CLT in seismic retrofits

By - , Build 146

US research has found cross-laminated timber with elastic steel wires is an effective and affordable solution for seismic retrofits of low-rise timber structures. Could this help to strengthen similar buildings here?

Figure 2: CLT panel retrofit assembly with elastic steel wires attached to foundation.

A RECENTLY COMPLETED research project in the US investigated methods of strengthening low-rise buildings with bottom storeys that could potentially collapse in a severe earthquake.

Many of the timber-framed buildings in the US, particularly along the Pacific Coast, are more than 1 storey, with the ground floor used either for parking or commercial space. This requires large openings and few partition walls, meaning the earthquake resistance of the first storey is lower than required to resist a major earthquake. Potentially, the lower storey could deform significantly or even collapse.

Looking for an economic retrofit

With large numbers of these types of buildings, the US National Science Foundation recognised the potential for extensive damage and funded a research project, Seismic risk reduction for soft-storey wood-frame buildings. The aim was to develop and validate economical retrofit concepts for these buildings.

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Full-scale model tested

A typical 4-storey full-scale model building was tested on an outdoor shake table at the University of California San Diego (see Figure 1).

The test building was designed to be representative of typical 1920s to 1930s San Francisco soft-storey timber-frame construction. The beauty of the shake table test is that it can shake the building in a similar way to an actual earthquake.

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Stressed CLT elements

One of the retrofit measures investigated to increase seismic resistance was the addition of rocking walls made of cross-laminated timber (CLT) at the ground floor level (see Figure 2).

This solution was an extension of the rocking timber shear walls concept recently developed in New Zealand by the Structural Timber Innovation Company (STIC). It basically uses CLT walls in conjunction with elastic steel wires to anchor the wall to the foundation.

As the structure leans sideways in an earthquake, the walls rock on their bases. Because the steel wires tying them down stretch elastically like stiff rubber bands, the walls return to their original position after the earthquake with no permanent deformation of the structure.

Additional elements can be added to absorb energy from the earthquake, helping to prevent it from rocking for too long as the earthquake passes.

Figure 2: CLT panel retrofit assembly with elastic steel wires attached to foundation.

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Retrofitting bottom storey only

The goal was to achieve an acceptable performance by limiting the retrofit to the weak bottom storey. This reduced the retrofit cost and time and avoided displacing occupants.

CLT panels were installed between the ground and first floors to provide extra stiffness and strength. These panels had to avoid interfering with the normal use of the space.

Plywood panels and additional framing were added to the underside of the first floor to ensure the forces from the shaking structure above were properly distributed to the tops of the CLT panels. The top of each panel was connected to the floor framing above using a wide flange I-beam.

Each line of shaking resistance consisted of doubled CLT panels. The panels were installed over CLT bases that were bolted to steel framing on the shaking table, representing the building foundation.

The test building was subjected to shaking similar to that recorded during the Loma Prieta 1989 and Cape Mendocino-Rio 1992 earthquakes. It was reduced to a level approximately matching the earthquake loading that would be used to design the building if it was new.

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Bottom storey bears the brunt

As expected, most of the damage sustained in the test was in the bottom storey associated with the rocking movement of the panels in the direction of the shaking.

Cracks often started at corners of openings and propagated during subsequent tests. Overall, moderate damage was observed in the bottom storey while the upper 3 storeys exhibited very little sign of distress.

The different types of damage were qualitatively classified to relate to a building performance level number. Very little damage is acceptable for a fully functional damage level, while some non-structural damage and minor repair works may be acceptable for continued operation.

Based on the observations, an inter-storey drift value of 1% – that is, 24 mm drift for a 2,400 mm storey height – was found to be the boundary value between the two levels.

Importantly, there was no damage observed during the testing that suggested the building had reached a state that threatened life safety or a collapse condition.

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Effectiveness proved

The shake table tests confirmed that CLT rocking walls can be used as an effective and practical seismic retrofit measure for timber-framed buildings with a weak bottom storey.

The researchers considered that the building could have survived even greater motions with only moderate non-structural damage and minimal structural damage.

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Could be used in New Zealand

Several buildings with similar configurations were significantly damaged during the Christchurch earthquakes, and the developed technology could potentially be employed in New Zealand.

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Note

Prior to coming to BRANZ, Asif Iqbal was involved with this project.

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Articles are correct at the time of publication but may have since become outdated.

Figure 2: CLT panel retrofit assembly with elastic steel wires attached to foundation.

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