Value is the ratio of function to cost. (V = F/C)
Of course, this is not a strictly numerical ratio, because cost can be defined in currency, but function is not objectively defined in dollars and cents. It can be easily seen that value is increased by increasing the function or decreasing the cost, or a combination of both. Therefore, two possible misinterpretations of value would be to turn it into yet another cost-saving mechanism, without a real evaluation of the function, or to incorrectly evaluate the function, or the potential function, either by setting the worth of some unexpected function too low, or completely ignoring a potentially valuable function.
In terms of structural engineering, the greatest danger in using value engineering in structural engineering building design methods would be to use measures to lower cost without realizing the corresponding loss of function, particularly in case of total failure.
In Bucharest, Romania, a civic center auditorium dome roof was built with a series of cost-saving measures which theoretically increased its value. Not long after being built, however, the entire roof “snapped” inside out under a relatively minor snow load. The roof was not extensively damaged but was found hanging suspended “inside-out” from the exterior walls.
Value Engineering Gone Awry in Structural Engineering Design
Domes have always used the principle of the eggshell, which when it is held in the palm with the long dimension of the egg parallel to the fingers, cannot be crushed by uniform pressure of the hand. Domes are built to enclose large open spaces without pillars blocking views, and are valuable for churches, sports arenas, auditoriums, and other large meeting places.
These domes — which are, like eggshells, generally rotated shallow parabolas — are tremendously strong compared to their weight, and exert only downward and outward forces at their outer edges, no internal support going to the ground is necessary … as long as the “eggshell” does not “crack.”
The cost-saving measures that were incorrectly evaluated as “value engineering” in the building design were that a network of steel tubes intersected at three angles to form a mesh to hold a relatively thin-shell roof above them. There were two sets of meridional tubes, which intersected at angles of approximately 60 degrees to each other, and another set of latitudinal rings that emerged from the center like ripples on a pond and were horizontal. These three sets of tubes were tied together at their joints by metal “saddles” which were just steel strands wrapped tightly around the intersecting tubes.
The Value Engineering Principle
The value engineering principle behind the joint design was that in Romania, the manufactured materials made in modern Europe were expensive, whereas labor was relatively cheap. The principle of cost-savings used in the building design was that joints in tube-frames are detailed manufactured items, and were entirely eliminated. The steel strands or “saddles” which replaced them, however, did not stiffen the pipes by offering true “bracing” — there was some slipping allowed at the joints — and the thickness of the pipes did not change at any of the joints, where the bending moments were exerted on the tubes.
Second, the steel tubes were as thin as possible, and the modeling did not accurately predict the way the tubes would behave in this network. Third, the roof material above the network was somewhat flexible and elastic. This elastic shell was extremely thin, compared to an eggshell. That is, the thickness to span ratio of the auditorium roof was approximately one-twentieth of that of an eggshell.
If we consider a rubber basketball cut in half and set on the ground, it may be seen that a fist can push one part of the ball in, buckling it to force it to be concave rather than convex in a localized area. If the push is continued far enough, the whole ball will essentially “snap” inside out, and maintain a concave shape. If the same ball had been covered with plaster of Paris and allowed to dry, it may be clear that this slow pushing inside-out of the ball would be impossible, and the stiffness of the whole “structure” of the ball-dome would be drastically improved.
This is what happened to the Bucharest auditorium—the force of the unevenly distributed snow load pushed one area of the roof “inside-out” by buckling a few of the unbraced tubes and overstressing some of the tied connections. Once a part of the roof was “snapped” inside-out, the force required to make this buckling a chain reaction was much lower, and even the portion of the roof with a relatively low snow-load “snapped” inside-out. Several of the steel tubes were found to be buckled, and the connections overstressed and “slipped.”
Of course, the cost savings in this case included improper design modeling or assumptions, the use of a much less expensive joint, and thin, flexible roofing material. The calculation of value falls apart when the cost is reduced, but the function goes to 0. Zero divided by anything is zero.
The costs of demolition, re-design, and reconstruction of the roof were no doubt much larger than the costs would have been to do it right in the first place. The costliest cost savings are those which cause disaster. The most drastic drop in function is to make a structure cease to function.
Therefore, value engineering in structural engineering building design must accurately assess the risks of total failure, and roll those into the calculation of “function.” Value engineering must not be used as a tool to do relentless cost-cutting, at the expense of the final function of the building standing and serving for its service life and beyond.
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