The building was constructed out of post-tensioned concrete, beloved of developers because it permits thinner slabs, longer spans and is more resistant to cracking. But it requires careful inspections and decent maintenance.
Illustration from the Seattle Times
According to an expert in Structure Magazine,
"Since the slab thickness is reduced, a developer building a high rise structure can easily add more floors without increasing the overall building height," said Scott Greenhaus, president of VSL. "Over the course of the building's life, this can represent significantly increase leasing revenue for the owner."
Unfortunately, it relies on cables being tightened after the concrete is poured, and that the cable ends be protected. In this case, according the owner, they were not properly sealed and water got in, rusting them out. (greater detail here)
The builder, one of the biggest in the country, says there's no problem:
We were surprised to read reports that the owner planned to evacuate tenants and demolish the building. Our experts, who are independent, highly-qualified structural engineers, have conducted sophisticated, thorough testing of the building components at issue and determined that it is entirely safe and that with reasonable remediation, maintenance and monitoring, long-term ongoing operations could continue.
Post-tensioned cables can be replaced; it has been going on for years, as they were often used in parking garages and exposed to lots of salt and moisture. But we are in the middle of a real estate crisis, and many investments are well under water. A lot of buildings are going to go to the dumps, a lot of embodied energy is going to wasted, and a lot of architects, engineers and builders are going to get sued as owners try to recover their losses.