By: Chris Urmson and Sam Toulmin | Alan Reay Consultants Ltd. from Tilt Up Concrete Association
Tilt-Up construction has been a popular method of erecting buildings in New Zealand since the 1950s. It is widely used in low-rise commercial and industrial buildings due to its low cost relative to other construction techniques and the speed and ease with which buildings can be erected. With a population of roughly 400,000, Christchurch is New Zealand’s second largest city and contains several thousand Tilt-Up buildings. Over the past two years, Christchurch has been subjected to more than ten thousand earthquakes, beginning with a magnitude 7.1 earthquake on September 4th, 2010. As of May 2012, four earthquakes with a magnitude higher than 6.00 have been recorded, as well as 47 earthquakes with a magnitude between 5.00 and 5.99, and 360 earthquakes with a magnitude between 4.00 and 4.99.
While many of the earthquakes have caused damage to buildings, the biggest and most destructive earthquake occurred on February 22nd, 2011. This magnitude 6.3 earthquake caused the deaths of 185 people, has caused upward of $30 billion of damage, and caused the complete shutdown of the central business district (CBD), some of which remains closed to this day. Accelerations recorded during the February event were some of the highest ever recorded in an urban area anywhere on Earth, with vertical accelerations exceeding 2.2g in some locations. Horizontal accelerations corresponded to those for return periods of between 2500 years and 10000 years – well in excess of the typical 500-year return period used in design of most buildings. In some cases, this equates to horizontal forces more than twice the design level.
In this article, we examine how Tilt-Up buildings have performed during these extraordinary events. An overview of common repair methodologies is presented, as well as a brief discussion of the lessons that have been learnt as a result of observations made.
PERFORMANCE OF TILT-UP BUILDINGS
Most Tilt-Up buildings have performed well during the Canterbury earthquakes, despite the accelerations in many cases exceeding those predicted by design codes. Some examples of Tilt-Up buildings which have performed extremely well include West Fitzroy (7-storey apartment building); The Terraces on Chester St. (2-storey apartment complex) andAdmiralty Courts (4-storey apartment complex). Many Tilt-Up buildings experienced minimal structural damage and downtime. Damage occurred most frequently to panels under face loading and connections to steelwork, and as a result of land damage, and these are discussed in more detail below. Some minor damage was observed much less frequently to steel framing.
Wall panels on single-storey Tilt-Up buildings generally performed well. Most wall panels exhibited very little in-plane damage due to their high in-plane stiffness and strength. In-plane damage that did occur consisted of minor residual deformation and spalling at corners, the latter occurring due to rocking. Ductile shear wall panels which supported one or more suspended floors exhibited textbook flexural-shear cracking patterns. Panel damage due to face loading was more common, particularly in single-storey industrial buildings with full-height panels, where cracking patterns generally matched those predicted by yield line theory. A feature of the September earthquake was the large number of industrial Tilt-Up buildings where wall panels were damaged by collapsing stock and racking shelves. In some cases the force was sufficient to displace panels at their base.
Many Tilt-Up buildings in Christchurch use ductile cantilevered end-wall columns to support panels for face loading. A section of longitudinal reinforcing in these columns is designed to yield under earthquake shaking, with the rest of the column, the panel, the connections and the foundations protected against damage. End-wall columns in buildings subjected to high accelerations have performed as expected, forming a flexural hinge at the appropriate location.
Panel connection failure was observed in some buildings, although the number of cases where panels completely collapsed is very limited. In general, ductile cast-in anchors exhibited superior behavior to mechanical anchors, some of which pulled out under the excessive loads. Failures in panel connections to purlins and steel rafters were often caused by punching shear, displacement incompatibilities and anchors being close to panel edges.
Large parts of the city, particularly in the East, were subjected to high levels of land damage due to liquefaction. Liquefaction affects areas built over layers of loose sands and silts with a high water table. Earthquake shaking causes the soil to act like a liquid due to an increase in pore water pressure, and buildings constructed on such land will tend to settle (often differentially) and spread apart at the foundations. This liquefaction-induced settlement has been the primary source of damage to many Tilt-Up buildings, particularly low to-medium-rise commercial and residential buildings in the CBD, industrial buildings in Eastern areas and the city’s main sports stadium.
REPAIR METHODOLOGIES FOR DAMAGE IN TILT-UP BUILDINGS
Most damage to Tilt-Up buildings was of a cosmetic to minor structural nature, and able to be repaired following the Christchurch earthquakes, even in cases where buildings were subjected to liquefaction-induced settlement. In most cases, Tilt-Up buildings were available for immediate occupancy following the earthquakes.
Cracks to concrete elements wider than 0.3mm can be injected with epoxy to protect reinforcing steel, and areas of loose or spalled concrete can be repaired using structural mortar. Elements such as ductile shear walls and ductile cantilevered end-wall columns have been designed to protect the building by absorbing energy through damage. In these cases, injection of cracks would potentially lock in strains and reduce ductility in future events, so lateral load paths have been redirected through new elements. Damaged connections are replaced with more robust connections such as drill-and-epoxy type connections.
Typically, prior to the earthquakes, low-rise Tilt-Up buildings were constructed on shallow posthole or pad type footings. To repair building settlement and minimise the risk of future damage, underpinning of the building structure to suitable bearing layers is carried out using steel, concrete or timber piles. These new piles can then be used to jack the building back to level. Other methods for re-levelling include compaction grouting of the subsoils beneath the foundations and polyurethane foam injection. It should be noted that suitable bearing layers in the Christchurch region vary in depth from between five metres and twenty-five metres below ground level. In many cases this has rendered buildings uneconomic to repair.
CHANGES TO DESIGN AND CONSTRUCTION PRACTICES
As a result of the ongoing Canterbury Earthquakes, there have been a few changes to design practice, with the aim of further improving the performance of Tilt-Up buildings. Firstly, in May 2011 the seismic load for Christchurch and surrounding areas was in increased by 36% for Ultimate Limit State design, and by 80% for Serviceability Limit State design. The higher serviceability value recognizes that the region is currently experiencing a high level of ongoing seismic activity. Temporary works design is being carried out for full current earthquake loads, rather than the 25-year return period permitted by design codes. This is due to the ongoing seismic activity and the much higher likelihood that a moderate earthquake will happen during construction.
Secondly, it is important to consider the effects on critical connections when subjected to loads that are higher than those prescribed by building codes. For example, connections between Tilt-Up panels and steel frames that are ductile have more capacity for deformation under high seismic demands. Similarly, the connection from suspended floors to shear walls requires detailing to avoid the slip that occurs at overload. In addition, it is vital to consider the effects of earthquake-induced displacements. For example:
Well-detailed stairs allow movement at one end to avoid strutting large forces between suspended levels;
Systems with different dynamic characteristics should be seismically separated, such as when a tall flexible system occurs beside a short, stiff system;
Suspended ceilings, architectural linings and suspended services require adequate detailing to allow movement to occur where it is required
Thirdly, the Canterbury Earthquakes have confirmed that drawn wire mesh is unsuitable for suspended floor toppings. Mesh has been observed to fracture, while ductile reinforcing has been more successful under seismic demand. Additionally, for Tilt-Up panels with high seismic demand, ductile reinforcing is used instead of mesh.
One notable aspect of the Canterbury Earthquakes is the amount of damage that has occurred to the land itself. Some areas of the city have been deemed unsuitable for rebuilding on, and in general a greater level of geotechnical investigation is needed to determine whether land is suitable for building on, and what sort of foundations are needed. Typically deep piled foundations are necessary where shallow pad type footings were previously specified.
Many designers are taking the opportunity to introduce new structural technology into the rebuild of the city. For instance, the use of Damage Avoidance Design (DAD) is being implemented in several new buildings. Previous design philosophy has involved absorbing seismic energy through damage of key structural elements; however it has become clear that the post-earthquake economic cost of this approach is unacceptable. DAD combines elements such as rocking post-tensioned walls and frames, hysteretic and viscous damping devices, base isolation and other innovative features to absorb seismic energy in a way that protects the building against damage. Some of this technology may find its way into Tilt-Up construction.
Finally, the importance of good quality control and quality assurance has become highly apparent as a result of the Canterbury Earthquakes. In particular, the following components require extra attention for ensuring adequate performance under earthquake loads:
Bolting, welding, grouting of ducts
Strength and arrangement of reinforcing steel
Dimensions of critical components
SUMMARY AND CONCLUSIONS
The Canterbury Earthquakes were very destructive and in many cases were well above code-predicted earthquake accelerations
Tilt-Up buildings generally performed better than anticipated
With the exception of obvious design or construction errors, no Tilt-Up panels collapsed
In most cases, Tilt-Up building damage was repairable, even in situations where liquefaction caused large movement
Most Tilt-Up buildings were able to be occupied immediately after the major earthquakes
ABOUT THE AUTHORS
Chris Urmson and Sam Toulmin are structural engineers at Alan Reay Consultants Ltd. located in Christchurch, New Zealand. Dr. Alan Reay established Alan Reay Consultants in the 1970s. Since that time, the firm has steadily grown and is now one of the largest consulting engineering practices in its field in the South Island. Visit www.arcl.co.nz for more information.