The Development of Technology for the use of thin concrete shells for Port Structures.
Several applications of thin shell structures were seen in the construction of buildings in the Port Development work. This opportunity was made use of for the investigation of certain problems of design and construction of shell structures, if this type of construction was to he used. In the nineteen fifties, when this work was started, the information available in technical literature was not quite adequate for this purpose and a large amount of studies had to be carried out resulting in sonic innovative solutions to design and construction problems.
There was a large number of coaling jetties constructed in reinforced concrete where the deck reinforcement had undergone serious corrosion. Investigations revealed that the concrete cover to the slab reinforcement had deteriorated due to sulphate attack resulting in heavy corrosion of the reinforcing steel. The deck slabs had to be removed and replaced as a permanent measure although, as a temporary measure replacement of the corroded steel and "guniting" a protective layer of cement sand mixture was carried out. However a more permanent solution had to be found.
The construction problems in shell construction were -
1. The necessity to erect shuttering for the whole shell before connecting.
2. The necessity to have three layers of reinforcement to take the bending stresses in two directions and the shear stresses at the end.
3. The difficulty of compacting the concrete near the steep edges.
4. The need to have adequate water proofing treatment to protect the reinforcement against corrosion and to provide adequate water tightness of the shell roof.
The R & D work carried out resulted in the development of a system of construction, which avoided all the above difficulties. The first shell roof incorporating the new technique, a shell of 50 ft. span and 20 ft. wide was built in 1957. The method consisted of assembling thin precast shell segments on to the edge beams by means of prestressing cables passing through the segments using a
travelling centering covering the arch over a short length. Once all the arches were assembled by prestressing on to the edge beams they were connected together longitudinally. the cables passing through the preformed ducts in the shell segments and anchored to the precast end frames (traverses) of the shell. This resulted in a thin arch of 20 ft. span and 50 ft. long. The two edge beams were tied together by transverse ties to take the arch thrust of the ties arch. The next operation was to tension the main post tensioning cables in the edge beams and anchor them to the end traverses, releasing the transverse ties at the some time. This converted the long arch to a cylindrical shell of 50 ft. span and 20 ft width. This entire operation was carried out at ground level with the edge beams resting on the ground. The construction was completed by jacking up the shell to the required height using jacks at the four corners and supporting it on columns at these points.
This system of construction was used for the construction of a series of shells, 1 00 ft. span and 33 ft. wide to form the warehouses at the deep water quay in Galle Harbour Three shells forming a roof 100 ft. x 100 ft. were jacked up at a time to the required height. No water proofing was used as the orthogonal system of prestressing ensured crack free conditions by avoiding tensile stresses under all loading conditions. The shell has required no maintenance other than the periodical painting even after more then thirty five years. This method of cylindrical shell roof construction has reduced the cost of such structures considerably by avoiding conventional form work and reducing the amount of steel and concrete and eliminating the necessity for water proofing.
Hyperbolic Paraboloid shells of the Saddle shape
Saddle shaped Hyperbolic Paraboloid shells have been found very suitable for the roofs of factories, workshops and similar buildings where large columns free areas are required. These had desirable features like a longitudinal curvature enabling drainage of water towards the ends, a curved cross section convex downwards and the possibility of generating the surface by means of skew straight limes moving along parabolic curves at the ends. however, this property which required the prestressing wires being placed skew to the longitudinal axis of the shell prevented their construction on the long line of prestressing. A wire which started at the left hand corner of the shell had to go to the centre of the other end. The practice had been to cast the shell in individual moulds made strong enough to take the reaction from the prestressing wires. The wires had to be released from the moulds before demoulding, which meant that the concrete had to attain the necessary strength for this purpose. This required heat curing as no effective accelerators were available at that time. (late fifties and early sixties) With the use of the long line, the demoulding could be done without the detensioning of the wires, but the problem was the changing of the wires from one point in a cross section to another along the length of the shell. This was achieved by introducing diaphragms at each end o f the shell which had anchor hooks to fix the wires in the correct position. This enabled the casting of one shell per mould every day and a large number of these shells were cast in a 400 ft. long stressing bed. The shells were 2 inches thick, 5 ft. wide and covered spans up to 50 ft. giving a very economical solution to roofing of workshops, factories and similar buildings. A number of buildings including the 50 ft. wide S.E.C. Workshops at Peliyagoda and the 100 ft. x 100 ft. columns free space for a Hydraulic Model Testing Laboratory were built in the sixties.
Hyperbolic Paraboloid Umbrella Shells: Solution of the Corner Droop Problem.
Umbrella shaped Hyperbolic Paraboloid shells are very suitable for roofing large areas economically in view of the small amount of concrete and steel required per unit area of floor. The resulting structure is also aesthetically elegant. The possibility of covering large areas by a repetition of these shells is also a desirable feature. There were however, certain problems which had not been solved by the time they were considered for application in the late fifties. One was the large edge beams required when constructed in reinforced concrete and the corner droop proh1cm. (the downward deflection of the four corners). These problems had not been solved at that time but solutions were necessary before they could be constructed successfully in the Port. The opportunity created by the necessity for constructing a covered car park, which was aesthetically pleasing, was made use of for constructing
three umbrella shells, each 4Oftx3O ft in plan. The techniques required for the construction were developed in the course of construction. The heavy reinforced concrete edge beams normally used, were replaced with lighter prestresed beams. the post tensioning cables being terminated at intermediate points, in addition to the ends of the beams. This induced a prestress varying along the beam which approximately matched the membrane stresses along the edges of the shell. This also corrected the corner droop of the shells, which had troubled previous designers of these shells including the famous Prof. T Y Lin of Berkley. California. He was carrying out model tests, which the Author saw later, to find the solution to this problems. It was a pleasure for the Author to inform him of the solution, when the Author met him at the Shell Conference in Madrid in 1959, where the Author presented his paper on "Umbrella Type Hyper Parabolic shell roofs with prestressed edge beams". This was a significant contribution to the design and construction of these shells.
A decision was made by the Port Commission to open a road along the sea front to connect Galle Face Road with the main access road leading up to the Queen Elizabeth Quay. This provided an opportunity to construct three, four hundred feet lengths of concrete road. One was a pretensioned prestressed concrete slab and another was a post-tensioned prestressed concrete slab. Both slabs were four inches thick and the post-tentioned slab used the "Kulasinghe CPC" system for the post-tensioning cables. The third section was in conventional unreinforced concrete. These sections of the Marine Drive, which is now known as the Chaitya Road, were completed in 1956. They have performed very well during the last forty two years. They have required no major maintenance in spite of the very heavy traffic along this road.
These concrete roads were built to demonstrate their suitability for climatic conditions prevailing in this country as against bitumen surfaced roads which have to be repaired at frequent intervals. Unfortun4tely the road authorities have not appreciated the valve of concrete roads and ignored their suitability for the conditions in
this country. They have the wrong idea that a very thick concrete is required arid therefore they are expensive. It has been demonstrated that concrete roads making use of Rollcrete", a roller compacted concrete process is very suitable for conditions in countries similar to Sri Lanka. It is also very economical to use and hence its wide use in India. It is interesting to mention that roller compacted concrete was used for the construction of jetty decks in the Port as far back as 1 950. This is probably the first time the process was used for the compaction of concrete. In this process, used for roads, a lean dry mix of concrete is compacted by a vibrating roller giving a low cost high strength materials having low shrinkages.
Other Shell Structures
addition to the shells mentioned above other important shells built
by the S.E.C. the NERD Centre and the CECB have to be mentioned.
The Kalutara Bodhi Chaitya
The Kalutara Bodhi Chaitya was built in the late sixties as a thin shell of 100 ft. diameter with a thickness of 51/2 inches. This was the first thin hemispherical shell built in this country. The design and construction were carried out by the State Engineering Corporation whose computer (the first in the country) enabled the complicated calculations to be carried out without resorting to approximations that become necessary when working with calculators. This was especially so during the time the design was carried out. The construction was by conventional methods involving the construction of form work for the whole shell before placing the steel reinforcing and concrete. It is one of the most beautiful monumental structures in this country.
The Mahaweli Maha Seya at Kotmale.
The construction of the Kotmale reservoir inundated a number of temples in the area which led to a decision to construct a monumental stupa in a prominent site close to the Kotmale dam so that it could be seen over a wide area. This required a large structure and the final choice was a chaitya with a diameter of two hundred feet constructed as a thin semi spherical shell to reduce cost and also improve its usefulness. The design of a shell of these dimensions presented many serious problems. They were the danger of buckling, the thermal stresses created by the difference in temperature between the side exposed to the sun and the sheltered side and the vertical and horizontal loading which the shell is subjected to. The calculations involved in the design were quite complex and the safety requirements demanded very accurate analyses. A unique method of constructions had to he developed to enable such a large and think structure to be built. The method adopted was to
build a skeletal shell to the same shape of the shell itself with the steel reinforcement required for the structure. The formwork for the shell concrete was supported on the reinforcement skeleton and concreting was carried out in rings of four feet height. This involved calculations to determine the stresses in the skeleton and in the concreted section of the shell at every stage. This was tedious and required a high degree of skill.
This unique method of construction was successful and economical. It was almost impossible to construct this 200 ft. diameter thin shell using conventional methods of construction. Setting out the shell was also a difficult task as an accuracy to ensure a maximum deviation of one inch from the true surface had to be ensured to avoid local buckling of the shell. The design calculations for the shell formed the M.Sc. thesis of the Structural Engineer who worked on it with the Author under very close guidance. This structure is one where those involved in the design and construction can be justifiably proud of. The design and construction was handled by the Central Engineering Consultancy Bureau.
Sambuddha Jayanthi Chaitya on Chaitya Road
Another monumental structure for which the Author was responsible in the Sambuddha Jayanthi Chaitya built on Chaitya Road partly constructed by the Colombo Port Commission and completed by the Colombo Port Authority.
Kulasinghe Auditorium of the NERD Centre.
The other interesting shell constructed using a novel method is the 100 ft. diameter tension shell of the "Kulasinghe Auditoriuni' of the NERD Centre. This shell is in the shape of a paraboloid of revolution concave upward with a thickness of less than one inch. The method of construction was as follows.
The outer reinforced concrete ring beam of 100 ft. in diameter and a similar concentric inner ring beam of 10 ft. diameter were built on the ground. The two ring beams were connected by closely spaced radial pre-stressing cables which were tensioned to a predetermined value. The outer ring beam was jacked up using sixteen jacks placed at the sixteen columns spaced along the periphery. The jacking up was continued till the inner ring beam lifted off the ground. The jacking was continued, while placing temporary weights on the inner ring beam, till the inner ring beam was five feet below the outer ring beam. The jacking up was continued further till the outer ring beam reached the designed height of 1 5 ft. The sixteen columns supporting the outer ring beams were extended as the jacking proceeded till it rested on the columns at the height of 15 ft. A skin of fibber glass was attached to the underside of the radial wires and a thin concrete of about one inch was laid on the fibre glass which acted as form work. The temporary weights on the inner ring beam were removed progressively to maintain its sag of 5 ft. below the outer ring beam. Water proofing of the thin shell completed the tension shell structure. This novel method of construction resulted in a very low cost. This shell provides a low cost roof for large areas without internal columns.