Upgrade Seismic Codes to Reflect Experience

* Your April 24 story “Design of CSUN Library Blamed for Quake Losses” highlights the need to carefully review our current seismic codes before we rebuild the same mistakes.

During the Jan. 17, 1994, earthquake, horizontal accelerations in excess of 80% of the forces of gravity and vertical accelerations in excess of 50% of gravity were measured. Current codes for seismic design are loosely arranged around peak horizontal accelerations of 40% of gravity and generally ignore vertical accelerations. It is time to mandate that codes reflect the kinds of ground movement which structures actually experience.

Had the CSUN library wings been designed for these substantially higher forces, especially the concurrent action of vertical uplift with horizontally induced uplift, the number and size of anchor bolts would have been significantly larger, and the base plate would have required vertical stiffeners, larger shear keys, better detailing of reinforcement near the shear key and larger foundations. These larger elements would not have cost anywhere near the $5 million to $7 million that the repairs will cost.

And sadly, when the repairs are done, the entire structure will not be much better off than it currently is. Some, if not all, of the other elements of the buildings such as the columns, beams, and bracing are still not designed for the real forces that these buildings experience in thrust-fault earthquakes.

Why do structural engineers continue to ignore vertical acceleration as well as higher horizontal forces? Two factors are at play here: First, there is the mistaken belief that it is more cost-effective to build a building that only provides life safety during a large earthquake but leaves an unusable building afterward. The earthquakes in Northridge and Kobe should have taught us otherwise.

Secondly, structural engineers are reluctant to admit that their designs for thousands of buildings that have been built since the 1971 Sylmar earthquake ignore the forces associated with the vertical-thrust earthquakes prevalent in California.

The earthquake, with measured horizontal acceleration in excess of 100% of gravity in some locations, should have prompted an immediate and substantial change in the philosophy of design for earthquakes in California. It did not fully achieve that effect. The current codes are significantly better than the codes in existence in 1971, but are still deficient if we want to have functional buildings after the quake hits.

We have been building ships for thousands of years that can easily resist accelerations far in excess of those generated by earthquakes. When will we finally accept responsibility to design buildings that can perform as well as ships?

DAN FEGER

Northridge