About science; About earthquake engineering

This is about science produced by the California Institute of Technology and originally broadcast by station KPCC Pasadena California. The programs are made available to this station by national educational radio. This program is about earthquake engineering. With host Dr. Robert McGregor Leon and his guest Dr. Paul Jennings professor of applied mechanics. Here now is Dr. McGrath on. The start of the christening you'd better do this tell me just what you mean by earthquake engineering. Well by earthquake engineering what we mean is a study of the special problems that are related to the protection of structures against earthquakes. Well certainly we're in this area. California Southern California we're very conscious of earthquakes. It seems to be the long history of such experiences. How frequent are earthquakes throughout the world. Well they are very frequent. It's been estimated that maybe a million of them occur a year but fortunately very few of

these perhaps 100000 of them are strong enough to be felt by anyone. And of the ones that are felt very few of those perhaps 100 of them are such that they might cause damage. 100 or so damage which occurs is measurable in terms of structural damage building and destruction wrath of human life. Is this what you mean in terms of highly populated areas. Well one of the problems is that the people are increasing and as a result of more and more of the seismically active areas of the world are becoming populated so that the number of earthquakes that could be damaging increases because people are living in more places even if the number of earthquakes that are happening is relatively constant. As years go by we can expect somewhat more more problems in this area because

people are now living in places they didn't live before. Just moving out to these more active areas. What are some of the active areas throughout the world. Well most of the strong earthquakes are about 80 percent of them occur around the Pacific Ocean and this is often called the Ring of Fire. Pacific Ring of Fire because of earthquake activity is also often associated with volcanism active volcanoes here and there many of these around. I don't remember the Pacific Ocean so extending all the way from what Australia through the eastern coast of Asia up through Alaska and back oh yes Australia not so much but New Zealand is very active nationally as of course is Japan. The chapter an insular Russia illusion islands are extremely active. On along. The coast Pacific coast all the way down to Mexico and beyond into South America down through the western coast of the United States and down into South America right

now but other areas into Europe and Africa yes there's another belt of earthquake occurrence called the Alpine belt that runs through Turkey India northern India Russia Tashkent is on the northern portion of this earthquake belt and into Outer Mongolia. Do we have any idea how recent this activity is is it something for example the Pacific Ring of Fire as we call it. Is this something that's been going on for millions of years. Is it more recent than the one through the Near East. Well you know I talked to a geologist to say for sure we have. Reason to think it's been going on for a long time in Japan where are historic records of the longest. There's been lots of earthquake activity at roughly a constant rate for several centuries. We have records back to 0 0 I think maybe three or 500 B.C.

So it's been going on for a very long time. So certainly we have a good bit of recorded history certainly in more recent times. But you say that you don't know personally the long geologic history of how these germs have developed their notices. Tectonics and so outside of my area. So I'd rather not comment on that. My turn to be damaging earthquakes even this is more the problem of the earthquake engineer. We're very interested in what type of motions cause damage and how they reach doctors respond to these motions. For one think how do you measure the magnitude of an earthquake. You mentioned the frequency of the question of the magnitude. Well the common magnitude measurement is it on the Richter scale which majors the maximum trace on it. That aside I want the size of

just instrument gives him this is not. To great advice for the earthquake engineer because what we're interested in is a slightly different concept we're interested in the strong motion right at the site of the structure. Now the sidewall use instruments will not give you this. These are very sensitive instruments that are set to major earthquakes that have occurred very far away. And what they're trying to do is find out something about the construction of the earth the origin of the quakes and so much of what is the physical quantity that's measured is an acceleration as it isn't it's an acceleration because this by Newton's Law is most directly related to the forces that the structure would experience during an earthquake. So the instruments that were in general use isn't earthquake design. I believe this must be the feeling which are immediately concerned here. Our device is much like that of the seismologist but specifically. Designed and located well yes we we have somewhat different breed of

cat in our strong motion instrument in that it's designed to record a motion much stronger first any size seismologist instrument and secondly it what we want to get a little better resolution so we have to run the paper faster it records on paper the instruments we have now. What you would mean by resolution you want more detail we want more. Yes we want more detail in the traces of the motion because we make rather extensive analyses of these record to find out how they would shake different simple structures to get an idea of. What happened at the site and what would happen if this earthquake occurred somewhere else to tell us what kind of effect it would have on certain structures. Well you mention that in that the humans are exaggerations what might be the magnitude of a large earthquake in terms of acceleration me magnitude might be as high as. 50 percent gravity in fact we recorded that Coast and Geodetic surveys instruments recorded that during the Parkfield earthquake. This was a smaller earthquake rocked a

magnitude about five. It was just one one or two pulses in one of those faults is an acceleration of half. You have a gravity which meant that if your shoes are nailed to the floor that you just receive a force sideways or roughly half your weight. So that would be considered an extremely large when it has a large acceleration. Fortunately this earthquake didn't last very long and there is very very little structural damage associated with this or earthquake so that the time period and if you have to ration the acceptation is important. We have reason to believe that even in a large earthquake such as the Alaskan earthquake that the accelerations didn't even at the fault didn't exceed 50 percent G perhaps or in that magnitude but the duration was quite long maybe 45 50 seconds for the strongest portion and the perceptible motion lasted for five minutes. Well you have the strong separation for a long time and then this can be very damaging to structures and of course

it was in the last minute. And you mentioned earlier that the. Have acceleration of effects and gravitational acceleration field that had been experienced in terms of short pulses that might be what he would have seconds then. Oh yes a second at the instrument. There was right close to the fault. The acceptation was just a couple three seconds long the strong part of it so that something that's approaching a minute or many minutes down is the one that we're very much concerned with right structures. In what way are structures affected by these accelerations. Well basically two things can happen there. One of the problems is with the foundation in the soil itself. For example during the Alaskan earthquake there were many slides larger slides you may remember the pictures in the papers near an arm slide for instance a very large slide in a nice residential district and there were submarine slides on the waterfront areas and some of the small towns around that means.

And under the earth immediately under the water surface near the shore of some of the small towns as I was on a very steep slope and slid into the ocean and taking some of the waterfront structures with it and as a result also I generated a local wave which was anywhere from 10 to 30 feet high and this weighed and came back over the land and caused great damage to the thing. So here you are talking about the displacement and material of the earth. Yes and what it does in disrupting to mandate it and you have a building on top of us. I have a major slide. There's just not much you can do right now is there any other aspect of this. Oh yes assuming our foundation behaves itself the soil doesn't fail in that manner. Then you can talk about the shaking of the building. And if the shaking are strong enough some buildings receive damage it's very hard to relate the damage you get or the damage that you see to the

acceleration unless you have an instrument because it depends so much on the quality of construction. He structures specially summit they have in foreign countries such as say Agadir Morocco or a disastrous earthquake while back consisted mainly of boulders piled on top of each other. And during the earthquake even a smaller earthquake is sufficient to set some of these down. I was making any large scale attempt to gain experience and records the nature of earthquakes and magnitude frequency of some of these vibrations which occur. Yes the frequency in occurrence of earthquakes is a study that has been taken care of pretty well by a seismologist there very interested of course in the results of earthquakes as well as what they can learn from them about why they occur and what the character of the earth is. But the recording of strong ground motion has been the province of the earthquake engineer and at Caltech

and elsewhere were very active in encouraging people to put in instruments Coast and Geodetic Survey has an instrument program and we received a set of very interesting records from the recent Parkfield earthquake. How many local governments have they been participating in this investigation. Yes the city of Los Angeles recently passed an ordinance whereby buildings older I believe 10 stories have to have three accelerometers in a Mexican accelerometer is the name we use for our instrument and this instrument records the acceleration and at the rough and metal in the basement building. Now in the event of an earthquake and we and these instruments work well we have records that will help us in this and determining the. The accuracy of the methods of analysis and design that we use. I should mention too that these instruments have been developed over a period of several years and are quite reliable. These are checked periodically if you talk about these as accelerometers and such and I

presume that these are the essential sensing element of what we call the seismologist instrument. Well it's it's a similar sort of thing it has a different technical characteristics because we're looking at something that is a little bit different in terms of our motion our instruments sit there and wait for the earthquake to set them off. I see so there's a sort of triggering. Yes he is a little painted on there that any acceleration reaches a certain set level that the instrument starts and then it will run for a while until it senses that there is no more strong shaking you know it shut itself off and shut itself off so that you have a continuous graphic display that right. You mentioned Los Angeles. Has this become widespread throughout the United States. No I believe that it was and yours is unique in requiring that more tall buildings at these accelerometers being strong because we certainly have that for motivation. Yes yes the San Andreas Fault is here to remind us that earthquakes have occurred in this area.

What sort of problems those premium earthquake engineering counter. In this matter of the foundation design what aspects of the problem can he contend with. Well he has a very firm rule problem there because soil is a very difficult thing to work with and it from me analytical point of view in trying to decide what its properties are this in itself is very difficult. Some of the things that can happen to the foundation in addition to the slide is the lip liquefaction. And this is especially prominent in recent earthquake in Japan. What you mean by liquefaction what happens is that the sand is considering our sand it's saturated with water. The water table is maybe three or four feet below the ground you have a couple hundred feet of sand. Well if the sand is not quite dense during the shaking the sand grains can come apart from

each other so that the whole soil then assumes much of the properties of the fluid and it can flow. For example in the recent got earthquake in Japan. Buildings tilted several degrees hundreds of buildings filled in some buildings and tilted to the extent that there they had to be taken down one of them just layed slowly over on its side. Yes I think I remember seeing some editors of one woman a hanger washed up on the roof of this building and was able to walk down the side of the building after it slowly rotated and walked off into the building and promptly fainted at least. Not with regard to the structure then you talk about the foundation problems and the displacement of the soil ready earth beneath the building and the core traction problem how to design of the building itself to withstand a vibration. What kind of forces for example were in the nature of the forces that are induced by this acceleration

rather than. The earth shaking it is three dimensional in that it shakes north south east west and up and down. So which are the predominant forces in the lateral forces. I think what you could call on are south and east west are the stronger ones. About two to three times usually the vertical accelerator going down sways back and forth inside out and up and down. Yes it's a swaying motion back and forth that we're worried about. Buildings generally have a. Sufficient aspect of safety for vertical vertical loads anyway so that this is not considered a critical problem. These are lateral forces that are distributed over the building. They are proportional to the weight of the structure because it isn't what we call the inertial loading and it's proportional to how strong of earthquake you have in attempting to design for this kind of a condition. Do the engineers and present times were wrong from what we call dynamic on houses literally.

No no now you have to realize that in civil engineering structures like buildings and dams are not unique and so the engineer has a limited amount of time to devote to the design of each one it's not like an aircraft we are going to design build down hundreds of them he can analyze one thing in detail. The design is based on building codes which represent a standard rules you have to your standard rules and Compendium of experience with Pastor earthquakes. However special structures where the risk might be very high and you want to be extremely sure such as say nuclear reactors in a dynamic analysis as me so that they literally do a computation of the motion of the building under certain types of forces or perhaps you have them a lot of free it is it would be lateral forces are based on. The same sort of configuration that you'd expect in a dynamic response. So you apply a lateral load is proportional the weight of the structure and whose magnitude is specified by the code.

Are there any factors such as the shape or the general configuration of the building which enters into such question. Yes from a dynamic point of view here thinking of your building a swing back and forth. It's important that the resistance of the building piece symmetric in that you want to have resistance where you have weight of the structure so that the building will not tend to by break torsion layer twist twisting motion when it occurs can be very damaging and Hastert quakes we've seen cases where this is cause that you're building. So you'd like to keep the structure. Dynamically symmetric go by that I mean the distances are distributed in the same way that the weight is regard to which direction the earthquake came from a motion team. The building would have uniforms and interaction. Another property of interest to me talking about building design is to separate buildings especially

those of different heights because when a earthquake comes along the building will tend to sway at a period determined by its properties. You know and the building next door will have different properties and sway at a different period. If these aren't separated by enough room they can hammer together and cause quite a bit damp and icy so that literally the proximity then leads to interaction or pounding on the ends of the lever hammering them. You want they want to generally are separated by a few inches to a foot depending on the situation. I don't in addition to these codes which you mention which sort of rules of design or certain principles in terms of the structural sense now that I applied in deciding whether or not a building is well configured for earthquake resistance. Wrong. We're grading kids been called the great inspector and that it seems to seek out any weak point in your structure. So what you want to have of course then is that a structure is an enterable unit and

to have a good structure for the great purposes it's very important to design the joint so that the structure does act together and doesn't come apart because of a weak join or a lack of attention some detail. This is very important in the Alaskan earthquake. Some of the failures could be laid to inattention to detail with a particular elements were joined together poorly and came apart during the earthquake and I want to starts going apart at a much weaker structure and if the earthquake continues for a long time as it did in Anchorage Well then it would shake the structure down. What is the level to which you design. You attempt to designed to resist earthquakes are 99 percent. Well no I was not kind of economically feasible on the present philosophy is it. For the smaller earthquakes which you might have many of in the lifetime of a building that you know you mentioned for example high five and with my family's remember that one would encounter

and well not in a particular that's 100000 in a row do it well. But the particular building depending on is where it's located and you might go shake it three or four times during his lifetime with smaller earthquakes. It's expected that a well-designed building would not exhibit any damage or something of that magnitude so it would be completely resilient to a smaller earthquake a smaller earth. However it's not economically feasible in art and in fact desirable to design it to remain completely elastic during the strongest earthquake because. Two reasons the cost of it would be prohibitive. Since you are designing against something that may or may not happen we don't know that much about the occurrence of earthquakes within any reason no time span like one just two years or hundred years we don't know if an ice quit going to come. Be. I would think consider this is the deflections. If you have a completely elastic structure and a strong earthquake it may shake quite a bit in the amplitudes may build up

so the idea is used that in the strong earthquakes were I upon be yielding of some members the plastic action is causing it to dissipate energy mainly meant bending. Yes the structures can be designed to be very duct also that they will retain or strain even after they're bending just like a Cuesta coat hanger. Yes you can twist it many times before it breaks and it takes energy to do that well this is the sort of thing that where it is considered and design of buildings with a strong earthquake comes along willing to accept some damage no loss of life or are a hazard to do the persons involved but some damage can be accepted in view of the rarity of the event. So for these more severe situations then you have designed a building so that it may suffer some permanent defamations literally be twisted out of shape but would not collapse. You know it would not collapse and it would be repairable.

What is the frequency of earthquakes and in this area can time unity yes sayang. Just quote done some studies of some other people have made Clarence and Dr Allen turned sour on the side on to collab and Richter and doing the scales in the name of some other people have done some studies on this and the data are very scant in you have to just kind of take a rough average over what you've experienced in the past but it appears that somewhere in southern California area including certain parts of northern Mexico you can expect a greater earthquake Richter magnitude seven and three quarters or greater. About once every 50 years went to 50. Would this be of the magnitude and to cause permanent defamation and buildings. Yes if they were close to the fault you'd expect. Some signs of distress do not collapse and even in a well-designed building. How about

these much lighter earthquakes and we experience them every so often. I've never really attempted a copy of it keep track of them. How many of those do we really experience in this area. Only depends on the size you're thinking of something that might be noticeable. Well you might say oh and i need to do three earthquake might be noticeable bending on the local area. Some places are even and so I got Fournier much more quiet than others. You might get three or four of those a year and be strong enough to so that you'd notice them readily. I see what sort of damage is caused by these major earthquakes. What are we talking about in terms of the economics. Well he turned out a very significant economic loss in the Alaskan earthquake the damage it's very hard to spin it down to somewhere between 300 and 500 million dollars. The earthquake of the same year in the God of Japan the damages been estimated about 1 billion dollars I think.

How about some of the more recent earthquakes in a silent California area. They have been damaged in an extremely minor I think. Course we haven't had a real strong earthquake either since a Long Beach earthquake and he's 33 was that 10 to me and I know it was about 40 million dollars dollars and at that time. What is the nature of the work that's going on today Paul. And in connection with the research associated with the design of the structures as well I should point out that at Caltech we don't actually design structures as a job of the practicing engineer what we're working on is the research problems associated with Earthquake Engineering attempt to understand a problem yes we're trying to understand the problem we're trying to understand what kind of motions can occur during earthquakes so we try to encourage people putting out accelerometers so they can measure the motion we perform analyses on this motion.

We try to understand some of the soil effects that can happen. And we spend a lot of time working on the structural dynamics to try to find out why given certain motion what can this do to special structures try to understand just what it is it goes on during during the earthquake. You mentioned. Building structures are unique and the response of these buildings will be different one from the other so that you don't really have a standard approach to the problem. When I feel not quite fair we have a standard approach but it has to be a rather general approach because it has to encompass for the practicing engineer has to compass encompass a variety of buildings that have come up again so that you can translate into the set a code where you as a set of principles to go by where they decide depending on the type of construction they use what kind of forces should be designed for and that is all of this work that's done in the research area and all the tickler mathematical didn't you know knowing one of the recent things

it's been done at Cal Tech is the development of machines to shake buildings this was done for the State Department of Architecture and these building the ship building shakers can be placed in a building and they will then put a very small force on this building and shake it back and forth. And this helps us determine what the period of the building as the oscillation curator's Yast the oscillation characteristics and what kind of shape it assumes when a sways back and forth. And I remember you did something like this would it do him. Yes we can and yes the. I can't recall a good book a Canyon Dam was shaken this way to see what kind of things would happen there to see if it were something that we could learn by shaking dams. It appears that if you could pick up some of the dynamic properties of a dam by shaking it. So there is literally field testing if you like. Yes this is very true and we learn a lot about structures when we can put these machines on and shake them.

Does it make sense to talk about the dynamical properties in terms of modeling and work with small scale devices. I wanted to go to the full sized building. You can work with small scale devices the problem is getting the scales correct because that of the dynamical characteristics or has one of the problems as I mentioned before was with the joints of the structure and to scale the joints satisfactorily is very difficult to get a little to duplicate the joints of a steel building would be very difficult on a very small scale so that a lot of problems in the service business of having it in or go. Which is uniform and it's well balanced in its characteristics throughout. There are not weak points in it. To simulate this in that model is a difficult person to figure out there are some significant work being done now in this area. However one of the things we like to do too and I think the Japanese are starting this problem is to build a big platform about 50 foot square so that I draw likely shaking the base of it you could build a full sized structure and shake it.

This was about science with host Dr. Robert McGregor Leon and his guest Dr. Paul Jennings professor of applied mechanics at the California Institute of Technology. Join us again for our next program when two more members of the Cal Tech faculty will discuss a subject of interest about science is produced by the California Institute of Technology and is originally broadcast by station KPCC Pasadena California. The programs are made available to this station by national educational radio. This is the national educational radio network.