SEISMIC PERFORMANCE OF LOW-RISE PRE-ENGINEERING BUILDINGS WITH TILT-UP AND MASONRY FAÇADE WALLS DURING THE CANTERBURY EARTHQUAKES IN NEW ZEALAND

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1 10NCEE Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska SEISMIC PERFORMANCE OF LOW-RISE PRE-ENGINEERING BUILDINGS WITH TILT-UP AND MASONRY FAÇADE WALLS DURING THE CANTERBURY EARTHQUAKES IN NEW ZEALAND A. Jain 1, C. C. Simsir 2 and B. Arya 2 ABSTRACT The Canterbury earthquakes sequence ( ) caused widespread damage and destruction in and around the city of Christchurch. At the time of the earthquakes, a large percentage of the low-rise industrial building stock in Christchurch was built using pre-engineered steel moment frame (portal) buildings and their exterior cladding walls were either concrete tilt-up panels or brick or concrete masonry blocks. Portal frame columns were typically supported on shallow concrete pads. Where tilt-up concrete cladding panels were used, only the tilt-up wall ends are supported on the steel column pad foundations, whereas the remaining tilt-up panel edge between the foundation pads are supported on continuous shallow concrete beds. The large magnitude of the ground motions from multiple events coupled with the extensive liquefaction that affected large regions in Christchurch had a devastating impact on these low-rise industrial buildings causing extensive damage to them. Severe damage occurred to the connections between the tilt-up wall panels and the steel portal frame columns. Even though the light-weight roof framing reduced seismic forces, liquefaction-induced ground settlement caused extreme stresses in the structures. Differential settlement and liquefaction within the structures destroyed the concrete slab on grade floors, racked the tilt-up walls and steel framing and caused extensive damage to the non-structural components and other building finishes. Simplified structural analysis can be used to evaluate the damage caused to these buildings by multiple earthquake events. Finally, lessons learnt from the observations of structural performance of these structures during the Canterbury earthquake sequence are incorporated into methodologies for repair and retrofit of these structures. 1 Principal, Weidlinger Associates, Inc., Marina del Rey, CA, Associate, Weidlinger Associates, Inc., Marina del Rey, CA, Jain A, Simsir CC, Arya B. Seismic Performance of Low-Rise Pre-Engineered Buildings with Tilt-up and Masonry Façade Walls During the 2010 and 2011 Canterbury Earthquakes in New Zealand. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

2 Seismic Performance of Low-Rise Pre-Engineered Buildings with Tilt-up and Masonry Façade Walls during the Canterbury Earthquakes in New Zealand A. Jain 2, C. C. Simsir 2 and B. Arya 2 ABSTRACT The Canterbury earthquakes sequence ( ) caused widespread damage and destruction in and around the city of Christchurch. At the time of the earthquakes, a large percentage of the lowrise industrial building stock in Christchurch was built using pre-engineered steel moment frame (portal) buildings and their exterior cladding walls were either concrete tilt-up panels or brick or concrete masonry blocks. Portal frame columns were typically supported on shallow concrete pads. Where tilt-up concrete cladding panels were used, only the tilt-up wall ends are supported on the steel column pad foundations, whereas the remaining tilt-up panel edge between the foundation pads are supported on continuous shallow concrete beds. The large magnitude of the ground motions from multiple events coupled with the extensive liquefaction that affected large regions in Christchurch had a devastating impact on these low-rise industrial buildings causing extensive damage to them. Severe damage occurred to the connections between the tilt-up wall panels and the steel portal frame columns. Even though the light-weight roof framing reduced seismic forces, liquefaction-induced ground settlement caused extreme stresses in the structures. Differential settlement and liquefaction within the structures destroyed the concrete slab on grade floors, racked the tilt-up walls and steel framing and caused extensive damage to the non-structural components and other building finishes. Simplified structural analysis can be used to evaluate the damage caused to these building by multiple earthquake events. Finally, lessons learnt from the observations of structural performance of these structures during the Canterbury earthquake sequence are incorporated into methodologies for repair and retrofit of these structures. Introduction The low-rise industrial building stock in Christchurch sustained extensive damage during the Canterbury earthquake sequence which impacted the country with devastating consequences. Extensive liquefaction also occurred in Christchurch during the February 22, 2011 and subsequent earthquakes. Low-rise industrial buildings which are the subject of this paper had a slab-on- grade (tied and floating) foundation. The roofs of these buildings typically consisted of timber framing between the portal frame beams and sheathed with corrugated metal sheeting. Typically two steel weld plates were embedded along each vertical edge of the tiltup wall panels and welded to the steel portal frame columns to secure them in place. The large magnitude of the ground motion (both horizontal and vertical) along with the 1 Principal, Weidlinger Associates, Inc., Marina del Rey, CA, Associate, Weidlinger Associates, Inc., Marina del Rey, CA, Jain A, Simsir CC, Arya B. Seismic Performance of Low-Rise Pre-Engineered Buildings with Tilt-up and Masonry Façade Walls During the 2010 and 2011 Canterbury Earthquakes in New Zealand. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.

3 accompanying liquefaction caused significant damage to these low-rise industrial buildings. Ground motion induced accelerations caused severe damage to the connections between the tiltup wall panels and the steel portal frame columns. This paper describes the damage observations made by the authors on several of these structures of different design and construction vintages and compare their relative performance. Description of Buildings The buildings investigated by the authors were located in throughout the greater Christchurch region, outside of the Central Business District (CBD), where the most severe damage was reported. These industrial-type buildings are usually tall one-story structures with a height of approximately 20 feet. Usually, a light-framed mezzanine level is constructed within the structure to house offices for building staff. The most common structural configuration of these types of buildings consists of steel portal moment frames spanning in the building s transverse direction. The floor plan of one such building is shown in Fig. 1 for ease in describing the structural system. The lower two-thirds of the steel portal frame columns are usually encased in concrete to provide composite action. Solid black lines along the building s perimeter and through the middle of the building represent the location of tilt-up wall panels. The location of the steel portal frame columns are indicated by blue I sections. The intermediate red lines indicate the location of control joints in the slab-on-grade floor of the building. Partition walls and floors in the mezzanine and office regions are light framed timber structures. An overview of a typical building s structural system can be gleaned from Fig. 2. Although Fig. 2 shows bracing elements in the roof diaphragm, the tilt-up wall panels along the building s longitudinal direction do not provide a consistent load path to resist seismic forces. Most buildings investigated by the authors were designed and built subsequent to the 1976 building standard [1], which introduced more stringent seismic design code requirements. Figure 1. = Portal Frame Column = Slab on Ground Control Joints Building floor plan showing structural system. = Slab on Ground Cracks (Larger than 3 mm)

4 Figure 2. Overall view of bottom portion of building in Fig. 1. Ground Motion during Canterbury Earthquakes Sequence The Canterbury earthquake sequence commenced with the magnitude 7.1 Darfield earthquake which occurred on September 4, The Darfield earthquake was followed by numerous earthquakes, three of which were magnitude 6.0 or larger, close to the city of Christchurch: Magnitude 6.3, February 22, 2011 Christchurch earthquake; magnitude 6.3, June 13, 2011 earthquake; and magnitude 6.0, December 23, 2011 earthquake. Fig. 3 presents the magnitude, epicenter, fault rupture, and aftershocks associated with the four main events of the ongoing Canterbury earthquake sequence as recorded until July 2012 [2]. As indicated in Fig. 4, one of the buildings investigated was located close to the Christchurch CBD, where several ground motion recording stations were located [3]. Table 1 shows a comparison of the peak ground accelerations (PGA) of recorded instrument data at the four stations from the four main earthquake events [4]. Vertical accelerations were larger than horizontal accelerations during the February 2011 event at the stations located to the south of the CBD and closer to the epicenter of the earthquake. Fig. 5 shows the acceleration response spectra for the ground motion recorded at the stations during the February 22, 2011 earthquake. The recorded data shown in Fig. 5 were rotated to match the north-south and the east-west orientations for consistency. In addition, Fig. 5 provides a comparison of the recorded spectra with the 2004 New Zealand building code design spectra for 2500-year and 500-year (ultimate limit state load case) return period earthquakes [5]. The spectral accelerations for the February 2011 event were twice as large as the 500-year design values in the range of fundamental periods of vibration for the five buildings of interest (The hazard factor for

5 Christchurch was revised after the recent earthquakes to reflect a 36% increase). The ground motions at building sites were estimated by using a spatially weighted averaging technique [6] for the acceleration histories recorded at the four closest stations. Figure 3. Seismicity map for the four main events of the Canterbury earthquake sequence. Figure 4. Location of the five buildings and the recording stations in Christchurch CBD.

6 Spectral Acceleration (g) Spectral Acceleration (g) Table 1. Vertical (V) and horizontal (H) PGA (g) from ground motion recording stations. Station Code September 4, 2010 February 22, 2011 June 13, 2011 December 23, 2011 V (g) H (g) V (g) H (g) V (g) H (g) V (g) H (g) CHHC CBGS CCCC N/A N/A REHS North-South CHHC CBGS CCCC REHS NZS1170:5(2004) 2500-yr NZS1170:5(2004) 500-yr East-West CHHC CBGS CCCC REHS NZS1170:5(2004) 2500-yr NZS1170:5(2004) 500-yr Period (s) Period (s) Figure 5. Response spectra at recording stations for the February 22, 2011 event. Damage Description Investigation of earthquake induced damage to the building performed by the authors included documentation of visual damage to structural and non-structural building components, levelness surveys of slab-on-grade floors, and measurements of wall plumbness. Wherever liquefaction affected the buildings, the most prominent damage was undulations in the slab-on-grade floors caused by differential settlement (Fig. 6). Remnants of liquefaction were still apparent in the wide cracks (up to 2 inches), which were frequently observed to have vertical offsets across the cracks. A floor levelness survey of the building shown in Figs. 1 and 2 (see Fig. 7) indicated that the floors were out of level by almost 7.5 inches (190 mm). These floor levels translate to localized slopes in excess of 1:50. Coupled with the extent of damage observed to the floor slab-on-grade along with the likelihood of voids below the uneven floors had rendered the floor space virtually useless for any industrial function. Per the Detailed Engineering Evaluation (DEE) guidance report prepared by the New Zealand Engineering Advisory Group [7] after the Canterbury earthquakes, these floor level variations places the building in the highest level of foundation damage.

7 Figure 6. Damage to floor slab on grade (m) Figure 7. Floor levelness measurements. = Lowest Points (0.0 mm)

8 Tilt-up wall panels used for cladding of these industrial buildings were typically 6 inches (150 mm) thick. Tilt-up wall panels and their embedded steel weld plate attachment to the steel portal frame columns exhibited damage in the form of displaced wall panels, concrete cracking (up to 2 mm wide), rupture and toe crushing, wide separations between adjacent wall panels, cracking and spalling around the steel weld plates, and separation and cracking of steel portal column encasements. Some of the damage observed to these tilt-up wall panels is shown in Fig. 8. The red arrows and numbers in Fig. 7 indicate the direction and angle of wall panels from the horizontal. The residual drift in the walls panels is clearly identifiable and is consistent with the floor slopes. Figure 8. Damage in tilt-up wall panels. Front elevations of these industrial buildings were usually lined with a combination of brick veneer walls, doors and windows. Similar to the tilt-up wall panels, the large magnitudes of the ground motion during the Canterbury earthquake caused the brick masonry walls to sustain large

9 cracks, separations along cold joints, and displacement various building components relative to the brick masonry. Fig. 9 shows some examples of damage observed to the brick masonry walls. Figure 9. Damage to brick masonry walls. Other types of damage to these industrial buildings observed by the authors included liquefaction-induced undulations to the building grounds, cracking of light-framed wall finishes, shifting and splitting of roof wood framing, racked door frames, damaged window frame, and water leakage through separations and cracks in wall and roof panels. Analysis of Damage The high intensity of ground motion during the Canterbury earthquake sequence subjected the wall cladding to large accelerations in all three directions. Most areas in and east of Central Christchurch and locations in proximity of the Avon and Heathcote Rivers, experienced significant liquefaction during multiple events of the Canterbury earthquake sequence. These seismic actions subjected the tilt-up wall panels and masonry walls to high in-plane and out-of-plane acceleration and differential settlement along wall foundations resulting in the damage previously described in this paper. The resulting overall damage to the buildings was often severe and in some instances

10 led to evacuation of the premises due to structural instability, unsafe work conditions or loss of functionality. Evaluation of the tilt-up wall panels indicate that their method of construction and the unanticipated seismic forces on them, frequently left them in a severely damaged state. The tilt-up wall panels were originally intended to carry in-plane seismic forces, however, other significant demands on them during the earthquakes and the lack of a positive connection to the foundations rendered them vulnerable to the damage they experienced. The large expanse of the region which experienced liquefaction during the earthquakes made the wall panels further susceptible to damage from differential settlement. As described in this paper, these industrial buildings were exposed to a series of large earthquakes and numerous small ones over the course of a year and a half. In some instances documentation of damage subsequent to each of the major events clearly indicated the progression of damage sustained by these buildings. Temporary repairs were implemented in some buildings with the intent of stabilization, however, the large number of earthquake events caused worsening of damage to both structural and non-structural components. Depending on building specific requirements, the relative contribution of each earthquake event to the total damage in the buildings was determined by analyzing the nonlinear response of an equivalent single-degree-of-freedom (SDOF) mathematical model of the building subjected subsequently to the estimated site-specific ground acceleration history of each event and by monitoring the energy dissipated through hysteresis. The nonlinear analyses accounted for the second-order (P-Delta) effects and the cyclic degradation of the strength and the stiffness of the structural system. Arya et al. [8] present this analysis technique and methodology developed in response to the Canterbury earthquake sequence to quantify the damage apportioned to each event. Fig. 10 shows the displacement response history of the SDOF system for the building shown in Figs. 1 and 2 and the force (kips) versus displacement (inches) hysteresis along its principal axis. Displacement-Time Force-Displacement Figure 10. Displacement and force-displacement response of building along its axis. Conclusions The concrete tilt-up wall panel and masonry clad steel portal frame buildings described in this

11 paper sustained significant incremental damage from each of the major events of the Canterbury earthquake sequence. The ground accelerations experienced during the Canterbury earthquake sequence were significantly higher than what had been anticipated for building design in the region, prompting the upward revision of design earthquake loads. Damage sustained by the buildings was further exacerbated by the widespread liquefaction that was experienced during the Canterbury earthquake sequence as well. The types of damage sustained by the tilt-up wall claddings panels included panel instability, severe cracking, separation between the wall panels leaving the building interiors exposed to the elements, and collateral damage to the steel portal frames components. Damage to the tilt-up wall panels was further amplified by severity of forces unanticipated in original designs. Similarly, the brick masonry walls experienced severe cracking. Very similar types of damage were induced in the various buildings of this type investigated by the authors. While most of these buildings investigated by the authors did not experience collapse as a result of the earthquakes, many had to be evacuated due to concerns for life safety. The authors also noted that the intensity of damage reduced in the west regions of Christchurch, which also had minimal liquefaction. Damage to the tilt-up wall panels and brick masonry walls can be remediated by locally repairing or replacing the damaged components. However, any repair methodology would have to first address the building s levelness and verticality which implies foundation remediation. Depending on the extent and location of damage, results from site-specific geotechnical exploration and building specific repair methodologies would need to be developed. Further, design and construction of remediation work would need to consider the high likelihood of liquefaction-induced building settlement during stronger future earthquakes. References 1. NZS 4203:1976, Code of Practice for General Structural Design and Design Loadings for Buildings. Standards Association of New Zealand: Wellington, GNS Science web site: 3. GeoNet project, GNS Science: 4. GeoNet project, GNS Science: ftp://ftp.geonet.org.nz/strong/processed/proc 5. NZS :2004, Structural Design Actions, Part 5: Earthquake Actions New Zealand. Standards New Zealand: Wellington, King SA, Hortacsu A, Hart GC. Post-earthquake estimation of site-specific strong ground motion. 13 th World Conference on Earthquake Engineering, Vancouver, BC, Guidance on Detailed Engineering Evaluation of Earthquake Affected Non-residential Buildings in Canterbury, Part 3, Section 5. Engineering Advisory Group: Revision 3, 16 May Arya B, Jain A., Simsir CC, Park, K. A simplified approach to quantify damage contribution of individual events in canterbury earthquake sequence. Proceedings of the 10 th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.