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Obituary: William S. Jewell, professor emeritus of operations research and early proponent of interdisciplinary research
I am sorry to hear about the passing of Prof. Jewell. I remember
that he had a reputation for being one of the toughest professors
in the IEOR department. I recall being very nervous prior
to enrolling in one of his courses. His reputation for being
tough was indeed justified, but after I survived his course
(I got a B+), I realized that he was among the top three best
teachers I had during my entire Cal career.
I hope the other professors in IEOR will maintain the level
of excellence Prof. Jewell expected of his students.
— Yohannes Hailu
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Bomb-Resistant Buildings
I read with interest the idea of using cables to strengthen building structures. Having been away from the engineering profession for many years as a result of some unplanned diversions of my career and long periods of unemployment, I have had an opportunity to observe many things I might not have otherwise been able to see.
I was living in San Francisco across from the construction site of a subsidized residence community known as "St. Francis Community" on Guerrero Street, and every day at meal time I observed the surveys, excavation, and techniques of construction in use on that building and later at another site of a similar nature at Jones Street and Golden Gate Avenue. The building on Guerrero Street is constructed using pre-stressed concrete flooring laid on reinforced concrete walls surrounding a massive central anchor foundation structure supporting elevators.
I was impressed with the manner in which the floors were constructed
and the method of their installation. They were prefabricated
and delivered to the site to be lifted into place on the supporting
walls, which are made of cinder block and filled with reinforcing
bars and concrete. Consideration of that manner of construction
leads me to believe that the use of cables as a structural
element is a "natural" to prevent the collapse of a building.
Retrofitting of buildings to establish seismic integrity is
established practice in San Francisco, and it seems that cables
might also be considered as an alternative or supplement to
massive external retrofitted structures I have seen in place
at several locations.
— David K. Eberly
The catenary cable system has many fundamental, negative characteristics
that may deem this system inappropriate for the prevention
of progressive collapse in a building. In simple terms, some
of the negative features of using the catenary cable alone
system are:
1. Since these high strength cables are cast into the floor's
concrete slab, the cable system can only be activated once.
A new cable cannot be reinstalled into the already hardened
concrete for protection against a future threat of progressive
collapse.
2. It would be very difficult to "tune" the correct
pretension in these multiple cables on each floor to assure
that each cable is participating equally in retaining the
corresponding floor loads. An unequal pretension would drive
only the most taut cables to carry most of the gravity loads.
This may lead to a localized overstress in these taut cables
and may lead to a secondary failure.
3. A floor deflection of 25+ inches at a location above the
lost gravity column may negatively affect other structural
systems that are not designed or fabricated to accomodate
this large amount of deflection. Assuming that a W16 wide
flange spandrel beam is being used along the building's perimeter,
and that the column-to-column bay length is 247 inches. A
drop of 25+ inches at one end of the beam would induce a 1-inch
horizontal movement in the beam web's top and bottom bolt
holes at the opposite end. Since there is only 1/16" of remaining
annular space in a standard bolt hole and 5/16" in a short
slotted hole, 1" of horizontal movement in the beam web may
yield the rigid bolted connection causing additional collateral
damage.
4. With a floor deflection of 25+ inches at a location above
the lost gravity column, the building's entire curtain wall
system along the two adjoining bays, from the ground to the
roof, would shed itself from its beam and column support systems.
This loss of curtain wall would be a danger to pedestrians,
the building's occupants and very expensive to repair in the
future.
5. With a floor deflection of 25+ inches at a location above
the lost gravity column, the building's entire floor system,
ceiling grid, and mechanical systems, along the two adjoining
bays, from the ground to the roof, would need to be demolished
and replaced. This loss of floor slab would also be a danger
to the building's occupants and very expensive to repair in
the future.
6. With a rapid floor deflection of 25+ inches at a location
above the lost gravity column, what happens to the building's
occupants and contents located within the vicinity of these
abrupt and large floor deflections (i.e., window offices)?
All floors, from the second floor level to the roof level,
would experience this 25+ inch deflection in the vicinity
of the lost column below — all occupants on all floors,
in the vicinity of the lost column below, would be affected
by the same rapid floor deflection that could tilt the once-flat
floor toward the now unprotected exterior (curtain wall has
shed) at a 10% +/- slope.
7. See ASCE 7-98, "Minimum Design Loads for Buildings and Other Structures," Sections 1.4 and 2.5 for recommendations on General Structural Integrity and Load Combinations for Extraordinary Events, respectively. Also read the ASCE 7-98 Commentary (page 215 and page 222) at the rear of this Standard for a good explanation on these Sections.
Conversely, a fully welded steel moment frame exterior wall
system, would deform into a vertical truss, deflecting only 2.5
+/- inches under the same blast scenario as noted above; approximately
one-tenth of the deflection calculated for a catenary cable
alone system. In addition to providing the building with a
structural system that responds better to lateral loads due
to wind or earthquake, a moment frame system would limit the
amount of collateral damage to the entire building, as a result
of only one column being lost at the ground level. "The structural
system should be designed in such a way that if an extraordinary
event occurs, the probability of damage disproportionate to
the original event is sufficiently small" — ASCE 7-98,
Section C2.5, Load Combinations for Extraordinary Events,
page 222. The question here may be — what level of damage
could be deemed "sufficiently small" to the building owner,
regardless of the recommendations noted in this ASCE Standard.
— Robert Levenson, P.E.
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