Solid wood buildings – layers of wood glued together – are gaining popularity as greener and faster alternatives to concrete and steel structures. With recently updated new building codes allowing the construction of more solid wood high-rise buildings in the United States, many have wondered how such buildings would fare in earthquakes.
The TallWood Natural Hazards Engineering Research Infrastructure (NHERI) project aims to demonstrate the resilience of tall timber buildings by simulating a series of large earthquakes in a full-scale, 10-story solid timber building at the [email protected] facility – the world’s tallest full-size building ever tested on an earthquake shaker table. The research project is funded by the US National Science Foundation. The main feature of the building is a prestressed swing wall system made of solid wood panels. Under lateral forces, the wooden panels swing from the base and the post-tensioning rods re-center the system after the earthquake. This new system aims for resilient performance, meaning the building will experience minimal damage from structural-level vibrations and can be quickly repaired after infrequent earthquakes.
The project is being led by Professor Shiling Pei of the Colorado School of Mines and the structure was designed and built by a large team of academic and industrial collaborators. Professor Keri Ryan, from the Department of Civil and Environmental Engineering at the University of Nevada, Reno, has played a key role on the leadership team.
Non-structural systems research
Ryan and her team coordinated the development and integration of all of the building’s non-structural systems. “Resilient design must also consider the building’s non-structural systems, which are not part of the structural load-resistance system but play an important role in the function of the building and its ability to recover from the earthquake,” said Ryan, Project Co-researcher and Engineering professor at the university. Recent earthquakes have shown that in regions with modern seismic engineering, damage and loss of function tend to be concentrated in the non-structural systems.
Some of these non-structural systems include interior walls, stairs and elevators, and plumbing, electrical, and ventilation systems. “Most seismic engineering research to date has focused solely on the lateral drag of structural components,” Ryan continues. “However, non-load-bearing components are often badly damaged in earthquakes. After the 1994 Northridge earthquake, nonstructural components accounted for about half of the cost of damage to buildings. This highlighted the need to address vulnerabilities in non-structural systems.”
Even if a building remains structurally intact after an earthquake, non-structural damage can ultimately compromise its structural integrity. For example, damage to the siding (building envelope) can mean that the underlying structure is exposed to the elements. Many non-load-bearing components are also crucial for the safety of a building. Damage to stairs, for example, can trap occupants in a building after a natural disaster, such as the 2011 Christchurch earthquake in New Zealand.
The walls and stairs of the TallWood project
The seesaw wall system only works if the other components can move with it. If the building structure is designed to flex during an earthquake, the other building components connected between floors must move with it or risk being crushed or fractured. This is known as deformation compatibility. “We focus on interior walls, the building envelope and stairs as the main non-structural components,” said Ryan. “We work closely with designers and other specialists to figure out how to make improvements wherever we can.”
For interior walls, they are investigating an innovative solution known as “slide rail”. “This occurs when the top of the wall is not directly connected to the floor slab above and can therefore move or slide independently of the floor above,” Ryan said. This works well until those walls meet at corners, at which point there is a risk of collision and damage. Now the team is working on the finer points of minimizing damage to the corners. Ryan’s team is also one of the first to extend the sliding concept to exterior facades. On the lower levels of the building, four distinct exterior facade assemblies are noteworthy, each connected to the structure in different ways to prevent or minimize damage.
Stairs are also a big area of interest. “Stairs are typically rigidly connected between floors and are often damaged in earthquakes and are then considered unsafe for future use,” explains Ryan. “US building codes have changed to address this, but a lot of research needs to be done to get the best design.” Similar to interior walls, the best solution is to detach the stair from the ground at one end, so the stair between the two is not stretched or compressed when the floors move independently.
time to shake
The implementation of the project was not easy. Three years ago, Ryan formed an industry working group that met every two weeks and over time turned ideas into plans so they could test and compare multiple solutions to solve this challenge. With industry guidance, graduate students William Roser and Yi-En Ji developed the plan sets, details, and design calculations for all cold-formed steel walls. (Other components were detailed by industry partners.) Gradually, the working group participants donated almost all of the materials needed to build the assemblies, and one organization even helped build the facades as part of their training program.
Ryan and her team (Roser, Ji and graduate student Sir Lathan Wynn) have been on site since the fall to oversee construction, prepare cables, install sensors and cables, and perform numerous other tasks to prepare the building for testing. CEE professor Mohamed Moustafa will also make a notable contribution to the project along with postdoc Luna Ngeljaratan and PhD student Mohammed Ibrahim. They will use digital image correlation with cameras and targets to try to measure the building’s movement from base to top. Traditional drift sensors, string potentiometers attached to an off-table benchtop sensor don’t work well when the building is that tall.
The four-week test program begins now on the world’s only outdoor shaker table at UC San Diego’s Englekirk Structural Engineering Center and can be followed via live stream. The research program will carefully scale up movements from very frequent (small) earthquakes to a very rare (maximum expected) earthquake in order to develop a good understanding of how well the details hold up when the shaking intensity is systematically increased.
The non-structural scope of work was also supported financially by the US Forest Service, Softwood Lumber Board, Computers and Structures, Inc, Allegion and Exponent. Materials were donated by Construction Specialties, Allegion, Ehmcke Sheet Metal, CEMCO, USG, Simpson Strong-tie, Winco Window, Innotech Window and Door. The non-structural assemblies were installed by Southwest Carpenter’s Union, Ehmcke Sheet Metal, Long’s Glazing and Door, and Pacific Coast Drywall. The airtightness test of the windows is carried out by RJC engineers. The project could not be completed without the generous contributions of industry.
The scope of the non-structural components of this project is sponsored by NSF Grant #CMMI-1635363 and USFS Grant #19-DG-11046000-16. The use and operation of the NHERI shake table facility is supported by the NSF through Grant No. CMMI-2227407. All opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.