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Styrene Roofs New concept of th in-

shell constuction uses Styro­foam for form as we l l as insulat ion

Ι ο Λ MATHEMATICIAN, a hyperbolic paraboloid is a fundamental geo­metric form that fits certain equa­tions.

The architect, on the other hand, envisions a hyperbolic paraboloid as a gracefully sweeping roof or roof segment.

Dow Chemical goes still a step further, sees it as one form of thin-shell construction which could pos­sibly be made easier by using styrene foam—not only as thermal insula­tion and vapor barrier, but also as a form for the permanent roof.

In fact, a study at Purdue's structural engineering laboratory (sponsored by Dow) has now shown that such a concept of thin-shell roof construction is feasible. And the Purdue group topped off its study by erecting a 20- X 20-foot roof to show how it might be done.

A conventional thin-shell roof is basically a concrete structural shell, perhaps just a few inches thick. Weatherproofing is applied to the outside, and this is generally topped with a reflective covering. Insula­tion goes on the underside along with some type of architectural ceiling treatment, such as plaster.

The concrete for such a roof is cast, thus the need for a mold or form. To begin with, the builder erects a forest of wooden supporting members (falsework) on top of which go wooden stringers and beams. Ply­wood panels, bent to the hyperbolic paraboloid shape, make up the form. When the concrete hardens and can support itself, the falsework and framework arc removed. Then

Here is an example of a structure that might be built using hyperbol ic pa rabo lo id sections. The right rear wing o f the pavi l ion shows the e labora te formwork and support needed fo r conventional methods of constructipn. The left rear wing shows the use of Styrofoam as the form

comes the job of covering the outside and inside.

All this costs money—both for the labor and for the material. This, says Dow, is one reason wider use is not made of the construction tech­nique in the United States.

Now that Purdue has demon­strated the feasibility of using Styro­foam, Dow hopes it can also be de­veloped into a less expensive ap­proach than the conventional one. The potential here, Dow feels, looks good : For pne thing, it eliminates much of the elaborate falsework and formwork. For another, it does away with a separate operation for applying insulation.

Offset Wires

A hyperbolic paraboloid is formed by straight line generators on two axes: the connection between any point on one edge with the corre­sponding point on the opposite edge is a straight line. The Purdue group figured it could make use of this by shaping the Styrofoam boards be­tween two sets of wire lattices, ex­plains Joseph L. Waling, professor

of structural engineering in Purdue's School of Civil Engineering.

But tightening the wires into straight lines while they are support­ing and shaping the Styrofoam sets up stresses in the edge beams which are too high. This results, Dr. Waling says, in sizable lateral de­flections in the edge beams.

Heading back to the geometry, Purdue looked into another charac­teristic of the hyperbolic paraboloid. A vertical plane intersecting the sur­face at an angle forms a parabola. This turns out to be the key to shap­ing hyperbolic paraboloids.

Here is how Purdue goes about erecting the shell. First a lattice of wires offset in the proper direction is stretched between the edge beams. Offsetting the wires causes them to form parabolic traces concave up­ward, according to Dr. Waling. Styrofoam boards are then layed on this lattice and aligned to each other with sheet metal clips.

Over the boards goes another wire lattice, with the wires offset in the direction opposite to those under­neath. This forms parabolas con-

3 2 A INDUSTRIAL AND ENGINEERING CHEMISTRY

Many ideas and projects—big and little—can influence you, give you ideas for use in your work, and provide useful information for "current awareness." Each month I/EC's field editors and Washington staff select for detailed report and analysis, designed for easy reading, some of the most timely, in research and commercial development, process design, engineering, production, and marketing areas in the chemical process industries. We present also our comments on other interesting happenings of business and professional interest.

The Chemical World Today cave downward. The upward and downward pressure of the wires, ex­plains Dr. Waling, warps the Styro-foam into shape with lower stresses in the edge beams.

Styrofoam wedges are inserted be­tween the boards and the edge beams. In effect, this prestrcsses the boards and exerts an outward pres­sure on the edge beams, helping to overcome any deflection that might occur. In addition, the prestressing strengthens the Styrofoam shell.

With its 20-foot square thus erected, Purdue ran load deflection tests. Unloaded, the shell varied from a true hyperbolic paraboloid by less than l/a inch over its entire sur­face. And under a 20 pound-per-squarc-inch uniform load, the center-point deflection was only 23Λ inches. Purdue finds, however, that coating the shell with about V4 inch of mortar (3 parts sand, 1 part cement) stiffens the shell and cuts the de­flection under load down to V2 inch.

Purdue now has a few more ideas up its sleeve. It plans to study adaptations of this construction method to other shaped structures. One it has in mind is a spherical-type shell made up of several trun­cated and skewed hyperbolic parab­oloids. Another is a barrel-type shell—an elongated structure com­posed of full, skewed hyperbolic paraboloids in conjunction with trun­cated, skewed hyperbolic parabo­loids. J.H.K.

Implosions Help Oil Wells Produce Shock waves produced by an implosion help open up rock formations to release petroleum

r^ TECHNIQUE causing the same effect as that which comes from breaking a light bulb—implosion— may be the best tool petroleum engi­neers have for coping with "tight" formations. The shock waves from an implosion cause selective fracture initiation near the implosion in oil wells. Result: Formations open and accept fluids more easily.

Implosions have .been developed by Dowell specifically for oil field use. In DowelPs case, the tech­nique is similar to breaking light bulbs inside a well, but on a much bigger scale. The shape and wall thickness differ for the bulbs as does the method of making them fracture.

The bulbs or capsules are made of thick-walled borosilicate glass. Dow­ell supplies them in diameters and lengths to fit a variety of jobs. They are designed to implode or collapse inward at different pressures.

It is this collapse that produces the shock wave. The violence of an implosion depends on the differen­tial pressure inside and outside the bulb before it breaks. In the first phase of an implosion, the pres­surized fluid rushes into the low-pressure cavity left by the collapsing capsule. Velocities range between 100 and 800 miles per hour, de­pending on the imploding pressure, according to D. D. Setser of Dowell.

Potential energy, stored in the fluid under pressure, is released when the fluid rushes to fill the void of a capsule. When the void fills, the inertial force of the column of fluid in an oil well exerts pressure against the face of the well bore. This high pressure is of short duration—about a hundredth of a second. Most of the force is limited to the section of the formation immediately around the capsule.

•4 Implosion capsule. Mr . D. D. Setser of Dowell examines a glass capsule to be imploded or col lapsed in an oil wel l bore to produce a shock wave at the adjacent format ion. Increased pressure resulting f rom the implosion breaks up t ight rock formations. At left a re capsules in a handl ing rig

VOL. 52, NO. 7 · JULY 1960 3 3 A