Seismic Isolators Protect an Undulating Timber Airport Roof

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New main terminal at the Portland International Airport is equipped with pendulum isolation bearings

In a multi-year project, the main terminal of the Portland International Airport (PDX) in Oregon, USA, was expanded, renovated and equipped with seismic protection. The existing buildings are not designed for the newly applicable specifications and earthquake intensities. The approximately 32,500 m² new timber roof will be visually distinctive. The roof is modelled in arches and scalloped shapes, a design of ZGF Architects and KPFF Consulting Engineers. The roof rests on 34 approximately 16.2-metre-high steel Y-columns filled with concrete, which are arranged at a distance of more than 30 metres.

Beyond the appearance, the seismic isolation of the wooden beauty posed a challenge. The seismic design aimed to cover all possible load cases during normal operation and in the event of earthquakes.

Per code requirements, the structure was analysed for the MCER (Risk-Targeted Maximum Considered Earthquake) case, which is approximately equivalent to the maximum conceivable earthquake in Portland in 2,500 years. In addition the structure was also analysed for the deterministic seismic hazard of a Mw 9.0 event on the Cascadia subduction zone which is approximately 100 km west of Portland. In the end, this resulted in large roof displacements of ±406 to ±572 mm.

These large displacements are accommodated by SIP®-D-Bearings; see below for details. The design of the bearings was developed in close coordination between KPFF Consulting Engineers and MAURER Munich.

SIP®-D: Double Sliding Isolation Pendulums

SIP® stands for Sliding Isolation Pendulum. They have four functions:

• They isolate and separate the terminal roof from the columns and allow horizontal movement in all directions.
• They limit the seismic movements through internal friction by converting kinetic energy into heat.
• They re-centre the roof back to its original position following an earthquake due to their concave sliding surfaces.
• They transmit vertical loads of up to 4,115 kN.

“D” for Double, indicates that the bearings have two concave surfaces (instead of one) between which a puck slides. The reason for this detail is that the large horizontal seismic displacements would require very large bearings. In the case of SIP-D bearings, the displacement is evenly distributed over two concave surfaces which reduces the required diameter by approximately one third. This means that SIP®-D-Bearings can be built smaller, which complied with the architects’ specifications.

High-performance sliding material MSM®

“Overall, however, it was not only about the design of our bearings. The requirements for movements and superimposed loads in a highly confined space could only be met with our high-performance sliding material MSM®,” reports Mark Kaczinski, MAURER USA.

MSM® stands for MAURER Sliding Material. In contrast to other alternatives such as PTFE, it is PFAS-free. MSM® also has a much longer service life (more than 50 years) and double the compressive strength. The bearings can therefore be built smaller and the largest have a maximum diameter of only 1,000 mm.

A total of 68 Double Sliding Isolation Pendulums from MAURER lie on top of the two tines of each Y-column. The bearings have been designed for the most extreme seismic loads and an extensive test programme was therefore carried out.

Several test series in special testing laboratories

Prototypes of all four bearing types were manufactured in their full-scale size and dynamically and statically tested at the EUCENTRE (Pavia).

To simulate real earthquake conditions, the bearings were tested at velocities up to 1 m/s and up to ±380 mm displacement with a maximum 420 tonnes superimposed load. Even more challenging, half of the prototypes were tested again in SISMALAB (Crispiano) to ensure that the subsequent production tests also provided valid results. These tests simulated more than three maximum earthquakes. “Even with these high requirements, the bearings showed no signs of wear or damage,” reports Mark Kaczinski.

The combination of a robust design procedure and SIP®-D-Bearing performance will ensure passenger safety and significantly reduce building damage from the worst possible earthquakes and allow the airport to resume operations without a long interruption.

Phase one of the new PDX Airport Terminal is scheduled to open in August 2024 with full completion in late 2025.

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