrete block masonry. The steel columns were grouted into place to provide lateral support to the block work. Figure 4 shows the new steel framing system, which includes the existing framing, new shelf angles and brick tie locations.

4 The designer and contractor felt that the existing sprayed-on polyurethane insulation along the cavity side of the concrete block masonry provided adequate insulation and was a practical, reliable method of sealing irregular surfaces, acting as the air-water barrier. The contractor repaired sections of damaged insulation by applying new foamed-in-place polyurethane. New continuous steel shelf angles were installed on brackets at the face of the columns (shelf angles were installed above the windows to eliminate loose lintels). Brick ties were installed to anchor the new brickwork to the existing concrete block masonry backup and steel framing.the brick masonry tie system consisted of corrugated, hot-dipped galvanized steel ties.the 2-mm (0.08-in.) thick by 32-mm (1.26-in.) wide ties were anchored to the backup wall and to the steel framing. A finite element analysis determined the required thickness and strength of the brick ties. Quality control testing was performed on sample ties to ensure they met the appropriate CSA standard CAN3- A370-M84 "Connectors for masonry". During the construction, the fasteners used to anchor the brick ties to the concrete block masonry and to the steel framing were tested periodically. Standard modular clay bricks. 90 x 57 x 190 mm (3.5 x 2.24 x 7.5 in.). were selected, with a compressive strength of 65 MPa (9.4 ksi), a rating of SW (severe weathering), and tested resistance to freezing and thawing. Steel shelf angles support the brick vertically at each floor. Mortar was Type "S", containing Portland cement, hydrated lime and masonry sand. New self-adhered bituthane membrane flashings, weep-holes and movement joints were incorporated as needed. Scheduling The retrofit was first expected to take two years. However, when the contractor removed the precast concrete panels, they showed serious deterioration of the concrete masonry wall. In the end, construction took place over four years, from 1988 to 1992.A number of factors seriously affected the retrofit methodology and ultimately, timing and cost. Newfoundland and Labrador Housing Corporation preferred not to evacuate the building and relocate the tenants for the duration of the repairs and asked that the work be carried out with as little bother as possible. Most tenants, however, were relocated to either short-term (hotel) and long-term (apartment) accommodations. Tenants who remained during the work reported cold temperatures, water infiltration and leakage into apartments. Between 10 and 20 per cent of tenants moved out of the building. Because the building remained occupied during construction, the contractor had to follow strict safety procedures to ensure tenant safety. The Costs

5 Newfoundland and Labrador Housing awarded the contract by public tender. The initial timeline of two years and budget of $4,190,000 did not meet the construction requirements. Construction lasted four years at an actual cost of $4,503,954.The following table shows the approximate breakdown of construction costs. Given the total exterior wall surface of approximately 6,975 m 2 (76,000 sq. ft.) the total retrofit cost of the exterior cladding is estimated at $600 a square metre ($55 a square foot).the figures do not include design and consulting fees, or owner's management and engineering costs. In addition, Newfoundland and Labrador Housing paid about $195,000 for a pre-construction evaluation of the building envelope. The Newfoundland and Labrador Housing Corporation, a Crown agency of the Newfoundland Government, financed the construction costs with a capitalized bank loan. Results Approximate Construction item cost: 1992 dollars Replace precast concrete panels with brick veneer, including structural $4,203,954 reinforcement and insulation Replace the roof on seventh and eighth floors $200,000 Install new windows on first floor $80,000 Install new exterior lighting $15,000 Miscellaneous exterior painting $5,000 TOTAL $4,503,954 The retrofit work brought the building envelope up to National Building Code standards and eliminated the safety hazard.the marketability of the building is similar to what it was before the renovation.the current owners feel that the brick veneer is not as aesthetically pleasing as the precast concrete panels.they agree that brick veneer was the right solution, given the materials and construction costs of the other options.the retrofit work enabled the Newfoundland and Labrador Housing Corporation to sell the property in Contacts Original Owner: Newfoundland and Labrador Housing Corporation Retrofit design and supervision: J.W. Cowie Engineering Limited Architectural design: MacFawn Rogers Condition survey: Clayco Construction General contractor: Seabord Construction and Management Ltd If you have comments or would like to submit your project for publication, please contact: ldemigue@cmhc-schl.gc.ca

6 Seabord Construction and Management Ltd If you have comments or would like to submit your project for publication, please contact: