 Welcome back everyone. In today's class, we are going to discuss about the application of structural dynamics to a field called seismic isolation ok. So, as the name suggests seismic isolation is a technology to basically isolate a structure from the seismic effects of an earthquake. So, we are going to see first what is the basic idea behind seismic isolation and we are also going to discuss in a subsequent lecture the structural dynamics of a seismically isolated building ok. So, let us get started. So, basically now we are going to discuss the principle of seismic isolation ok. And first I would be introducing you like you know the overall idea and then discussing about some type of seismic isolator application, mechanical behavior and modeling. But as part of this course, I would mostly be focusing on the basic idea behind the seismic isolation and giving you some of the structures that have been famously isolated using different type of isolators ok. Now, before I get into the idea of seismic isolation, first let us get into idea of seismic force or earthquake force and then see how do we actually incorporate the idea of seismic isolation into it. Now, as a very simple formula you know I mean and like you know this is oversimplification but overall we know that the total force on any structure can approximately be written as mass of the structure times acceleration ok. And if you write mass of that structure as W by G ok and combine G with A I can further write it as basically this here ok. Now, this W is what is called the seismic weight ok and you would have come across this term if you would have doing seismic design and everything. This is the seismic weight now you might come across this term many times ok, but let me just discuss what is the idea behind the seismic weight like like you know what is the dead weight of a structure ok. And you might estimate what are the live load acting on the structure. Seismic weight is typically defined as the weight that might be present during an event or an earthquake event for which the structure has been designed ok. So, it would definitely include the dead weight of the structure plus certain portion of live load because you typically consider live load for the extreme scenario in which you consider the structure to be fully loaded ok. And then you determine or estimate the live load, but during the normal operation it is unlikely that your structure would be under the capacity of the full live load. So, we typically take 20 percent sorry 25 percent or 50 percent of the live load. So, dead weight plus let us say 50 percent of the live weight live load is basically considered or included in the seismic weight of the structure. And this A by G which is the acceleration of the structure not the ground motion as such ok. This is the acceleration of the structure the A by G is basically called seismic coefficient. Now most of the seismic analysis procedure like linear static procedure or response spectrum procedure ok. As per Indian codes the seismic codes and the outside code they give you procedure how to calculate the seismic coefficient here ok. And there you have like you remember this factor you might see it as Cr ok or other places this factor is written with some other notation, but basically this is the seismic coefficient through which if you multiply with the seismic weight you get the lateral load that is being applied on the structure ok. Now this is basically the earthquake force or the lateral earthquake force. Now this A which is the acceleration of the structure I can also say this is the assay or the spectral acceleration let us say ok. Now we know that spectral acceleration of any structure is a function of time period of that structure and time period in turn is a function of mass of the structure and the stiffness of the structure and of course what I am writing here is valid for single degree of freedom system, but we can extend this same concept for multi degree of freedom system as well ok. Now if you look at the design spectra and this is the typical shape of a design spectra ok due to an earthquake or at a particular location of a building or a structure you initially have a increasing spectral acceleration with time period then there is a constant phase and then there is this decrease. Now most of the structure ok if it is a rigid structure it would have typical time period of around 2 second ok. If it is a like you know moment frame building you know tall building then you can go it can go up to 1 second or even 1.2 second and further up, but typically we say that a typical structure is in range of 0.25 second to 1 second which on the response spectra falls in this zone ok. Now let us say I have a technology ok or I want to reduce the force transferred to the system so what should we do I mean I know that the seismic weight of the structure is fixed so that cannot be changed right similarly the mass of the structure is fixed so that cannot be changed the only thing that I can tinker with or I can change time period right and how would I change the time period mass or stiffness ok. But mass remember typically mass of a structure in a design is fixed right it is determined by weight of the infill beam and column weight ok life weight and those things does not depend on the structure I mean it depends somewhat on the structural properties the geometric properties but overall there is not much you can do about the mass, but what you can do is about the stiffness ok and let me just delete that here I know that time period is nothing but 2 pi under root m by k and I know that if I want to reduce the force seismic force I would need to reduce the seismic coefficient or spectral acceleration how do I reduce that let us say initially my structure was here if I want to reduce that then I have to go in this range right not this because there is not decrease in that part so I need to come along on this part of the response spectra now if I do that ok that means I need to decrease the time period ok and I know time period is 2 pi mass by k or the stiffness to decrease the T n ok what can I oh sorry in this case not decrease I am sorry increase the time period to decrease the force ok so to increase the time period what can I do so basically our goal is to decrease the stiffness ok so we have found out one conclusion that if I want to reduce seismic force on a structure we should have a system or the structural system which has low stiffness ok alright so the design objective for a seismic design is basically reduction in seismic forces ok so increase T n through reduction in stiffness ok now increase the time now remember if this is your acceleration response spectra ok remember there is also displacement response spectra right so for a typical acceleration response spectra like this a deformation response spectra I have drawn on the right hand side ok so what happens as you decrease or as you increase the time period and decrease the forces what happens to the spectral displacement let us say initially I was here T 1 I increase the time period so that my forces decreased ok so let us say initially I was here and then T 2 I was here so my force or the acceleration is decreasing but what is happening to the displacement here my displacement and this should be actually SD here it is incorrectly written as SA ok so this is SD which is the displacement ok displacement is actually increasing and that might be a matter of concern especially if you consider closely spaced building in a dense urban environment ok so we need to control displacement as well now we know by increasing the time periods we can decrease the force but increasing the time period is creating a problem for me because it is going to increase the displacement so I need to address that issue so let us see how do we do that so as I discussed if I increase the time period it would lead to decrease in the acceleration and subsequently decrease in the forces so initially if let us say this was acceleration SA 1 it becomes SA 2 when SA 2 is much smaller than SA 1 however I was initially ok so initially I was here my displacement and this was SD 1 as I increase the time period to T 1 to T 2 remember this is the spectra I have I increased the displacement from SD 1 to SD 2 so in order to control the displacement what do we need to do we need to increase the damping in the system now what does damping do it basically decreases the displacement and my response spectra or the displacement response spectra that I was having it here by increasing the damping I can reduce it to here now if I do that then I am able to bring my displacement down either below the initial level or maybe control the displacement to an extent that it is not detrimental to my structure anymore ok. So period lengthening lead to force reduction ok and if we provide damping which is basically energy dissipation it leads to displacement reduction and through these two ok we basically control the response of the structure and that is why at many places you will also see that seismic is isolation is studied as a part of seismic response control or seismic control of structures ok there are other technologies also involved in seismic control like you can have viscous damper or other type of tuned mass damper or you can have other technologies but basically seismic response control means applying technologies so that I can control the response of the structure to a desired value ok. So if you increase the mass it will increase the time period ok but consider a typical example let us say you have a concrete building ok and if you want to design it can you do much about the mass of the building that you are designing structure are more suitable for seismic isolation rather than light structure so basically you would not apply seismic isolation to a good structure there is I have not come across any application like that although many of the houses outside are made up of wood structure because wood structure itself is very light and the stiffness is very low ok and they adequately perform during the earthquake as well but even steel structure when we apply we try to stiffen the super structure ok because the steel structure is also lighter but for concrete structure seismic isolation is very good if you have heavy let us say masonry structure for that also it can be applied and any type of structure so for heavy structure of course your seismic isolation is more effective because we already have mass ok that helps in increasing the time period of the isolated structure ok ok. So let us say you estimate you know some loads ok and you estimate let us say live load and you know that there are certain uncertainty associated with that ok so those factor would still need to be there because the uncertainty associated with that load is taken into consideration with the load factors ok and uncertainty associated with the strength of material is taken with the strength factors ok that cannot be addressed through seismic isolation it is an entirely different technology that is more often statistical and risk perspective and what here is basically in terms of the technical aspect of it ok alright. You might think that seismic isolation is a very new technology ok and you know I mean you might have seen the recent application but the overall idea behind seismic isolation technology is actually not very new and to like you know just support my argument what I am showing you right here is a very old structure ok it is a basically a tomb built on stone blocks ok and this is basically in a Persia and as you know that region is actually very prone to earthquake and see this is like 530 BC ok this is a tomb of King Cyrus the Great. Now this is basically very big stones ok being kept over each other and the tomb is kept on the top of the structure. This has survived ok even it is there right now so you can imagine 2000 and then maybe 2500 years this structure has survived and how now the idea remains simple during an earthquake these stones are actually not filled with mortar ok and these can slide over each other so when the earthquake comes it does not allow the transmission of the force from the ground to the tomb and the overall force that is transmitted to the structure ok is significantly smaller because when this slide over each other you can assume or you can imagine that it would have very small lateral stiffness ok and this was built very long time ago. Similarly this is a very famous structure you might have seen in photos movies or some other place this is a Parthenon in Athens ok it has survived earthquakes for 2100 years ok and when it was finally damaged it was not damaged due to an earthquake it was damaged due to an explosion ok. Now as you know Greece is a very seismically active region there are lot of earthquakes and over the 2100 years it is still survived. Now the design of these structures are very simple basically what you have you have modular blocks so each of the column if you see these contains modular blocks which are kept over each other and then there is a central hole through which you can have let us say post tensioning kind of mechanism or you can have a single rod to keep it in place but effectively what it does it reduces the lateral stiffness significantly. So you can see I mean in all these structures what the engineers as you can see at that time what they simply did is basically they change the lateral stiffness of the system so that overall stiffness reduce significantly ok. Now the stiffness is reduced overall force transferred to the structure is also reduced and that is why they have survived for so long ok. Now you might ask when was the first patent related to seismic isolation technology we do not exactly know because you know I mean there are different type of technologies will look like seismic isolation technology and people file the patent for that one of the first known instances is basically this US patent that was issued to basically Jules Stolia of San Francisco and it was issued in February 1870 and the figure is from that patent application if you look at it he has simply shown a schematic of a house and this house is kept on a foundation and if you look at the magnified view here ok. This is nothing but rollers ok so you have like you know cylindrical rollers and then you have overall base mat you can see and your foundation and when the earthquake come again it can roll over each other so it provides the restoring force now as well as well as sliding ok and this was one of the first known instances of you know patent issued in this area ok. So let us look at basically seismic demand ok on a regular structure and base isolated structure ok so what I am showing you here is basically a traditional conventional fixed base building now what you have seen till now we can achieve seismic isolation if you if you provide a horizontally flexible layer and I will show you different type of seismic isolator but let us just for right now assume that this is an isolation layer so the left hand side you have a structure that is fixed to the ground on the right hand side you have the same structure but now it is isolated through an isolation layer ok. Now what typically happens during an earthquake ok the overall seismic demand is accommodated by the superstructure is it not? So deformation acceleration and the velocity demand is basically accommodated by the superstructure because of deformation demand it leads to deformation in these beams and columns and that leads to basically cracks development plastic hinge formation and if the demand is too large may so that seismic design technology what it tries to achieve is to provide enough ductility in the system so that it can accommodate such seismic demand in the superstructure so it can go in the non-linear range but it can still accommodate ok this much of drift demand without collapse and basically the performance goal is typically collapse prevention ok. Now compare that and that I only talked about in terms of drift or displacement let us say now if you look at it while you might protect against drift demand what will happen to the you know objects and machinery and people inside this building if you look at this and you might think like you know those things might not be expensive but just think of a hospital building or a hospital structure in a hospital the structure cost you know it is typically nothing most of the cost is actually machinery a typical MRI machine might cost north of let us say 10 crores 15 crores like that and there are several such machines scanning machines so those are of much more equal like you know value that needs to be protected ok now you might protect by let us say making the structure very strong but if you make the structure very strong it would have again large stiffness and acceleration so acceleration is not reduced because if your this is more then the acceleration would be more so you still have the acceleration demand and that would lead to acceleration in the internal components so with the traditional design philosophy seismic design philosophy you may be able to like you know protect your structure against the drift demand up to certain extent but the acceleration demand or the velocity demand might not be accommodated that efficiently as we would compare with the base isolated structure now what happens in a base isolated structure you have this super structure right and this isolator layer now the all of the because this is very flexible layer all of the drift demand is actually accommodated at this isolation level here so right here now isolators have properties that it would have sufficient restoring forces that it can bring back the structure to its original position it would have sufficient buckling capacity ok it would not have any permanent or like you know very minimal permanent damage so that those things I can do by designing the isolator like that but the super structure there is no drift or very small drift compared to the fixed base so there is very less damage to the frames of the super structure at the same time because my stiffness here is very small for this isolation layer the acceleration is very small so that's why internal and so is the velocity ok so the internal components are also actually protected ok in a base isolated structure so you can see the benefit of applying isolation here so now you have seen ok some of the structures and examples what should be the requirement of a seismic isolator ok now you know that seismic isolator must be horizontally flexible to increase the time period because only with the increase of time period I would achieve reduction in the seismic forces and reduction in acceleration ok so I need to have for this isolator so through this slide what I am trying to find out what should be the requirement of a seismic isolator and then we will see different type of isolators that satisfy those requirements ok so to achieve this horizontal flexibility we can use horizontally flexible material or we can use sliding surface ok when I see one surface is sliding over each other what I basically means in horizontal direction it has very small stiffness or almost negligible stiffness ok now it need to be horizontally flexible but at the same time it need to be actually stiff or vertically stiff because see your whole structure is still there right so even when it is moving flexibly in horizontal direction at every point it would still need to sustain the vertical load isn't it so it need to be actually stiff now how do we increase the actual stiffness we will look at later but basically the action we know that let us say if you have a simple column and if you have line axial load ok I know that a shorter column would have higher capacity or higher buckling capacity isn't it or if I provide lateral restraint it would have higher capacity ok so basically I would need something that is either laterally restrained ok in terms of axial stiffness or it has large compressive modulus ok if I would have those property then I can accommodate large vertical load so the total load of structure ok now it should also accommodate large displacement because as I said in the previous slide when you did not have an isolator ok the whole drift demand this much was being accommodated by the super structure now imagine displacement demand of the similar order now need to be accommodated by the isolators ok so it should be able to carry large displacement so it should have elastic property or even if it does not have elastic property it should have property which should be recoverable ok so some of the materials have that kind of property like recrystallization especially like lead rubber bearing ok now we also want damping because we don't want too much of displacement otherwise it would lead to pounding effect to the next building or instability so to control the displacement and other response quantity we basically want some dissipation of energy or I can say damping now there are different ways in which we can achieve damping one is friction so if you have a sliding surface friction like you know sliding over each other it leads to generation of heat because of friction and ultimately leading to loss of energy ok and that is what is damping is you might also dissipate energy through hysteresis so if you have a material let us say elastic plastic material or bi-linear material will dissipates energy that would also allow dissipation of energy ok then at the end what you don't want that after the earthquake your structure is remaining in the displaced position ok because then residual deformation would lead to serviceability issues so you want your structure or the isolation layer to have adequate restoring force so that it can bring back the structure to its original position ok so you want either elastic restoring force let us say with the rubber or you want maybe through a curvature ok so that structure slides back to its original position but there should be some adequate restoring force in the base absolute structure ok so these are the basic requirements from the seismic isolator ok any isolator that satisfy these requirements may be investigated further for application or to be utilized as a seismic isolator ok two of the popular type of isolator and these remember these are not only the two type of isolators that are available right now there are many other isolators as well but these are very popular because of their reliability and basically application to wide range of structure the left hand side what you see here is a cut view so this is not overall you know this isolator what they have done they have cut it so you are seeing basically the internal section of a lead rubber bearing now what you see here is a multiple layer of rubber constrained by steel shims in between ok and then there are bearing plate which is this is the internal bearing plate and then there is a bolted external bearing plate now this bolted external bearing plate is how this is connected to the sub structure and superstructure ok this rubber layers these rubber layers are the one that provide if provided horizontal flexibility the steel shim the role of steel shims here is basically provide lateral restraint to the rubber and I will show you why do we need that the lead here lead is a very good energy dissipation material plus it recrystallizes when you bring it back to its original position and temperature so its properties are actually recoverable so it dissipates energy but after sometime when you leave it it will acquire its original property back so it is not exactly elastic but if you bring it back to original position at molecular level it recrystallizes and again would give you similar kind of energy dissipation capacity like the one you had before okay so this is the one of the example of elastomeric bearing on the right hand side okay what you have actually is basically a sliding bearing okay and remember this is not again the whole sliding bearing what you see here is the internal surface on which this block here slides and the top plate is kept on the in the corner as you can see okay so this kept on the top of that and again this is bolted so it is connected from the top and bottom using these bolts and when the structure comes is basically slides over this now this sliding surface provides you very low horizontal stiffness okay the curvature provides you restoring force okay because it is like a pendulum if you take a pendulum the restoring force is mg sin theta okay now these two type of system either rubber based on sliding based elastomeric systems are typically used in today's seismic isolation applications okay and we will look into these type of systems further so what I want you want to show you here is basically example of a building okay it's a full-scale building I don't know whether you know this or not this is a shake table facilities in University of California San Diego so this is one of the biggest shake table facility in the world and what you see here is basically a structure that is built on top of it on the base there is a basically a platform a shake table platform so the ground motion is being applied through this shake table platform and this structure is isolated now on the right hand side okay you see the magnified view of the isolator below the base mat okay so here keep an eye here okay I will start it the video and then we'll see okay overall seismic isolators can be broadly characterized and what are like now typically used in nowadays basically these types elastomeric bearings and in the elastomeric you can have low damping rubber bearing or lead rubber bearing and as we will look at later low damping and lead rubber bearings are basically almost same except in lead rubber bearing you have a lead to allow higher damping or energy dissipation capacity that comes because of the lead similarly you can have sliding bearings now in sliding bearings you can have a flat slider bearing you can have friction pendulum bearings okay in friction pendulum you can have single double or triple and the way we categorize these bearings are basically the number of sliding surfaces based on that we say either these are single sliding surface then single pendulum bearings and then double friction pendulum bearing or triple there is also like another type of bearing which is like you know tension and compression friction isolator but those are not very common okay now throughout the course I will show you some like you know some sliding bearings as well or friction pendulum bearings as well but our focus would be on elastomeric bearings rocker bearings look something like this you have a surface like this and on this one you can have let us say column and here basically a slotted this thing so because of this rocker thing basically it leads to reduction in the horizontal if you as a building designer would have to use would you go for this type of bearing then why not go for directly sliding bearings okay this would be much more stable it would provide you enough capacity as well okay and the force transferred would also be smaller now we are mostly like you know if we size for the prospective of seismic isolation we are mostly utilizing either rubber elastomeric bearings or friction pendulum or sliding bearings okay bridge rocker bearings are a little bit different okay those are still used those are not seismic isolation bearings okay so in general term bearings are used like in at many places like bearings are used ball bearings you know it is also used in mechanical engineering the rocker bearings that are used for bridges they are still used okay those are not those are not for seismic isolation application like even the neoprene pads neoprene pads that are used in bridges they are not for seismic isolation they are only for providing or accommodating expansion joint or accommodating traffic load deformation and those things for seismic isolation it needs to accommodate large deformation capacity because even for seismic isolation application if you look at it okay these steel surfaces that accumulation of dust is a big issue so here what they do they use a cover so the overall thing is actually there is like you know cover whole thing is actually covered inside a closed this thing and this surface here is polished and there is a layer that is applied on this basically a polymer layer that is applied on this surface because change in the value of friction is a major issue with the sliding bearing because let us say you are designing your sub superstructure for certain load transfer and if your if your coefficient of friction increases then the overall load that is being transferred to your structure would also increase and then that becomes a problem so to maintain that they take some counter measures in sliding type bearings by making it very polished and then covering it completely so there is no sand or dust accumulation and it is of stainless steel so those type of counter measures that it but if you have a single rubber block of this much or if you have same thickness and if you have a steel plates in between this would have less bulging than this one okay and this is what you can see here this is the internal construction of an isolation bearing in which you have intermittent steel plates which are called steel shims and these are used to prevent that bulging or effectively you can say to increase the vertical load capacity so this is done basically in isolation as well and coming to the issue of neoprene longevity basically what happens neoprene is basically a synthetic rubber now after 10 15 years you have to keep basically inspecting these neoprene bearings that they are still like you know providing adequate performance otherwise you need to replace this in isolation we are now moving towards natural rubber bearings what has been seen with natural rubber bearing that these type of bearings actually would have less strength degradation or less environmental issue or degradation issue over the time so that's why in US they have almost shifted to natural rubber bearing in Europe they are still using little bit but overall natural rubber bearing if you replace neoprene with a natural it performs much better for seismic isolation of course astromeric bearings have other kind of issues like you have ingress issues in the sliding bearings or like you know change in the coefficient of friction the problem with rubber is that over the time the property of the rubber actually changes it changes because of environmental condition like sunlight or like you know it might change because of it might change because of rubber which is basically a material that keeps on getting cured over like you know long amount of time so the shear modulus might change so those are issues are there with both of these but typically those are being taken care of in the design itself by estimating that let us say over 15 20 years the shear modulus is going to increase by 10 percent so we do bounding analysis saying that even after changing the property my structure should still be able to sustain those loads and deformation demand from that quick so let us now discuss elastomeric bearings now as I said in terms of elastomeric bearings we would be discussing low damping rubber bearings and lead rubber bearings and lead rubber bearing is nothing but low damping rubber bearing with a lead core and the role of the lead core here is basically to provide energy dissipation capacity okay and then there are also other type of bearings which are high damping rubber bearings construction wise high damping rubber bearings is same as low damping rubber bearing these do not have lead core the source of energy dissipation actually comes from the rubber itself okay so by changing the composition of the rubber you can achieve damping properties okay in high damping rubber bearing this used to be popular at the beginning but now people are moving more towards lead rubber bearing and low damping rubber bearings because what happens we use basically natural rubber and natural rubber do not have inherent damping capacity as much as neoprene bearing because remember natural rubber bearings basically it is naturally cured rubber from the trees so I do not know whether you know the manufacturing process of rubber rubber is actually natural rubber is actually obtained from the trees if you cut like you know the outer layer of tree and basically from that whatever you get from there and you can go have watch some videos those rubber are actually obtained from those trees neoprene rubber which is also synthetic rubber can be produced in the lab by changing the composition of the materials and chemicals okay and by adding different type of additives you can increase the damping or decrease the damping the problem with that is that over time as we talked about the longevity issue high damping rubber bearing and neoprene rubber bearing have more longevity issue and then over the time degrades more compared to natural rubber which does not have any kind of additives because with the carbon filler sulfur or other type of materials that are used to basically obtain neoprene rubber they keep on reacting over the time okay over the years okay so these dormant reactions are like you know keep on going over the time and that is why it is changing its property now as a designer if you are designing something that is changing its property over a long time then you would prefer other type of material which have less change in the property over the time compared to neoprene which have more change okay so that's why for seismic isolation we are going with the natural rubber now okay and you see you have foundation pedestals here so below this you might have a rough foundation or something else on the top of that you have basically this foundation pedestal which are typically very rigid and this isolator is connected through this external bearing plates and bolted to this pedestal here and on the top you again have same bearing plate and then you can connect to the concrete columns or steel column as the case may be okay and you would have several of such bearings in plan so let us first show you what is the low damping rubber bearing now what I have seen you the internal construction of a low damping rubber bearing okay there is no lead core here okay and on the left hand side what is there is basically this rubber bearing is actually cut in half so what you are seeing here the internal construction multiple rubber bears remember in the end what you see here is basically rubber cover cover is provided to rubber bearing as well to protect the debonding at the interfaces because it is very vulnerable at those points okay and sometimes you might see a central hole the whole idea behind providing a central hole is like you know from the manufacturing perspective it allows proper distribution of heat during the vulcanization or curing process okay because if you have internal you know hole basically it allows uniform distribution of heat so that it cures properly and you get good bonding between rubber and steel shims okay so this is a low damping rubber bearing similar construction you would have lead the damping lead rubber bearing but you would now instead of a central hole or without hole you will have a lead core now okay and there are methods through which you can decide that what is or how to find out the diameter of the lead core okay in terms of sliding bearings you can have a flat sliding bearing you can have single friction bearing for with a single sliding surface you can have double friction bearing double fp bearings which double sliding surface as you can see here okay or you can have triple in which you have one sliding surface here one sliding surface here and then one sliding surface here okay so this is a magnified you have a single friction bearings okay you have a sliding surface then this is a double friction pendulum bearings now a question you might ask why do I need to go if I have this single friction pendulum bearing why should I go for double friction or triple friction pendulum bearing anyway now what happens if you go for double friction or triple friction pendulum bearing these would provide you enough or larger displacement capacity with similar or smaller footprint because you have multiple sliding surface now okay in this case this would just slide to the end okay and that is the end of it but on this one first this would slide to the end then this can again slide over the top of this once the displacement capacity is actually increased okay if you increase the number of sliding surface further again your displacement capacity is further increased okay so if the structure where you want to apply seismic isolation have very small footprint let us say very small columns and you cannot fit larger bearings then you can go for triple friction pendulum bearing because it would have smaller footprint or smaller dimension and it would provide you similar or higher capacity than single friction pendulum bearing or illusory bearing bearing okay