53MARCH 2012 The IndIan ConCreTe Journal

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Are heritage structures in Tamilnadu seismically vulnerable?

A. Veerappan

This article is written with the sole purpose of starting a healthy discussion and providing a different point of view on the earthquake vulnerability of Chennai’s historical buildings such as Fort St. George and Madras High Court, following the revision in IS 1893:2002 changing Chennai’s earthquake proneness from Zone II to Zone III.

The basis for this discussion is a report published in a Chennai daily using somewhat debatable observations made by a well-known Professor and structural engineering consultant. The expert is reported to have said that:

From colonial structures such as Fort St. George to single pier flyovers and 20-storey high rise buildings springing up in the suburbs, there is a need to check their earthquake resistance and carry out seismic retrofitting

While delivering a lecture at a well known University in Chennai, the expert is reported to have further stated that:

Fort St. George, seat of power in the state, would not withstand earth quakes of Zone 2 intensity”. (When the codes for the earthquake were revised in 2002, Chennai was moved up to Zone 3 up from Zone 2 as it was found to be more vulnerable). “No effort has been made to strengthen it” said the expert.

Similar is the case of the Madras High Court buildings. In Ripon Buildings, modifications have been suggested as part of extensive rehabilitation, but as of today it was also vulnerable, said the retired professor.

The thickness of the shear walls, the ground floor car park and the use of flat slabs in high-rises were also quite vulnerable to earthquakes, as they would not be able to resist the “lateral load”.

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The expert claims that his study on eight modern apartment buildings in the city has revealed that seven of them were not earthquake-resistant. He further states that the single-pier bridges in the city on Pantheon Road, Royapettah High Road and TTK Road would suffer from “functional vulnerability” in the event of an earthquake. To reinforce his point he showed slides featuring quake damaged bridges that were designed with single-piers. For some other bridges in Chennai his recommendation was for providing seismic arresters to prevent their collapse in the event of an earthquake. He listed inadequate planning, design and application, poor code compliance, non-engineered buildings, inadequate detailing of reinforcements, extensions, alterations and encroachment as the reasons for the vulnerability of the Chennai monuments and buildings of historical importance to earthquake forces.

In my opinion these observations must be examined in detail and the context understood.

Perhaps these statements were made with the purpose of creating an awareness among civil engineering students and faculty about the importance of designing structures, both load bearing and RC framed structures, taking the seismic forces into account and following IS 1893 : 2002. Perhaps the expert was also right in pointing out that many of the multi-storeyed apartment buildings with stilt (soft storey) constructed recently were not designed to resist even an earthquake of moderate intensity.

However, to me most of the observations appeared to be generalised statements made without any specific data on building distresses and corresponding past earthquakes.

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In my opinion, even though many of buildings the expert referred to were designed 100 years ago, without taking earthquake forces into consideration, they might not get distressed in the event of an earthquake simply because their layout, wall thickness and other systems for transmitting loads were adequately constructed based on the knowledge available then.

And it is not only true for Chennai buildings, throughout the world century old buildings and monuments were not designed constructed considering earthquake forces.

However, renowned architects and builders of the yester years constructed buildings by designing them following the norms prevailing then. They especially included wider foundation base, thicker walls (say min 1’6” to 5’0”) that distributed loads more or less uniformly; more particularly they ensured that the centre of gravity (CG) of the load system was below 1/3 height of the structure, thereby providing stability, safety and serviceability to the structure. They also made sure that the buildings were without any tilt or overtapping. Many such constructions have performed well until now.

Chennai‘s previous classification of earthquake proneness was under zone II. In 2002, it was changed to zone III in IS 1893 (Part – I): 2002 based on the digitised data of Survey of India.

Would such a re-rating of earthquake proneness cause any severe effect on the existing century old structures? Should a mere change in the classification be the basis for worrying about the stability of century old structures?

The answer to these questions lies in examining the structures and analysing their construction data. To begin with, TN never experienced an earthquake exceeding 6 on the Richter scale. The quakes of 1966 and 2001 measured 5.4 and 5.6 on the Richter scale respectively.

The change in classification means that in future the buildings have to be planned, designed and constructed providing adequate safety against the likely Zone III seismic forces (3.5 to 4.20 in Ritcher Scale)..

In order to effectively resist the seismic and lateral forces, one of the important criteria specified in the building construction in general and in the framed structures in particular is the provision of strong column and weak beams. In other words, the stiffness of columns, piers or pillars should always be greater than the stiffness of beams and supporting floor or roof slabs.

An examination of the old buildings in Tamil Nadu suggests that this theory was indeed followed in Chennai’s Fort. St.George Secretarial buildings, Rajaji Hall, Cheppakkam PWD Buildings, Presidency College, High Court Buildings, Madurai’s Thirumalai Nayakkar Mahal, Thanjavur’s Serfoji Palace and in many multitiered Hindu Temples.

In sizing piers and the thickness of the load bearing walls the engineers of yester years have taken care to effectively prevent distress in the structure due to lateral loads including from seismic forces by constructing stronger pillars, piers, walls compared to floor, roof beams and slabs.

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Similarly, the foundation base of these buildings are symmetrical either as a square or rectangular, without any eccentricity or cantilever projections. Building such foundations is one of the criteria specified for earthquake resistant structures. The Fort St. George Secretariat buildings and multistoreyed Building (MSB) satisfy this important criterion and thereby minimize the effect of earthquake forces. Many of them are supported on a serried of well foundations?

To resist lateral forces, some other factors that play an important role in the foundation system are the width of the foundation and thickness of the load bearing walls. In many of Tamil Nadu’s historical and monumental buildings including the palaces and temples, the base width of the foundation is in the range of 5’0” to 10’0” or more and the ground floor wall thickness is in the range of 1’6” to 3’0”. Further the CG of the entire load system is ingeniously arranged to be at 1/3rd of the height of the buildings. Further, Height/Breadth/Width or Length/Breadth value is also kept well within 2 to

impart safety and stability to the structure against lateral forces including earthquake.

The Fort St.George and New MSB Secretariat buildings also satisfy these criteria. They are therefore are not likely to get distressed due to earthquake forces of the intensity as experienced in the past.

Yet another important aspect in these structures is the load bearing masonry construction with allowable compressive and tensile bearing stresses.

The factor of safety in many of these old constructions is 10 (not 3) in compression and 8 in bending /tension as against ( 1.5 x 1.5 =2.25) 2.25 specified in IS: 456 - 2000. Due to this higher factor of safety adopted in the load bearing masonry structures such as Fort St. George Secretariat and other historical buildings, the safety and stability of these structures against

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seismic forces is adequate. These details explain why these buildings did not get affected during the earthquakes of 1966 and 2001.

The design concept for structural use of unreinforced masonry is given in TN Building Practice, NBC of India – 2005 , IS 1905 – 1985 - Code of Practice and SP : 20 (S & T) – 1991 Hand book on Masonry Design and Construction – BIS. Although the present day structural engineers and design experts are highly knowledgeable about framed RC structures, their exposure to the nuances of load bearing masonry structures is not enough. It would not be a surprise if many of them are not even aware of what the guidelines provide in IS : 1905 – 1985 and SP : 20 (S&T) – 1991.

More than theory, analysis and design of structures conforming to the latest codal provisions, the performance of the structure during its life time should be the basis for vulnerability comments.

Needless to say that buildings such as St. George, Chennai High Court and other Secretariat buildings

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withstood and performed excellently during the earthquake in 1966 and 2001.

The expert’s comment about the vulnerability of single piered flyovers on Pantheon Road and TTK Road, casting doubts on the ability of these structures to withstand the earthquake forces corresponding to Zone II / III is also debatable. Those involved in the construction of these structures were expected to follow the applicable design codes in designing and constructing the flyovers’ structural elements including single piered columns (with double cantilevers). The design of these structures follows IRC Standards and includes seismic forces. Only after satisfying themselves about the competitiveness of the designs do the authorities approve them for execution. Therefore, these flyovers are also not likely to suffer from any distress during earthquakes of the intensity as experienced in the past.

For the sake of this discussion it is presumed that the expert has analysed and checked the stability of these single piered columns as free standing cantilever. However, design practices suggest that these single piered columns, from their real edge conditions, are to be treated as propped cantilever (since sway is adequately resisted and restrained by the heavy beam elements, under which the intensity of moment due to lateral load is reduced to ¼ of that of the free standing cantilever). In view of this fact, the approving civic authorities are encouraged to check the design details of the structures and reply to the expert based on the data.

Notwithstanding the above, it is necessary to check the safety and stability of the historical buildings and present day RC framed structures with respect to likely earthquake

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forces and take suitable measures as specified in the following codes.

IS 1893:2002 Indian Standard criteria for Earthquake Resistant Design of Structures

IS 4326:1993 COP – Earthquake Resistant Design and Construction of Buildings.

IS 13828:1993 Indian Standard – Guidelines for improving Earthquake resistance of Low strength masonry buildings

IS 13920:1993 Ductile Detailing of RC Structures subjected to Seismic forces.

In this connection it is worthwhile to consider the opinion of Prof. S.K.Duggal, Prof of Civil Engineering, Motilal Nehru National Institute of Tech, Allahabad and author of the book titled “Earthquake Resistant Design of Structures” published Oxford University of Press (2007).

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“Severity of ground shaking at a given location during an earthquake may be minor (occurs frequently), moderate (occurs occasionally) or strong (occurs rarely). The probability of a strong earthquake occurring within the expected life of a structure is very low. Statistically, about 800 earthquakes of magnitude 5.0-5.9 occur in the world, while only about 18 of magnitude above 7.0 are registered annually. If a building is designed to be earthquake – proof for a rare but strong earthquake, it will be robust but too expensive. The most logical approach to the seismic design problem is to accept the uncertainly of the seismic phenomenon. Consequently, the main elements of the structure are designed to have sufficient ductility, allowing the structure to sway back and forth during a major earthquake, so that is withstands the earthquake with some damage, but without collapse. An earthquake-resistant structure resists the effects of ground shaking; although it may get severely damaged, it does not collapse during a strong earthquake. This implies that the damage should be controlled to acceptable levels, preserving the lives of the occupants of the building at a reasonable cost. Engineers thus tend to make the structures earthquake resistant”.

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Finally, the following may be suggested for making the buildings and structures resist the seismic forces to a reasonable extent.

Construct RC / Concrete Tie beams in both directions at Grade beam level (just below the EGL)

Provide RC continuous lintel at lintel level

Have RC Floor / Roof beams in both directions at Floor level / Roof level.

Reinforce the four sides / circumference of large openings

Separate the asymmetrically loaded / projected cantilever elements from the main structures

Reinforcing load bearing masonry walls at corners and junctions as shown in the enclosed sketch.

Have compulsory provision of Ductile detailing of Rebars in foundation footings, Columns, Beams, Slabs and beam – column junctions as specified and detailed in IS 13920 – 1993 (Sketches enclosed)

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A. Veerappan, ME(Struct) FIE, MICI, Dip LL and AL is former Chief Engineer, Tamil Nadu Public Works Department. Presently, he is a structural consultant and state secretary of TNPWD Senior Engineers Association. He has delivered more than 2500 technical lectures to practising engineers (government engineers in particular).

He has edited and published 40 technical handbooks for day-to-day use of field engineers and engineering students. He has won several prizes for his engineering writings, which have highlighted the importance of adopting modern constructions techniques and cost effective construction materials including the use of construction chemicals. He has many years of experience in restoration and strengthening of distressed buildings and foundations.

Adopt Grade of concrete and steel Rebars according to exposure conditions as specified in Table No. 3 & No. 5 of IS : 456 -2000 Code of practice for Plain & Reinforced concrete – BIS

These measures are sufficient to protect buildings and structures against the seismic forces expected to occur in and around Chennai. So the learned Professor’s statements should be seen as a statement of caution and his stress on the importance of designing and constructing buildings and structures against the codified earthquake forces must be understood, appreciated and adopted in practice to have durable structures in the state of Tamil Nadu.

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