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International Symposium on Earthen StructuresIndian Institute of Science, Bangalore, 22-24 August 2007

Load Carrying Capacity of Brick Masonry Dome in Mud Mortar

N C Balaji1

and G Sarangapani2

Department of Civil Engineering, N.I.E., Mysore, India1

[email protected],2

[email protected]

Abstract

In this paper the results of an experimental investigation carried out to determine the load carryingcapacity of brick masonry dome in mud mortar is reported. Dome has been constructed without using

formwork. Dome of span 3m, thickness 0.075m, and central rise 0.6m has been considered in the study.The entire outer surface of the dome was subjected to uniformly distributed load of upto an intensity of

2.845kN/m2. Further more, the dome was also subjected to partial uniformly distributed load around thecrown upto an angle of = 6.309 degree measured from the crown.

Keywords:Masonry Dome, Mud Mortar

Introduction

The conventional roofing consists of reinforced concrete slab, the cost of which is around 20% of totalcost of the building. The brick masonry shell, in particular the dome has proved to be more economical

than the conventional reinforced concrete slabs for spans upto 5m. This is mainly due to ease ofconstruction, reduction in cost of formwork and the use of locally available materials and labour. The use

of dome can be mainly attributed to their unique structural behaviour and for aesthetics. Masonry domesare more durable than reinforced concrete slabs. Masonry domes represent a very ancient structural

typology, developed both by Eastern and Western architecture. The use of brick/stone masonry shellssuch as vaults and domes for roofing is an age old practice followed even till recent times and thesestructures have performed well for long periods. The dome history can be traced back to about five

millenniums, during which some decisive innovative applications have taken place. The Roman Pantheondome built from 118 A.D. to 128 A.D. with its inner diameter of 43.30m, remains one of the largest

domes ever built until the modern era. The material of Pantheon is Roman Pozzolona opuscaementicium, which sets by combining chemically with water, used together with irregular stonesgiving the first application of concrete in domes. During the period 532 A.D. to 537 A.D. the Byzantine

Hagia-Sophia dome of diameter 46m was constructed. The dome of Hagia-Sophia is much thinner and ismade of bricks instead of Roman concrete. One of the most striking early Islamic

Examples is the Dome of the rock built at the end of the 7 th century, in Jerusalem. It is a double shell

dome made of wood, the outer shell being covered by metal sheets and the inner shell by fine plaster. Theoldest masonry dome, still standing is the mausoleum of Ismail the Samanid in Bukhara constructed in

907 AD. The earliest European examples of the double dome are St.Marcs in Venice and St.Antonys inPadua, both built in timber at the end of the 13 th century. A number of masonry domes have beenconstructed by Turkish building master Mimar Sinan during period 1490-1588. Sinans primary concern

was the architecture of domed structures. The creation of the optimal internal space together with thehighest aesthetics of the external form was the aim of his experiments. Infact Sinan tried all three main

types of supporting systems of the dome, namely square, hexagonal and octagonal support systems withfour, six and eight equal arches, respectively, creating all together 12 variants, The Selimiye Mosque with

its 32m diameter dome and octagonal supporting system being accepted as the climax of his innovativesolutions. The structural performance of Sinans domes is excellent, as none of them have suffered any

damage in spite of nearly 100 earthquakes. Sinan reduces the thermal effects covering the brick masonrydome by mud of 20cm thick and lead plates. Both materials are good insulators and protect the dome in

an excellent manner. One of the most striking example domes in India is the 41m diameter dome of GolGumbuz built by of Muhammed Adil Shah (1625-1660) in Bijapur. The wall thickness of this single

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International Symposium on Earthen Structures148

dome changes between 3.05m at the base and 2.74m at apex. The dome is constructed of horizontal bricklayers glued to each other by using lime mortar. This huge dome rests on the crowns of eight intersectingarches and on pendentives.

There were remarkable structural innovations in dome construction during the industrial revolution of 19th

century. During the first 6 decades of 19th century reinforced concrete became the dominant material for

domes. The innovation introduced by this material is the richness in the shape and type of supporting. The

spherical form was rarely used. Preference was given to shallow shells with curved edge beams supported

by slender columns. Nervi, Torroja, Dischinger and Isler have created the most striking examples of

domes in reinforced concrete. In the beginning of sixties the costs of labour, formwork and scaffolding

increased rapidly, which resulted in decline of R.C. domes. On the other hand metal space frames of

different systems were developed and were preferred to reinforced concrete. The innovation of the

eighties is the membrane dome either air-supported or air-inflated. Another important innovation is the

use of cables to stiffen and support the dome.

The Structural behaviour of the masonry dome has been studied at length since the end of 17 th century.

The modern study of masonry dome starts from a series of well known papers by Heyman. In his paper

the plastic theory which was used for the masonry analysis has been extended to the analysis of shells by

considering the collapse mechanism (Heyman 1967). The analysis of the hemispherical masonry dome

has been done by considering the dome as an arch of increasing width from the crown to the spring. The

value of minimum thickness to span ratio as a safe solution has also been provided by Heyman (1977),under the assumption of infinite compressive strength and friction resistance and zero tensile strength.

Pesiullesi et al. (1997) theoretically studied in detail, the optimal spherical masonry domes of uniform

strength. They have proved that change in the sign of circumferential stresses can occur for considerably

larger angles, depending on the shape of the profile for masonry domes of variable thickness.

Experimental and computational work on a radially symmetric model dome subjected to foundation

settlement, was studied by Livesley (1992) to investigate the effect of support movements on the

behaviour of the dome. Recently Ragunath et al. (1994) have conducted an experimental investigation to

obtain the load carrying capacity of partially constructed masonry dome. As regards to the experimental

work on the masonry domes not much has been done. In this study, the load carrying capacity of brick

masonry dome in mud mortar has been experimentally determined. The masonry dome has been

constructed without formwork.

Test Program

In this experimental investigation the load carrying capacity of brick masonry dome in mud mortar has

been determined. Table moulded bricks of compressive strength of 4.0N/mm2 and 2:1 (Soil: Sand) mud

mortar of compressive strength 0.803N/mm2 (dry) have been used for the construction of dome. Sand

conforming to zone II as per IS 383-1970 and locally available red soil has been used for preparing the

mud mortar. The soil has 8% clay, 40% silt and 52% sand. The dome has a 3.0m span, 0.6m rise, 0.075m

thickness, and semi central angle of 43.603 degree. Bricks have been placed on edge to achieve 0.075m

thickness of dome. The R.C. ring beam of size 0.45m x 0.23m with 0.5% steel has been constructed all

round the dome. To make sure that the failure is through the masonry dome the ring has been made very

strong and rigid. The ring beam rest over six stone masonry columns of size 0.45m x0.45m. The stone

masonry columns have been constructed in 1:6 cement mortar. Masonry columns have been constructed

over stone masonry footing of dimensions of 1m x 1m x 1m. Footing has been constructed on 1:4:8

cement concrete bed of 0.15m thick. The soil over which the footing has been placed has a safe bearing

capacity of 300kN/m2. Figure 1 shows the masonry dome under construction. The dome is constructed

without any formwork. In this method each course or ring of brick was laid in mud mortar. The bricks

have to be temporarily supported until the entire ring is completed. To facilitate this, the bricks before

completion of the ring were supported by S-shaped G.I.wire hooks. One end of the hook was fixed to the

brick while the other end was loaded by suspending a counter weight as shown in Figure 1. The hooks

were removed after every course and further reused for laying the next course. This procedure was

continued till the completion of the dome. Each day not more than 3 courses were constructed. This

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Load Carrying Capacity of Brick 149

innovative method of constructing the dome reduces over all cost of dome. The masonry dome was air

dried for seven days till the mortar achieved dry condition. Figure 2 shows the completed masonry dome.

Two types of loadings viz uniformly distributed load over the entire outer surface of the dome and

partially uniformly distributed load around the crown upto an angle of = 6.309 degree measured from

the crown have been applied on the dome separately. Partially uniformly distributed load is spread over

an area of 0.18m2. The load was applied through sand bags. Initially the dome was subjected to uniformly

distributed loads under dry condition. The dome did not show any signs of cracking or failure even

through it was subjected to uniformly distributed load of intensity 2.845kN/m2. As such uniformlydistributed load was removed and partially uniformly distributed load was applied over the crown of the

dome under dry condition. This was continued upto a load of 6.13kN. For every increment in the load the

deflections were measured. In case of masonry dome subjected to uniformly distributed load over the

entire surface the deflections were measured both at the crown and quarter span. However, in case of

masonry dome subjected to partially uniformly distributed load the deflections were measured only at

crown. Figure 3 shows the arrangement made for measuring the deflections. The dial gauges used for

measuring the deflections were fixed on a rigid platform. The dome did not shown any signs of cracking

even for partially uniformly distributed loading in dry condition of the dome. In order to induce failure of

the dome, water was sprinkled on both the surfaces of dome for a duration of 6 hours with an intention of

reducing the strength of brick masonry.

Results and Discussions

The results (i.e., load and deflection) of the tests conducted on the brick masonry dome subjected to

uniformly distributed load are presented in Figure 4. The masonry dome subjected to uniformly

distributed load over the entire dome did not fail even upto a load of 2.834kN/m2, in the dry condition.

Cracks also did not appear upto this uniformly distributed load of 2.834kN/m2. Deflections were 0.75mm

and 0.62mm respectively at crown and quarter span at a load of 2.834kN/m2. The load-deflection curve is

almost parabolic. As the dome did not fail and no cracks appeared upto a load of 2.834kN/m2, it can be

conveniently used for roofing upto a span of 3.0m. As per IS 875-1987 the maximum live load to be

considered on curved roof is 0.75kN/m2.

Figure 1 Masonry Dome under Construction

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Figure 2 Completed Masonry Dome

Figure 3 Arrangement made for measuring the Deflections

Figure 5 represent the load deflection curve for the dome subjected to partially uniformly distributed load

at crown. Partially uniformly distributed load was applied on masonry dome under dry condition after

removal of uniformly distributed load. A load of magnitude 6.13 kN was applied on the crown of the

dome. Under this load of 6.13kN also the masonry dome did not fail. However cracks appeared in the

dome. The first crack appeared in one of the mortar joints of the crown portion at a load of 1.72kN. On

further loading cracks appeared in several mortar joints in the crown portion. The crack patterns are

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shown in the Figure 6. There was also a loss of bond between the brick and mortar. The cracks developed

continued in the mortar joints along the meridional direction. The deflection were measured only upto

3.43kN. As there was no sign of failure, the masonry dome was sprinkled with water continuously.

Further the masonry reached the saturated condition after absorbing sufficient water and the dome

collapsed under this sustained load of 6.13kN. Figure 7 shows the failed dome with the first three courses

from the ring beam remaining intact. The failure is of punching shear type at the crown as shown in

Figure 7.

Figure 4 Load-Deflection Curve for Masonry Dome Subjected to Uniformly Distributed Load

Figure 5 Load-Deflection Curve of Masonry Dome

Subjected to Partially Uniformly Distributed Load over Crown

0

0.5

1

1.5

2

2.5

3

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Deflection (mm)

Loadintensity(kN/m

2)

At Quarter span

At Crow n

0

0.5

1

1.5

2

2.5

3

3.5

4

0.00 0.20 0.40 0.60 0.80 1.00

Deflection (mm)

Totalloadatcrown(kN)

At Crow n

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Figure 6 The Crack Pattern

Figure 7 Failed Masonry Dome

In the recently conducted experimental investigation by Raghunath et al. (1994) on a brick masonry dome

with opening at the crown in 1:6 cement mortar of span 3.0m, thickness of 0.075m, and octagonal in plan,

the dome did not fail upto a uniformly distributed loading of magnitude 4.48kN/m2. In their experimental

investigations also failure could not be achieved even after application of a load of 4.48kN/m2 over the

dome with central opening.

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Conclusions

1. The brick masonry dome in mud mortar when subjected to uniformly distributed load over the entireouter surface of dome did not fail upto a load of magnitude 2.83kN/m2 in dry condition.

2. The brick masonry dome in mud mortar in wet condition failed under a 6.13kN partially uniformlydistributed load around the crown upto an angle of 6.309 degree measured from the crown.

3. Single course brick masonry dome in mud mortar of 0.075m thick can be effectively used asalternative roofing upto spans of 3.0m.

References

1. Heyman, J. (1977).Equilibrium of shell structures. Clarendon Press, Oxford, England.2. Heyman, J. On shell solutions for masonry domes, J. Sol. Str., 3, 1967, pp. 227-241.3. I.S. 383 -1970,Indian Standard Specification for coarse and fine aggregates from natural sources for

concrete, BIS, New Delhi, India.

4. I.S. 875 (Part 2) -1987,Indian Standard code of Practice for Design Loads (Other than Earthquake)for Buildings and Structure, BIS, New Delhi, India.

5. Livesley, R. K. A Computational model for the limit analysis of three-dimensional masonrystructures, Mecc. 27(3), 1992, pp. 161-172.

6. Pesciullesi, C., Rapallini, M., Tralli, A. and Cianchi, A. Optimal Spherical Masonry Domes ofUniform Strength, Journal of structural Engineering, Vol. 123, No. 2, Feb, 1997, pp. 203-209.

7. Raghunath, S. and Prasanna Kumar, P. Experimental Investigation on Brick Masonry Domes, 1994,B.E., Project, Dept. of Civil Engg, B.M.S.C.E., Bangalore.

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