Retrofitting using Base Isolation Techniques

for Masonry Buildings

MASONRY STRUCTURES

(CE 625)

By:

RAHUL YADAV SAKET KUMAR SHUBHAM TRIVEDI

(Y8127391) (Y8127439) (Y8127509)

Instructor:

Prof. D.C.Rai

IIT KANPUR

Contents

Abstract 31 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

1.1 Sliding Friction Isolator Systems . . . . . . . . . . . . . . . . . . . 51.2 Elastomeric based systems . . . . . . . . . . . . . . . . . . . . . 61.3 Rocking pillar base isolation systems . . . . . . . . . . . . . . . . 9

2 Case Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112.1 Salt Lake City Hall . . . . . . . . . . . . . . . . . . . . . . . . . . 112.2 MacKay School of Mines . . . . . . . . . . . . . . . . . . . . . . . 12

3 Inferences and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . 15

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List of Figures

1 Mathematical Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Rubber bearing isolator components . . . . . . . . . . . . . . . . . . . . 73 Schematic layout of the beam connecting the column bases.[Luca et al., 2001] 84 Geometry of Rocking Pillar [Kawamata et al., 2004] . . . . . . . . . . . . 105 Movement of Rocking Pillar [Kawamata et al., 2004] . . . . . . . . . . . . 106 Building after completion, circa 1900 [Allen and Bailey, 1988] . . . . . . . 127 Typical Isolator installed in the building [Allen and Bailey, 1988] . . . . . 128 Elevation Mackay school of mines [Way and Howard, 1992] . . . . . . . . 139 Building section and isolators Mackay school of mines[Way and Howard, 1992] 1310 Isolator system: Bearing [Way and Howard, 1992] . . . . . . . . . . . . . 1411 Isolator system: Sliding [Way and Howard, 1992] . . . . . . . . . . . . . 14

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Abstract

This report covers a literature review on the development and implementation of the baseisolation technique for the seismic isolation of historic or old masonry. To preserve theold and historic buildings with minimal changes, the base isolation technique presentsa good fit to the situation. A basic study of the base isolation is done to understandthe underlying concepts involved in this idea as implemented to masonry buildings. Thebasic idea of base isolation is reviewed with implementation using geosynthetics andmarble plates. An advanced technique of base isolation in masonry using short masonryrocking piers is then reviewed. With this background of knowledge, we then review someinteresting cases of implementation of this concept in real world projects involving someold masonry buildings.Keywords: Masonry Retrofitting; Base Isolation; Sliding Isolator; Rubber Isolator

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Retrofitting of Historic Masonry BuildingsUsing Base Isolation

1 Introduction

Retrofit of historic structures is of foremost importance as they need to be preservedat all costs. Earthquakes provide a serious hazard that must be dealt with. Old build-ings have little seismic resistance as earthquake resistant design was not well understoodback then. Many ancient pieces of architecture today stand at the risk of demolition ifmajor earthquakes hit. But due to amount of structural detail and these buildings andthe complications in the structure make traditional retrofitting very difficult. Moreover,traditional procedures of strengthening often require bringing down certain parts of thestructures and major restructuring in others. Such procedures are unacceptable as theydamage the architectural beauty of the historic buildings. Base Isolation technique pro-vides an excellent alternative to the conventional strengthening procedures in this case.The basic approach underlying this technique is not to strengthen building, but to reducethe earthquake-generated forces acting upon it. Base isolation ensures no major changesin the structure of the building; rather few changes are required at the foundation level ofthe building which is less critical as opposed to major changes in the more visible partsof the building. It also ensures the safety of the nonstructural parts of the building whichare not considered in the conventional design procedures.

Most of the structures in our country are unreinforced masonry. These types ofstructures suffer the maximum damage in earthquakes and also cause most casualties.Hence sufficient procedures are necessary for retrofit all such highly vulnerable buildings.Base isolation can provide an effective and relatively cheap technique to ensure the safetyof weak and inappropriately designed masonry buildings in the country. Mass productionand subsidized distribution of base isolation systems by the government can result in areally cheap method improve the performance of many poor buildings in the country.

We have discussed the following types of implementations of base isolator systems:

1. Sliding Friction Isolator Systems

2. Elastomeric based Isolator systems

3. Rocking pillar base isolation systems

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1.1 Sliding Friction Isolator Systems

Base isolation system based on sliding was first proposed in 1909 by a doctor JohannesAvetican. These kind of base isolation systems are the simplest kind of base isolators.Isolation was first considered as the seismic design strategy by the Italian government afterthe Messimo-Regio earthquake of 1908 which killed around 160,000 people due to collapseof the unreinforced masonry building. URM buildings were the typical building beingconstructed at that time. He proposed two kinds of base isolators one was interposingthe sand layer in the foundation and the another was by providing rollers to the baseand apart from these they also specified a design with low height and lateral forcesconsideration and this design method was used majorly but not the base isolation.

Later in after the earthquakes in Bihar(1934) and Dhubai(1930) some of the smallmasonry buildings which slid on their foundations performed well in the earthquake.Since the elastomeric base systems like rubber bearings may not be that economical asfar as smaller buildings are considered , Arya(1984) [Arya, 1984] proposed sliding systemsto decouple the structure from the ground.

Similar studies on the base isolation was done in China after devastating Tangshanearthquake in 1976. Chinese engineers observed that the some of the masonry buildingssurvived which slid on the ground (This was not because they were having the slidingisolators but because of the significant crack at the base due to larger base shear whichresulted in around 6cm slip). Dynamics of the structure in such kind of isolators havebeen done considerably and in most of the cases Coulomb damping is assumed as theenergy dissipating mechanism. This energy dissipation through the friction decreases theresponse of the structure (65% response reductions compared with the a rigidly supportedbuilding, P.Nanda). In some of the cases it has also been found that the friction doesn’tnecessarily implies the reduction in the response, for instance Westermo and Udwadia(1983) studied the periodic response of a linear oscillator on a coulomb friction slidinginterface and found that the response may be larger than that for the same fixed basemodel and that the single degree of freedom model had sub-harmonic frequencies gener-ated by sliding interface. Coulomb friction is generally used in these theoretical analysisbut is unlikely to be accurate in the real behavior.

P.Nanda, Pankaj Agarwal and M.Shrikhande [Nanda et al., 2010] have done similarstudies on isolating the structure by providing pure friction based isolation systems. Theyhave used green marble or geosynthetic sheets at the base. Geosynthetic sheets are easilyavailable in the Indian markets and so it’s a cost effective method. Previous studies haveshown many types of friction reducing material are used in the friction based isolatorsfor example sand, grease, oil, mobil oil, Teflon etc and they should be having a properrange of friction coefficient i.e. it shouldn’t be much less and of course shouldn’t be large,smaller friction will cause undesirable larger displacement at sliding level. Empiricalvalues suggest a value between 0.05 to 0.15. Teflon is a material which has such frictioncoefficient against steel and that’s why in the bridges Teflon is significantly used for theseismic isolation. Apart from these in long term use sand crushes after the shock whichincreases the friction coefficient , oils used may get contaminated with dirt , graphitemay be affected by the chemicals and so using these materials for reducing the frictionis not effective sometimes in long term. While on the other hand geosynthetic/marblescan be a good substitute in the long term. Equations of motions are developed for thecalculation of response in the two cases:

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1. Non SlidingMt(xg + xt) + Cxt +Kxt = 0 (1)

2. Sliding

Top Mass-Mt(xg + xb + xt) + Cxt +Kxt = 0 (2)

Bottom Mass-

Mb(xg + xb)− Cxt −Kxt + µ(Mt +Mb)g × sgn(xb) = 0 (3)

where sgn() is the signum function.

Figure 1: Mathematical Model

The non sliding conditioned is determined when the horizontal inertia force does notexceed the opposing friction force, i.e.

|Cxt +Kxt −Mb(xg + xb)| < µ(Mt +Mb)g (4)

In the present case the to analyze the effect of the ground motion on the isolated structurea synthetic GM compatible with the IS1893(I):2002 is generated with PGA value 0.36g(Maximum Considered Earthquake in zone V). In the result it has been found that thepeak absolute acceleration at the roof level was 0.8g in the fixed base case while in theisolated structure with geosynthetic it was around 0.4 g which indicates a 50% reductionin the response due to friction isolation.

1.2 Elastomeric based systems

Natural rubber bearings were first used for the earthquake protection of buildings in1969 for the Pestalozzi School in Skopje, Macedonia. Characteristic of the isolationsystems of this kind, the horizontal motion is strongly coupled to a rocking motion, so

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that purely horizontal ground motion induces vertical accelerations in the rocking mode.Many a times a steel plate is used as reinforcement inside the bearings which increasesthe vertical stiffness and also it prevents the lateral bulging of the rubber bearings. Suchlaminated rubber bearings are again classified into Low damping natural and syntheticrubber bearings and high damping rubber bearings.

Figure 2: Rubber bearing isolator components

Low damping rubber bearings have been widely used in the Japan with other dampingdevices like viscous dampers , frictional devices etc. It contains many thin steel plates(sometimes bars are also used) joined together with vulcanized rubber which results invery large vertical stiffness and the lateral stiffness is lesser due to less modulus of rigidityof rubber used. The material behavior in the shear is almost linear up to strains levelmore than 100% , with the damping in the range of 2-3% of the critical. There are manyadvantages of such a bearing, they are unaffected by the rate, temperature history, oraging and are also easy to manufacture. Disadvantage is that a supplementary dampingsystem is generally needed which always requires the use of many connections and alsothe metal used in it are prone to low cycle fatigue.

The rubber isolators have a very simple design procedure.

1. Select a suitable isolation time period (T) for the structure.

2. Determine the required horizontal stiffness for the rubber isolators from the isolationtime period.

3. Calculate the cross section area for the rubber from the compressive stress require-ments from the loads of the superstructure.

4. Estimate the height for the required horizontal stiffness for the calculated crosssection area.

5. Check for shape factor to determine the final isolator dimensions.

High damping natural rubber bearings are having higher inherent damping and sothe need for supplementary damping elements can be eliminated. It was first made in1982 by Malaysian Rubber products Research Association of UK. Damping is increasedby extra fine carbon blocks. The material is nonlinear at shear strains less than 20% andis characterized by higher stiffness and damping which tends to minimize the response

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under wind loadings and low level seismic loads unlike in case of the low damping rubberbearings which needs foams in the gaps to prevent displacements under wind loads orother secondary smaller loads.

Using Rubber bearings for base isolation for retrofitting historic buildings:

We have reviewed the paper [Luca et al., 2001] titled “Base isolation for retrofitting his-toric buildings: Evaluation of seismic performance through experimental investigation”by Antonello De Luca, Elena Mele, Javier Molina, Guido Verzeletti and Artur V. Pinto.

Base isolation is an exciting prospect in the retrofit of monumental buildings. But theexact behavior of the old building material and ancient load transfer systems of system ofarches and columns is not very clear. Hence study was focused on studying the behaviorof typical arch-column system that is common throughout the monuments in Europe tostudy the behavior and discuss the possibility of implementing a similar mechanism inthe retrofit of the major monuments in Europe.

This is because arches are good only in compression. This experiment involved the fullscale testing of arch-column assemblage sitting on high damping rubber bearing isolators.The model is made of three columns, two complete arches and two external half archesmade from stone block masonry. The material for this construction is carefully chosento represent the actual monumental conditions as closely as possible. The vertical loadis simulated by actuators. This typical assemblage is provided with a connection beamat the base of the columns to ensure a uniform action of the isolators at the columnbases. The high damping rubbers used for the isolation in this case have high stiffnessin the vertical direction but the very soft in the horizontal direction. This results in alow natural frequency of the assemblage for vibrations in horizontal direction in the firstmode, thus isolating it from the seismic effects.

Figure 3: Schematic layout of the beam connecting the column bases.[Luca et al., 2001]

The test is performed using pseudo dynamic procedures. The model is full scale butmanufacturing the rubber isolators is very expensive, so they have been made to one-fourth scale. The superstructure and the isolators are thus tested separately and theirinteraction is accounted for analytical modeling. The pseudo dynamic procedure involves

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calculating the displacements from the measured forces and the known quantities of thesystem and then using the displacements to determine the forces to be applied, hencerequiring no shake table in the tests. This procedure is implemented by fast computationsso that the calculation time step is same as the implementation time step, resulting infairly accurate results.

The results of this test indicated significant reductions in the earthquake forces reach-ing to 15 times less force in some cases. No cracking in the masonry was observed for anyof the cases. The lateral displacements at the isolator level were also under the acceptablelimits. Hence this test suggests immediate implementation of rubber bearings to isolatemajor monuments from seismic forces.

1.3 Rocking pillar base isolation systems

This is relatively new technique for isolation of masonry houses. This kind of baseisolation technique is easy to implement in the field. Only to settle down the caisson into ahole dug in the ground finishes the site execution of the isolation foundation. Maintainingthe integrity of the structure is the primary requirement during earthquake. Masonrywalls should be constructed in RC base beams and the RC lintels should also be providedat the floor levels. These RC members combined with RC floor slabs forms a rigiddiaphragm which prevents the out of plane failure of the masonry walls. Restorability ofthe rockling pillar is generally very weak and so additional damping devices to restrictthe excessive motion of the rocking pillars should be provided. The top end of the caissonas shown in the figure acts as a stopper against the excessive rocking motion of the pillarand also rubber cushion is provided against it to relieve the impulse due to collision.

One additional advantage of this system of rocking pillar is that it’s free from torsionalvibrations. In rubber bearing base isolators the center of their stiffness is matched to thecenter of mass of the superstructure by properly adjusting the positions of the rubberbearing in the base, to avoid torsional vibrations. But here since the restoring moment ofthe rocking pillar is proportional to the vertical load, the center of distributed restoringmoment automatically coincides with the center of gravity of the masonry structureand so torsional vibrations are avoided. Implementation using rocking piers technique[Kawamata et al., 2004]:

This paper explores the possibility of a new form of base isolation suitable for ma-sonry houses. It is suited well for small masonry houses. This is very cheap techniquewith a pretty simple installation procedure. It involves minimum site execution for theimplementation of the isolators at the foundation level.

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Figure 4: Geometry of Rocking Pillar [Kawamata et al., 2004]

The isolator includes a rocking pier formed by steel tube filled with concrete andprovided with spherical caps at the end contained in a caisson to allow free rockingmovement and keep free from any disturbance. The pier is allowed to rock betweenstainless steel plates at top and bottom. The pier is inserted at the foundation level justbelow the superstructure columns. The rocking piers require some additional dampingto be implemented by applying lead dampers at the top of the rocking pier as the piershave low restorability and can be excited low wind forces.

Figure 5: Movement of Rocking Pillar [Kawamata et al., 2004]

Derivation of Natural Period of the rocking piers: As the pillar rotates by angle ? dueto ground motion, the superstructure displaces by,

x = 2Lθ (5)

and the eccentricity between the vertical load from the top and the bottom reaction is,

e = 2(R− L)θ (6)

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Now, if the pillar is supporting a mass m, the moment of inertia acting on the pier is,

M = −2mLx (7)

And the resisting moment due to the eccentric vertical forces is,

M ′ = −2mg(R− L)θ (8)

Therefore the equation of motion becomes,

Lx+ g(R− L)θ = 0 (9)

Using the relation between x and L,

x+(R− L)g

2L2x = 0 (10)

So the natural period of this system can be given as,

T = 2π

√2L

(R/L− 1)g(11)

This period is independent of the mass of the superstructure on top of it. Any torsionalvibration is also avoided as the restoring moment is proportional to the vertical load, i.e.,the restoring moment passes through the center of mass. The effect of friction is neglectedas the rolling on steel plates is expected generate little friction.

Results of the testing on a shake table indicate a reduction in story forces of more than80 percent. The undamped configuration shows large displacements while the dampersresult in only small displacements. The dampers also result in effective damping of thevibrations over the undamped case.

2 Case Studies

We have discussed two case studies where the concept of base isolation was used in orderto retrofit the structures.

2.1 Salt Lake City Hall

Salt Lake city was constructed in 1890-94 using the unreinforced masonry. This buildingis situated in the high seismic zone where the expected PGA value was estimated to beas high as 0.55g. Proposal to resist these forces according to UBC like codes requiressignificant construction throughout the building, demolishing of the unreinforced walls atmany places in the building , shotcreting , extensive anchorage of floor diaphragms. Onthe other hand seismic base isolation approach requires lesser efforts. Additional dynamicsoil tests were also conducted to determine the period of the structure site and it wasestimated to be around 1.4 s which was different enough from the base isolated buildingperiod of 2.5 s which will preclude any resonance between the isolator and the ground. Inthe final design 447 bearings were placed on top of the original spread footings with a newconcrete structural system built above the bearing to distribute loads to the isolators.

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Figure 6: Building after completion, circa 1900 [Allen and Bailey, 1988]

Figure 7: Typical Isolator installed in the building [Allen and Bailey, 1988]

A new concrete floor is located above the isolators to act as a rigid diaphragm, con-necting all of the new side beams and linking all of the isolators so that they can acttogether.

One of the drawback of such a method is that the whole building remains in the dangerif any earthquake comes during the process of removal of walls to install the isolators. Toovercome this problem some temporary supporting structures like buttresses should beprovided against the building.

2.2 MacKay School of Mines

In appearance, the building has been designed as a national monument. It is constructedmainly of the URM. Conventional strengthening was also suggested but from the preser-vationist point of view there were definite advantages in the isolation design. According

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to Douglas Way and Jack Howard paper on phase III retrofitting of this building isola-tion system could filter out most of the earthquake forces and so none of the URM wallrequired strengthening. As a result many of the original architectural features of theoriginal building could be salvaged.

Figure 8: Elevation Mackay school of mines [Way and Howard, 1992]

Figure 9: Building section and isolators Mackay school of mines[Way and Howard, 1992]

A combination of elastomers and sliding system was developed and tested on the shaketable at the Earthquake Engineering Research Center. In this system the interior columnsof the structure were carried on Teflon on stainless steel sliding elements and the exteriorcolumns on the low damping natural rubber bearings. The elastomeric bearings helpedin the re-centering of the structure against the wind loads and other smaller secondaryloads and the sliding elements provided damping in the structure. Similar kind of systemwhich was a slight variation was used in the retrofitting of the Mackay school of Minesat the university of Nevada.

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Figure 10: Isolator system: Bearing [Way and Howard, 1992]

Figure 11: Isolator system: Sliding [Way and Howard, 1992]

Isolation system used here is high damping rubber bearings with sliding elements.The building rests on 67 high damping rubber bearings. As already explained ratherthan including the supplements like fuses and damping gadgets the high damping rubberdoes everything because it shows high stiffness at the low strains and also high dampingat low strains. A suspended concrete slab serves as the base floor over which the structurelies and this slab is placed on top of cardboard forms. To provide support for the flat slabso that the spans becomes manageable 42 Teflon sliders in total have been provided. Thesliders were targeted at friction coefficient of 0.10 (as already explained that it shouldbe between 0.05 to 0.15). The inplane strength of the URM building was calculatedto be reached around 0.25g to 0.4g so to avoid the collapse forces associated with theMaximum Credible Earthquake below this PGA threshold was filtered out during the 3Dtime history analysis which was done in programs SAP-81 and N-PAD.

We can see that base isolation systems can be very effective and suitable for suchheritage buildings or other buildings which are URM constructions and are vulnerableto the earthquakes (and also wind loads sometimes) because such isolation systems notonly mitigate earthquake damage but most importantly also prevents the original fabricand appearance of the building. Seismic retrofitting from inside of the building and baseisolation together can be implemented to achieve better safety and mitigation againstearthquakes.

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3 Inferences and Conclusions

In our review of the state of the art research on use of base isolation as a mechanism toreduce seismic demand forces on vulnerable structures is studied. Base Isolation found tobe an exciting prospect in this regard, but one with certain practical limitations. We havefocused upon the use of these techniques in monumental buildings made of masonry. Weare under obligation to protect our cultural heritage. Historic buildings have complicatedelements like arches, domes and vaults whose behavior under earthquake loads is notunderstood well presently. Retrofit of these structures using conventional procedureswould thus require a lot of inspection and destructive measures that would disturb theoriginal architecture of the buildings. Techniques like base isolation can be extremelyeffective; especially complicated elements like domes cannot be analyzed and retrofittedby conventional methods. This also ensures the artifacts and relics stored within themonument are guaranteed safety. Such nonstructural components are often neglected inconventional design procedures. The case study also shows successful implementation ofthe rubber isolation technique in American buildings.

The method of base isolation also comes with some serious limitations and practicalissues that may restrict immediate use of this technique. The assumptions of unre-stricted lateral shift of the structure, and no overturning are not realistic. Stability ofthese structures under normal loadings like wind and other lateral forces is not directlyimplied. Additional measures are always required to guard against these issues and candramatically increase the cost. Exact installation procedures of these isolators can alsobe extremely cumbersome; especially if historic masonry structures are involved where alot of walls are present no stiff diaphragm is present at bottom to effectively use isolation.The isolation measures only reduce the lateral forces to a certain level, hence some mea-sures to ensure enough out of plane strength and diaphragm strength would always bedeemed necessary. The critical issue of an earthquake event during isolator installationis also not answered well enough. This may cause serious controversy in implementingthis technique to symbolic architectural monuments in our country. The rubber isola-tors studied in the case study implementation of this technique do avoid some of theselimitations, but involve huge costs which may not be affordable in all countries.

Hence we conclude that base isolation is an exciting new way to protect our culturalheritage from earthquake damage. But this technique still has some questions unansweredbefore we actually proceed to implementing it in our symbolic monuments.

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Bibliography

[Allen and Bailey, 1988] Allen, E. W. and Bailey, J. S. (August 1988). Seismic rehabil-itation of the salt lake city and county building using base isolation. Proceedings ofNinth World Conference on Earthquake Engineering,Tokyo.

[Arya, 1984] Arya, A. S. (1984). Sliding concept for mitigation of earthquake disaster tomasonry buildings. Proceedings of Eight World Conference on Earthquake Engineering.

[Kawamata et al., 2004] Kawamata, S., Funaki, N., Hori, N., Fujita, T., and Inoue, N.(August 2004). Base isolation system suitable for masonry houses. 13th World Con-ference on Earthquake Engineering, Vancouver Canada.

[Luca et al., 2001] Luca, A., Mele, E., Molina, J., Verzeletti, G., and Pinto, A. (2001).Base isolation for retrofitting historic buildings: Evaluation of seismic performancethrough experimental investigation. Earthquake Engineering and Structural Dynamics.

[Nanda et al., 2010] Nanda, R., Agarwal, P., and Shrikhande, M. (2010). Retrofitting ofmasonry buildings by base isolation. Proc. of Int. Conf. on Advances in Civil Engi-neering.

[Way and Howard, 1992] Way, D. and Howard, J. (1992). Rehabilitation of mackay schoolof mines with base isolation. Earthquake Engineering, Tenth World Conference, Rot-terdam.

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