Avoid catastrophe

Another firm of structural engi­neers, Casper, Phillips & Associ­ates (CP&A), reports that a crane designed to meet its catastrophic load condition is able to resist the largest earthquake. A heavier and hence more costly steel structure is required but, says CP&A, when owners are asked if this is a price they are willing to pay for added protection, they usually accept it.

Having said this, the massive Kobe reconstruction project will be a fantastic consumer of steel. Steel prices seem set to rise sharply. This is bound to impact on the cost of large steel struc­tures such as cranes and the dif­ferential between a light and a heavy crane is going to widen.

When a crane is not tied down, says CP&A, the earthquake loads are limited to the load needed to tip the crane onto two legs. Once the crane starts to lift, says Bill Casper, the load cannot increase further because the addi­tional seismic energy is absorbed by the raising of the crane’s cen­tre of gravity.

The crane cannot tip over be­cause a fraction of a second later the load reverses. Earthquake tip­ping load is between 50 and 100 per cent of the crane weight de­pending on railspan and crane height. When the crane tips onto two legs they have to support the entire weight of the crane; other­wise the crane collapses and falls to the ground.

If the crane has been tied down, continues CP&A, there is no limit to the earthquake load­ing except that it will peak when the structure is absorbing energy as fast as seismic energy impinges on it. In a large earthquake this happens when parts of the steel structure are being plastically bent back and forth.

Earthquake forces increase in proportion to the strength of the crane, up to the severity limit of the earthquake itself. If the earth­quake is strong enough to bend all four legs, the crane is likely to collapse to the ground.

From the point of view of earthquake protection, then, tying down the crane is the worst thing to do. The situation is exactly analo­gous to the concept of “seismic iso­lation” for buildings.

Exactly one year to the day be­fore the Kobe disaster, on 17 Janu­ary 1994, a 6.7 Richter scale earth­quake severely damaged 31 Los Angeles area hospitals but the USC University Hospital, the world’s first “seismically isolated” building, rode out the earthquake with no dam­age at all.

Similarly, in the Kobe quake, the 6-storey West Japan Computer Center located 20 miles west of the city cen­tre and one of the world’s largest seismically isolated structures (ie the equivalent of a crane free to lift on the rails), suffered no damage at all. The peak force accelerations at the roof were lower by a factor of up to 9 (ie 0.07g-0.01g compared to 0.67g-0.97g) than those of a similar-sized but conventional fixed base building (equivalent to a tied crane