UPGRADING HISTORIC BUILDINGS TO RESIST EARTHQUAKES

By Richard Swift of Gifford and Partners

Summary

Cintec anchors installed in traditional materials provide enhanced shear resistance to seismic forces. The anchors induce low bond stresses in the masonry and yet because of their considerable length and perimeter can sustain very large forces without failure.

The anchor installed in a masonry structure can improve (i) the integrity of the wall construction (ii) the wall to wall connection (iii) increase the out of plane bending strength of walls and (iv) improve the capacity of the wall/roof and wall/floor connection.

The anchors may be installed vertically to assist in the resistance to vertical accelerations.

Base isolation requires the installation of a discontinuity between the foundations and the superstructure of a building. For a traditional building this means introducing a layered material which has a low resistance to shear at the base of the full bearing perimeter of the structure. This is expensive.

Vertical stiffness is generally required for gravity loads, seismic isolation is only appropriate for horizontal motions.

Base isolation is normally appropriate to framed buildings.

The anchor body is generally of stainless steel with a high ductile capacity.
The anchor is invisible when installed and is therefore particularly applicable to historic buildings.

The number, size and dimensions of the anchor may be "tuned" to the requirements of the anticipated seismic forces and to the nature of the building.

The installation of the Cintec anchor increases the redundancy of a building and this means that the loss of a particular building element may not necessarily mean the total collapse of the building.

There is no doubt that the installation of an appropriate pattern of anchors into a traditional building be it framed or cellular masonry will reduce the risk of loss of life. The extent of the damage to the building obviously depends on whether or not the seismic forces exceed the ultimate capacity of the anchors.

Statistically there is little recorded evidence of the behaviour of the Cintec anchor in an earthquake. The system is a relatively recent innovation and the benefits of the improved seismic resistance of the anchor are only now being appreciated.

English Heritage are the conservation guardians in the United Kingdom and are recommending the use of the Cintec anchor very widely. It has a particular merit in restraining fractured structures without re-building of the parts. It can be used in very delicate and distressed buildings to restore the structural integrity. It is one of the guiding principles of conservation in the UK and the principles of the Venice Charter 1964 and the Burra Charter 1979 that the original fabric of the building should be preserved. This is why the Cintec anchor is so useful.

The Cintec anchor may be used in a wide range of materials and the anchor body size and even material type can be adjusted and the diameter of the cored hole will be adjusted depending on whether the parent material is concrete, clay, terracotta, adobe or even timber. The costs will vary greatly for each application.

Research is showing that the introduction of a suitable array of anchors may also be used to dramatically improve the resistance of traditional buildings to explosive blast.

The conservation of historic buildings is a specialist aspect of structural engineering. Many times it is difficult to assess the capacity of weak masonry or friable lime mortars or accurately judge the strength of a decayed timber.

The natural inclination of the structural engineer is to dispense with the uncertain and replace it with a material for which there are well recorded structural properties. The conservation engineer must resist this temptation and understand that if the structure is still standing then the structural elements have combined together to adequately support the loading regime to which they are subject. The conservation engineer is frequently required to make an engineering judgement rather than rely upon published structural properties.

In the United Kingdom the Society for the Protection of Ancient Buildings (1) was established in 1877 by the artist poet and socialist William Morris with his friend John Ruskin. The society was established as a protest because many churches and cathedrals were being drastically altered and restored by Victorian architects such as Sir Gilbert Scott who saw the decay as a disgrace to God and therefore swept away the historic fabric and created a new structure. The manifesto (appendix A) of the SPAB called for "protection in the place of restoration to stave off decay by daily care, to prop a perilous wall or mend a leaking roof by such means as are obviously meant for support or covering and show no pretence of other art and otherwise to resist all tampering with either the fabric or ornament of the building as it stands."

Later the Venice Charter of 1964 and the Burra Charter of 1979 confirmed these principles and emphasised the necessity to conserve the historic fabric in their natural setting.

The problem is that there are times when we think we can improve upon the construction of a monument and in the process alter it. In 1993, I was requested by UNESCO to assist with the stabilisation of the core of the second pyramid Chephren on the Giza Plateau. Unauthorised excavations had been commenced to discover a supposed second burial chamber in Chephren. These excavations had caused some instabilities in the core. The core rather disappointingly was found to be a random shaped rubble loosely packed together and not the regular shaped blocks which I had expected. Some localised propping was undertaken to restore structural integrity.

On November 20 1992 there was a fire at Windsor Castle which resulted in 105 rooms being destroyed, 9 of them were principal rooms. Gifford and Partners were appointed as structural engineers for the project (2).

Windsor Castle is a Scheduled Ancient Monument and a Grade I listed building. This means that any dismantling of the existing fire damaged structure would be contested. A major part of the project therefore was the assessment of the surviving structure and undertaking minimum repairs to enable it to support the imposed loads again.

One of the principal rooms at Windsor is St Georges Hall. The existing 17th century roof which had been strengthened by Smirke in 1841 was virtually destroyed. The fire-damaged masonry gave cause for concern in several respects. Firstly the delicate nature of the surfaces, particularly at window reveals required consolidation. Secondly, the removal of the panelling revealed the many poorly bonded joints between the periods of different construction. An example of this and where the above two features were evident together is the north east window in St Georges Hall. The masonry structure around this window was in very poor condition. Sections of the wall date from the fourteenth century.

The wall was strengthened by using CINTEC anchors up to 12 metres in length to strengthen the friable weak masonry. The anchor consists of a stainless steel anchor body contained within a woven polyester-based sock. The anchor body may be a solid bar, hollow section or threaded rod according to the designer's requirements. A hole is diamond cored into the masonry and the anchor introduced into the hole. A cementitious grout is pumped at very low pressure into the sock around the anchor body. The sock expands to fill the hole in the masonry. A very good bond is created between the parent masonry and the grout and between the grout and the anchor body. If movement of the masonry is likely to occur it is resisted by low shear stresses between the parent masonry and the grout and hence into the anchor body. The system is very effective for weak masonry.

The CINTEC anchor was also used at the top of Brunswick Tower, where the fire had resulted in the movement of the crenellated parapet. The merlons had rotated outwards like opening up the segments of an orange. Initially it was considered that the only effective means of repair was the complete taking down and rebuilding of the parapet. This would involve loss of the patina of age and important architectural details of joint relationships and historic stone faces. An alternative solution was proposed, which involved the installation of CINTEC anchors like a corset around the faces of the octagonal tower. Holes of 40mm diameter were drilled, to accommodate the 20mm anchor body, at the centre of the thickness of the parapet. The anchor was installed and grouted and the entry hole pointed up so that there was little evidence of the remedial work. The movement cracks which had been created by the fire were monitored over a period of time to ensure that the anchors were effective in preventing any further movement.

The CINTEC anchor is capable of sustaining very high pull out loads on friable, delicate masonry. This was proved by the use of the anchor to resist the very high uplift loads exerted upon the temporary scaffold roof by wind forces. Windsor Castle sits high upon a chalk mound well above the general level of the surrounding countryside. Wind forces generated upon the nearly flat scaffold roof were anticipated to be very high. The only method of anchoring some of the scaffold legs down was to connect them by some means to the surviving walls of the fire damaged areas. A 2 metre (6 feet 6 inch) long anchor was drilled vertically into the thick masonry wall and tested to 170kN (17 tonnes). The anchor passed the pull out tests and was used to resist the uplift on the scaffold roof.

On October 12 1992 an earthquake measuring 5.9 on the Richter Scale struck Cairo. The city stands on very deep alluvial sand, silts and clays, forming either side of the Nile delta. The peak ground acceleration was estimated at 10% of gravity. The nature of the quake with its relatively high frequency motion meant that damage to low structures of up to 5 storeys was more likely.

I was requested by UNESCO to inspect a sample of the 150 national monuments in Cairo which were reported as being damaged by the quake. The majority of these were mosques. In truth, much of the damage evident could be attributed to decay, the leaking drainage system and the contaminated perched water table. However Cairo has a history of earthquakes, one significant event every 30 to 50 years.

The Egyptian Antiquities Organisation has a staff of 23,000 inspectors, architects, engineers and ancillary staff. The immediate response was to erect various types of propping and scaffolding by an organisation eager to show that it was doing something. I saw minarets dating from the thirteenth century taken down because someone said that they were leaning. No one had made any measurements. I saw scaffold towers erected 20 metres high in the middle of a busy street in a vain attempt to provide some bracing to a minaret 50 metres high which someone had said was moving. Near vertical timber props had been put alongside apparently leaning walls.

The monuments which are all unreinforced masonry buildings experienced different types of damage including (1) diagonal cracks due to in-plane shear failure, (2) lack of lateral restraint resulting in serious damage or collapse of walls or parapets, (3) lack of adequate connection of the walls to the floors or roof leading to collapse, (4) lack of adequate anchorage between walls resulting in vertical cracks and separation in the corners at the connection between walls at right angles.

More recently we have prepared repair solutions for a few particular mosques. The Mosque of al Ghuri, monument No. 189 (909AH/1504AD), has just been repaired (3). The initial repair scheme anticipated stitching the whole of the south wall which had separated from the lateral walls over its full height and length to leave it as a freestanding wall. In the event the Egyptian Antiquities Organisation renewed the stone at this junction. However the four severely distressed arches of the sahn were stitched as well as the severely distorted walls of the north iwan or open porch. CINTEC consolidation anchors were used widely to restore the integrity of the wall construction. These anchors are invisible in the repaired structure but serve to tie inner and outer skins of the wall together as well as the core. Anchors were installed into the decorative stones of the arch. By careful pelleting (removing a circular surface disk and setting it aside for re-fixing after installation of the anchor) the decorative integrity of the masonry voussoirs was retained.

The Mosque of Sarghatmish, monument No. 218 (757AH/1356AD), was also severely fractured by the earthquake. The presence of the heavy masonry dome led to the large movement of the south west wall and the whole of the south wall had separated from the intersecting walls. A scheme of CINTEC anchors to resist an acceleration of 0.15g has been designed for installation.

In 1996 a European funded project called the TOSQA (Town Scape Quake Assessment) (4) research project was undertaken. This involved the assessment of four historic town centres, Naples, Rhodes, Lisbon and Castiglione Causaria. The perception is that the historic centres of Europe's cities are under threat from general decay and deterioration from earthquakes and natural hazards and from insensitive alterations. Appropriate upgrading strategies are needed which will fulfil the following criteria: life safety for occupants, limitation of damage from future earthquakes, limitation of alterations to the appearance of fabric of buildings, responsive to economic needs.

The project looked in particular at the case study of Alfama in Lisbon. More than 35% of the buildings could be expected to experience the total or partial collapse of one wall in the event of an earthquake of intensity VII a one in 100 year occurrence. The introduction of ties to attach the facades to the floor beams or to the cross walls can have a very significant effect on vulnerability. This is especially true of the taller buildings 4 to 6 stories high.

In the research done by D'Ayala Spence Oliveira and Pomonis 1997 (5), a survey of 200 buildings in Alfama was undertaken to investigate structural features and condition followed by analysis of key collapse mechanisms to define static collapse loads under horizontal forces for each building. The results obtained in terms of earthquake ground motions likely to produce equivalent damage led to the development of vulnerability functions for the case study. The method was used to predict the reduction in losses achieved by the introduction of low cost unobtrusive strengthening techniques such as tie-rods connecting façade walls to floors and crosswalls. Cost benefit analysis, considering only structural costs indicates that the return on the investment would be considerable.

Two classes of failure mechanism were considered in the study out of plane (OP) failure and (IP) in-plane failure. These yield two values of equivalent shear capacity, calculated with equivalent static models as the critical acceleration causing the mechanism to take place.

For each building the equivalent shear capacity was calculated in terms of the a/g ratio (peak horizontal ground acceleration versus gravity acceleration) needed to trigger one of the two mechanisms. These were grouped into 7 bands (roughly corresponding to MMI intensity level resisted). It was found that only about 30% of the sample have a shear capacity exceeding 0.16 a/g. In most cases the shear capacity is governed by the overturning out-of-plane mechanisms (existing shear capacity). If the out-of-plane mechanism is prevented by some strengthening device, the in-plane mechanism IP would become the effective limit on the shear capacity (maximum shear capacity). The difference between the two cumulative curves can be interpreted as a first qualitative estimate of the potential benefits of such strengthening: it could be expected that nearly 70% of the buildings would achieve a shear capacity equal to or greater than 0.16 a/g and about 50% would have a resistance comparable with the value chosen by the code for the design of a new building 0.33 a/g.

Consideration was given to a particular building, the Rua Guillermo Braga No. 5. This is one of the buildings that was most vulnerable in the study and one which would benefit most from a strengthening intervention.

The front and rear elevations which are free from connection to other buildings and present several openings are the most at risk failing in out- of- plane bending. This may be resisted by connecting the frontal and rear elevation to the party walls by means of ties.

Two magnitudes of equivalent ground acceleration have been considered as design values; 0.10g and 0.25g (elastic limit value). In figures 1 and 2 typical strengthening at each floor is shown for the two options. In the first case only ties to connect the frontal walls to the lateral are used at each floor and the resulting shear forces are equilibrated by the shear strength of the party walls.

In the second case this strength will not be sufficient and therefore the floor structure at each level has also been strengthened to act as an horizontal diaphragm able to redistribute the shear forces among both frontal and lateral walls. In order to achieve that smaller ties connect the front and rear walls directly to the floor.

In December 1989 Newcastle cathedral in New South Wales, Australia was severely damaged by an earthquake having a Richter magnitude ML 5.5 or ML 5.6. Newcastle was located in a 'zero' seismic zone.

The effect of the earthquake on the building was largely as could be expected; high set stone crosses and other decorations were dislodged and fell to the ground or lower roofs; flying buttresses were dislodged but none fell completely away; shear cracking occurred in the nave walls which lie roughly parallel to the direction of the seismic wave and out-of-plane movements occurred in the east wall and dislodged windows.

A conservation plan was produced for the building, recording the history of the development of the building. Structural design parameters were established using experience gained mainly from New Zealand and California.

An Australian consultant, Bill Jordan and Associates undertook the structural design (6).

The prime aim of the structural design was to turn the building into a ductile structure. It was accepted that different parts of the building would end up with different degrees of ductility and so would be likely to suffer different degrees of cosmetic damage in a future earthquake. However this had to be accepted in order to maintain the aesthetic significance of the building, but with the over-riding criterion that the risk of personal injury to occupants of the building during a future earthquake would be reduced to a minimum.

Ductility could be introduced into the building either by the addition of ductile frames with the obvious visually intrusive consequences or by fully reinforcing the brickwork. In the event a combined system was chosen which used some frames where they were hidden from view: behind the parapets, in the roof space and inside the upper section of the tower.

With the design parameters set, a two dimensional finite element analysis was carried out using a readily available computer package. Different combinations of reinforcement strength and spacing were considered and costed. The CINTEC anchor selected was a high strength Grade 316 stainless steel deformed bar used in sizes from 16mm to 32mm diameter. In order to limit the amount of drilling that had to be carried out 'Hi-proof' bar with UTS in the range 790 to 920 N/mm2 was specified.


The total length of reinforcing installed was 3770 metres. Anchors up to 35 metres in length were used.

Research is currently being undertaken in the UK and in the USA of the effectiveness of the CINTEC anchor installed in masonry walls to resist blast. Masonry panels have been constructed and then retro-fitted with CINTEC anchors. The wall panels have then been subjected to various sizes of explosive charge at different distances from the panel. The results have been very encouraging and not only have strengthened walls survived but also the volume of masonry shard projectiles from the back of the wall been greatly reduced.

Conclusion

In conclusion the CINTEC anchor is a valuable method of repairing historic structures. It is particularly useful in the seismic strengthening of traditional buildings. Its introduction into the building of our historic town centres will reduce the loss of life in the event of an earthquake and will increase the chances of survival of our historic monuments.

The Cintec anchor is an alternative seismic strengthening device to base isolation. The Cintec anchor introduces ductility into the brittle masonry structure and may be used to resist vertical and horizontal accelerations.

The Cintec anchor may be used in a wide range of materials and the anchor body size and even material type can be adjusted and the diameter of the cored hole will be adjusted depending upon whether the parent material is concrete, clay, terracotta adobe or even timber.

Research is showing that the introduction of a suitable array of anchors may also be used to dramatically improve the resistance of traditional buildings to explosive blast.

References

1. The SPAB. The Society for the Protection of Ancient Buildings, 37 Spital Square LONDON E1 6DY.
   
2. 'Restoration - The Rebuilding of Windsor Castle' by Adam Nicholson published by Michael Joseph 1997. ISBN 07181 4192X.
   
3. Islamic architecture in Cairo - an introduction by Doris Behrens-Abouseif published by the American University in Cairo 1989 ISBN 977 424 2033.
   
4. The Tosqa Project. Earthquake Protection for Historic Town Centres: the case of the Alfama, Lisbon. Contract EV5V CT93-0305. The Martin Centre, Cambridge University. Instituto Superior Tecnico, Lisbon. 
   Final Report September 1996.
   
5. Earthquake Loss Estimation for Europe's Historic Town Centres by Dina D'Ayala, Robin Spence, Carlos Oliveira M.EERI and Antonios Pomonis M.EERI.
   Earthquake Spectra, volume 13, No. 4 November 1997
   
6. Earthquake damage repair and strengthening of Christchurch Cathedral Newcastle NSW. By B J Collins and J W Jordan. Paper presented at Structural Faults and Repair - 97 conference, Edinburgh, Scotland.

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