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DESIGN AND CONSTRUCTION OF THE DOME OF THE MAHAWELI MAHASEYA (STUPA) AS A THIN SHELL ANALYSIS AND DESIGNThe Structure was analyzed by solving compatibility and equilibrium equitation for the redundant forces occurring at edges where shell and ring beam meet. The resultant bending forces were super-imposed on the membrane forces to obtain the total stresses. The procedure was repeated for various Loading cases and various Loading combinations.
The structure was analyzed by analytic methods as well as by the finite. Element method and the results were compared and found to be compatible. The maximum values of the forces obtained under either method were adopted in the final design. The dome was also checked for intermediate construction stages.
DESIGN AND CONSTRUCTION OF THE DOME OF THE MAHAWELI MAHASEYA (STUPA) AS A THIN SHELL ANALYSIS AND DESIGN
The following nine loads and loading conditions were considered for the design analysis:
(i) Self weight (DL),
(ii) Live Load (LL),
(iii) Construction Loads (incorporated in LL),
(iv) Loading due to Differential Temperature rise and fall between shell and beam elements (TL),
( i) Loads due to mid-plane temperature rise in shell elements (TL),
(ii ) Through thickness Temperature (TL),
(iii ) variation of Temperature along any parallel circle of the dome (TL)
(iv ) wind Loads (WL),
(v ) seismic loads (SL).
Combinations of Loading Cases
From. the above loading cases following loading combinations were obtained which were critical for the final design forces.
(a) 1.4 x DL + 1.6 x LL
(b) 1.4 x DL + 1.6 x LL + 1.4 x TL
(c) 1.2 x DL + 1.2 x LL + 1.2 x WL
(d) 1.2 x DL + 1.2 x LL+ 1.2 x WL + 1.2 x TL
(e) 1.2 x DL + 1.2 x LL + 1.2 x WL + 1.2 x TL + 1.05 x SL
(f) 9 DL + 1.4 WL
The final envelope of stresses as derived from the above is shown in the Figure (2)
Site Investigations and Foundations
A number of alternative modes of construction both conventional as well as non-conventional were studied in detail to evolve the most suitable method which is economical and speedy. As a result of these studies, the following sequence and mode of construction was adopted:
(a) Constructing a working platform at lower ring beam level (i.e. 10.82m above the bottom elevation) by cutting and leveling part of hill.
(b) Drive 32 Nos. piles from this earth platform while the work on the platform is in progress and construct the lower ring beam over the top of piles progressively as the piles-are completed.
(c) Concurrently with pile driving and lower ring beam construction, erect a central tower of steel frames temporary construction) and lay the upper ring beam at the top of this tower about 28.5m above the platform.
(d) Erect the reinforcement (longitudinal and transverse) to precise profile within a tolerance of 25 mm and thus get a skeleton cage for the dome (see fig. and fig. 5) which was proposed to, be used for supporting the formwork and progressively the weight of concrete as it was laid. No scaffolding was thus required.
(e) Concrete the dome in circular lifts of one meter height.
(f) After completion of the dome concrete, construct the tower structure over the dome, consisting of square chamber, God's chamber spire and pinnacle.
(g) Construct the floor and other accessories of the stupa below lower ring beam after excavating to a depth of 10.8m from the construction plat form initially left at a relatively higher level for case of construction.
Upper Ring Beam
In the proposed planning for construction of the dome, prior construction of the Upper Ring Beam was an essential prerequisite.
Form work for the Upper Ring Beam was cast in 20 segments of' ferrocement moulds each weighing 'y T. Segments were assembled at the site prior to lifting to the final position at top of the circular tower with the help of' crane. Reinforcement was installed within this formwork and the ring beam was concerted (mix with 340 kg/m3 of cement and admixture 1.2 1/m3. Part of the ferrocement which was to be in contact with the dome concrete was removed after concerning of the ring beam the other portion was left in tact.
An epoxy bonding agent (BONDKOTE) was applied to the outside of the upper ring beam prior to concerning the joint between the dome and the upper ring beam to ensure better bond.
Fabrication and Erection of Skeleton
Reinforcement for the dome was divided into 81 segments for purposes oI pre -fabrication and erection. Each segment was fabricated on a horizontal Jig approximately 37 m long set out on a concrete pad to the true profile of the skeleton components with angle iron elements.
General
In the traditional method, form work is usually supported on scaffolding / pops from the ground, prior to fixing the reinforcement and concreting. this is time consuming, cumbersome and susceptible to inaccuracies, developing during erection of steel reinforcement and formwork, as well as while laying concrete due to end flexibilities, whereas in the adopted method, the complete ring of the cage had a self adjusting tendency.
The various alternatives such as fiberglass, plywood and steel were studied for the formwork. Finally a decision was taken to use steel formwork , with suitable bracings , as it was found to be economical.
Fabrication of panels
2 mm thick M.S. panels duly stiffened with M.S. angles and flats were fabricated for use as formwork . Seven Jigs were made, with adjustable vertical RSJs in order to bend the plates to the required curvatures. Curvature of the plates was retained by tie-bars and stiffeners welded to the angle iron frame. 12 mm and 13 mm diameter holes were drilled in the angles and flats for fixing bolts.
Fixing of Formwork
The steel panels were fixed to the reinforcement skeleton by using 10 mm dia. Through bolts encased in rubber hoses. The rubber hoses prevented the bolts from getting embedded in the concrete. The holes in the concrete formed by removing the bolts and the rubber hoses were used to fix the bottom edge of the next higher panel in precise position and to obtain necessary cover to reinforcement within a tolerance of + 10 mm.
Concreting of the Dome .. for the dome shell . Cement content was 320 kg/m3. Weigh batching was done at the batching and. mixing-plant and concrete was transported to site in- transmixers. A retarded was used to increase the setting time.
Placement of concrete was done with the help of skips and crane in one meter rings. Angle iron tie bars fixed to skeletons during erection were cut and removed prior to fixing the formwork. Near the top .where the slopes were rather flat, windows were made in the exterior shutters to feud concrete. Vibration was done by Poker Vibrators. Curing of concrete was carried out for 14 days by keeping gunny bags ver the concrete and spraying water over them. To ensure continuous availability of water, a temporary water storage was provided at the top platform. Covering with gunny bags also helped to prevent to sun concrete from high temperatures during the day due to direct exposure to sun. Concreting of the dome took about 11 months for completion up to the upper ring beam.
High standard of quality control and quality assurance was needed and maintained during concerning to obtain concrete of uniformly high quality to meet the demanding. Design requirements. Design requirement was to have concrete of 25 N/mm2 .The actual. strength achieved was found to be in the range of 30- 35 N/mm 2.
In keeping with the cultural traditions of Sri Lanka, it was intended to build the stup4 near the top of the hill along the Kotmale reservoir so as to get a full panoramic view of the dam and the reservoir. After reconnaissance, a suitable area for siting the stupa was selected and investigations were undertaken for pinpointing the exact location of the stupa, which was topographically acceptable and geologically stable.
At the chosen site, the firm rock was approximately' 12 m below the intended finished ground floor level of the proposed stupa. The overlying strata was soil with boulders. It was necessary to found the stupa over the firm rock foundations. Two alternatives were possible for this purpose namely:-
(i) Excavate and remove the overburden and build up the substructure from the rock level up to the desired floor level of the dome.
(ii) Drive piles to the rock and found the structure over these piles.
The latter alternative was chosen because excessive cutting along the hill. slope would have caused instability of the hill along side the proposed stupa requiring expensive and time consuming stabilization s measures
Piling
After considering various alternatives for the piles, it was decided to have 32 Benoto piles of' 1090 mm and 1200mm diameter (alternately) for which the equipment and know-how was locally available.
Presence of boulders in the strata, large diameter piles were piles were preferred to enable inspection of rock strata at any stage of driving and easy access for drilling, and removal of blasted material whenever the boulders were encountered. piles were set out along the periphery of the intended bottom ring beam of the dome in a manner that when extended at the top, they could be used as columns supporting the ring beam 10.8 m above the ground floor of the finished stupa. The piles thus served -he dual purpose of structural foundation support and architectural requirement of columns around the floor of the main hall.
After any pile was driven to the bed rock prefabricated reinforcement cage was lowered inside the pile casing and the pile was concerted while pulling out the casing concurrently.
Pile caps and Lower Ring Beam
The concrete at the top of the piles was found to be invariably weak due to the formation of laitance. These portions were removed by breaking and were re-concerted.
When three adjacent pile caps were completed, the piles were joined4 at the top forming a ring beam of size 1090 mm x 1200 mm leaving suitable starter bars for the dome. A set of curved steel shutters was fabricated for the inner and outer sides of the ring beam and was fixed the curve set out on the ground. The reinforcements were suitably arranged within the shutters on a screened concrete surface. The mix design used was 25/20s with 320 kgs/M3 of cement with w/c ratio 0.5 An electric poker vibrator was used for vibration. The concerning of one unit took approximately 3 to 31/2 hours duration. lasted of applying mould oil on the steel form work and screed concrete, a thin Polyphone layer was 'used which was of immense advantage in removing the form work in a shorter time and to obtain a finer finish.
