10: Discussion and summary
10.1 Overview and funding sources
Gujarat has a very rich heritage of buildings dating from the fifteenth to early twentieth centuries, many of which are of international importance. Many were affected by the earthquake, some quite seriously. The heritage buildings in Ahmadabad are generally under the care of the Architectural Survey of India (ASI), which is funded by government and despite limited resources have been well maintained. Damage to the Ahmadabad buildings was generally not excessive; temporary repairs have been instigated and permanent repair works are already in hand. The policy of ASI's senior conservationist for Ahmadabad, V S Nair, of developing a permanent group of skilled masons appears to have been effective in limiting damage and aiding the ability to carry out repair works rapidly.
The situation with the palaces of the former princely rulers of Gujarat is different, in that they have not previously benefited from public funding and many were in a poor state of repair before the earthquake. Some of the buildings have been converted to public use as schools, museums or hotels and this process is likely to continue in order to attract the funding needed not only for earthquake repair work but also (equally importantly) the maintenance necessary to ensure that these priceless monuments survive for the benefit of future generations. Unless this maintenance occurs, they will be lost with or without seismic activity.
10.2 Damage patterns
Typically, damage in engineered structures is greater in the lower half of the structure, reflecting the increased shears and bending moments at lower levels. The damage patterns seen in the Gujarat heritage buildings were exactly the opposite; there were numerous examples of buildings which were heavily damaged at roof and upper levels, but suffered very much less severely on the ground floor. Four possible reasons for the higher damage at the top of masonry buildings are as follows.
1.Weathering
This applies particularly to brick mortar construction, where the mortar joints tend to be more exposed to wind and rain at the top and so tend to deteriorate more. This weakens their seismic resistance.
2.More open structure
Both for reasons of aesthetics and because of the reduced gravity loads, masonry buildings often become much more open toward the top than is the case for modern buildings. This weakens their lateral resistance and may make them more prone to damage.
3.Consequences of increased accelerations with height
The swaying in an earthquake causes accelerations to increase towards the top. This especially affects decorative elements, such as parapets and external ornamentation, which were observed to be heavily damaged. However, it also affects main load carrying walls, because the 'out of plane' seismic forces (ie the earthquake pushing the wall out at right angles to itself, which is its weak direction) depend directly on the acceleration at that level. By contrast, the 'in plane' seismic forces (acting along the length of the wall in its strong direction) increase towards the bottom of the building, because they are accumulated from the floors above.
4. Shear strength of dressed stone masonry depends primarily on friction
The horizontal strength of dressed, good quality stone depends mainly on the vertical load from above, tending to clamp the stone blocks together. This vertical clamping force is less at the top of the building because there is less of the building weighing down from above. Therefore, the seismic resistance is also less towards the top of the building. However, the influence of vertical clamping force, and hence friction, on seismic strength is probably much less important in random rubble masonry dependant on weak mortar to bind it together; this is an aspect on which further investigation is needed.
There are some interesting consequences of this clamping effect. Firstly, it means that adding weight to the top of a building - something that earthquake engineers are always warned not to do - may be much less harmful in dressed masonry buildings than in modern reinforced concrete buildings; it may even be beneficial. It remains true that the additional weight increases the seismic loads, but for masonry buildings it also increases the resistance. There is support for this view in the palace described above at Dhranghedra, where the addition of heavy concrete roofs had served to tie together the tops of two tall masonry walls. These walls suffered very much less damage than identical walls poorly restrained by a lightweight timber truss roof. Similar circumstances are described at Halvad Palace.
The second consequence points to the benefits of prestressing as a means of strengthening good quality dressed masonry construction. It increases the clamping force - and hence shear strength - with no penalty in weight. Although it is not a technology currently in use in Gujarat, it may repay serious consideration for the restoration of historic buildings.
10.3 Maintenance
Many of the buildings that were studied showed clear signs that lack of maintenance aggravated the damage they suffered. This finding is common to many other earthquakes (see for example Feilden, Ref 1). Properly maintained buildings have always been found to stand a much better chance of surviving major earthquake shaking than buildings whose structure has been weakened by the effects of ageing. The original construction quality of all the buildings studied was generally high, but in some cases there were deficiencies observed in construction which also contributed to a higher degree of damage. The quality of construction was also related to the level of craftsmanship involved, which is another factor determining the degree of damage.
Restoring the buildings to their original condition is in many cases as important a task as repairing damage directly caused by the earthquake. Setting up maintenance programmes for the future is also a vital task.
10.4 The role of indigenous materials and crafts in restoration
This is of primary importance. Detailed restoration plans are useless without the necessary materials and skills to implement them. There is a great need to develop a body of trained, accredited craftsmen to carry out the large amounts of restoration work required in Gujarat.
10.5 Use of modern materials and technologies
Modern methods have a potentially important role to play. There were examples of the use of reinforced concrete elements added to historic buildings at JamNagar, Dhranghedra and Halvad, which undoubtedly reduced the local damage suffered in the earthquake, although the longer term consequences of this type of intervention may be more questionable. However, it is essential that use of modern materials and technology is integrated with traditional techniques in a harmonious and compatible manner to inject new life into the buildings, as discussed by Feilden (ref 2).
The following suggestions regarding the techniques and modern materials may be of particular value in order to develop specific restoration proposals for heritage buildings in the Gujarat region.
1) Stainless steel ropes introduced into the courses of cylindrical masonry structures to secure them against lateral failures.
2) Metal straps with bolted connections to secure polygonal forms.
3) Epoxy injection grouting for strengthening weakened and cracked masonry.
4) Use of corrosion resistant reinforcement in concrete slabs; this can take the form of galvanised steel, or reinforcement in non-ferrous materials such as carbon fibre reinforced plastics
5) Jacketing of disturbed wall surfaces by means of applying a mesh of non-ferrous or galvanised steel reinforcement to both sides of the wall, bonded to the wall with a mortar matrix.
6) Vertical and horizontal drilling for the insertion of grouted steel reinforcing bars.
ACKNOWLEDGEMENTS
The following report was made possible by generous travel grants from the Charles Wallace Trust and INTACH (UK) Trust, which financed the travelling expenses of Professor Vasavada and Edmund Booth. The local assistance of members of the Gujarat chapter of INTACH is also gratefully acknowledged. The authors benefited from helpful discussions with Sir Bernard Feilden.
REFERENCES
Ref 1. Feilden B. Between two earthquakes. Cultural property in seismic zones. ICCROM/Getty Conservation Institute, 1987.
Ref 2. Feilden B. Conservation of historic buildings. Butterworth London. Paperback edition 1994.
Edmund Booth (edmund@booth-seismic.co.uk ) & Rabindra Vasavada (rajavada@hotmail.com) welcome feedback, comments and corrections to this report
