Some discussion of recent (last 20 yrs) developments in codes 2

[TowerEngineer]

This was posted by MikeVV:

On engineering history: I have been studying the Uniform Building Code and often wonder why the requirements exist as they are written - what event promulgated the requirement of interest? I would love to see a compilation of the major catastrophies that drove the various requirements in the code. This would give me an understanding of the intent and help justify the expense of compliance. Is anyone aware of such a listing? How about for the International Building Code?

Mike Van Voorhis
MJVanVoorhis@CS.com


My response was:



To MikeVV on the subject of engineering history:

I began building and non-building structural design in 1981. We were using the 1979 UBC for wind and seismic loading. Back then it seemed like a "major code revision" involved simple (or not so simple) change of a single force factor in determining wind or seismic forces. For example, the "K" factor for seismic loads was reduced from 1.33 to 1.0 for wood frame construction in the 1982 UBC.

But then along came ANSI A58.1-1982 and the whole applecart was turned upside down, wherein not only were the wind forces dramitically increased, but the methodology in determining the lateral force (lateral force design procedure) became much more complicated and time consuming. Those changes in the design procedure and lateral force coefficients were quickly adopted by ICBO and incorporated into the 1982 UBC. There were such gross errors in the wind design procedures contained in the 1982 UBC (like forgetting to include a 2/3 factor for cylindrical objects) that I for many years chose to use the source document for design purposes instead of the UBC.

Things went along pretty smoothly until Applied Technology Council came out with its proposed seismic design procedures which were a complete computational nightmare as far as the structural engineer was concerned. This prompted SEAOC to take the initiative in undertaking a complete re-write of their "Blue Book" so that SEAOC would remain the driving force with respect to seismic design procedures. The major seismic design revisions contained in the 1988 UBC are more or less the result of turf protection on the part of SEAOC. I served on the SEAOCC Seismology Committee in 1983-1984 and have some good insight into the battle that has continued ever since between ATC and SEAOC.

The question is: "Other than the fact that the total base shear required under the more complicated and more recent wind and seismic design standards and codes have significantly increased in the last 20 years, are the buildings designed under the newer code provisions (say since 1982) significantly "safer" than the buildings that were designed under the old lateral force design procedures?" Other than the Northridge Earthquake, we really haven't seen any empirical evidence that there were any deficiencies in the older design codes and standards with respect to wind or seismic loads. With respect to Northridge, that seismic occurrence was not evidence of any shortcoming in the seismic design standard, but rather convincingly demonstrated that the design detail of welded Steel Moment Frame Connections was lacking. This single deficiency does not justify the plethora of code revisions since 1982 and is not addressed in the new codes anyway. You still have to look to the SEAOC "Blue Book" or other Advisory documents or standards in order to design a competent ductile welded moment connection.

I think an Engineering History forum can serve as a very useful tool helping to answer the foregoing question. It can also serve as a tool in discovering how things used to be engineered "in the good old days". I am currently doing some research on wind and seismic design procedures dating back to the 1850's. I am more than a little curious to know what lateral forces were applied to the Brooklyn and Golden Gate bridges in their design and how those forces compare to the lateral forces required by today's codes.

I am fortunate in that I have a set of engineering plans for a structure that was designed in 1937. Those plans contain wind and seismic lateral force diagrams on the drawings. It was very interesting to discover that the applied lateral loads were between 1/2 and 1/6th of the lateral force required under today's standards (based on the same reference design wind speed of 90 MPH). Unfortunately, there isn't enough detail in the calculations to determine how the wind speed translated to lateral force. I just found a reference to a book published in 1930 by John Wiley and Sons that may shed some light on the subject. I will share that info if I can locate the book.



Please add your comments to this thread so that we may keep the discusiion going (especially if you have some experience with design codes and methodology prior to the 1970's).

TowerEngineer

 

[JAE]

I just have exposure to some "older" engineers when I first started in 1979. One fellow had started out designing major structures in 1924 and was still designing in 1985! There were others at that firm that had started out in 1940's as well, who learned under the firm's founder (firm started in 1909).

What did I learn from them?

First, a great deal about being really careful. Their designs were incredibly conservative and almost always simplified to the extreme. I am sure that the thought of re-distributing lateral shear with respect to the various levels of rigidity in a building never entered their mind. Calculations were done on small pads of grid paper and were mostly made up of vast quatities of numbers....very little verbage to indicate what the numbers were trying to show.

Second, they rarely thought of lateral design. In south Texas, there was basically zero interest in seismic (UBC Zone 0 at the time). Even in the 1980's, I was taught to design numerous commercial buildings (one story) with no lateral force procedure....just told that the masonry infill (structural clay units) would keep it upright.

The engineer from 1924 had designed a 30 story tower but I'm not clear as to what his procedure with regard to wind was. It's still standing.

 

[TowerEngineer]

JAE,

That's the interesting thing about the lateral force design procedures of today and historic structures. Most all structures, whether designed with lateral force considerations or not, are still standing today.

One question I have in mind, and to which I hope to find an answer, is why are so many historic structures still standing if the standards under which they were designed were so inadequate. From what I have found out so far... in recent years (since 1982) the lateral force requirements contained in both the wind and seismic sections of all design standards have dramatically increased. This recent increase amounts to about a factor of 2 or so. In contrast to lateral force procedures dating from the 1930's, it appears that wind and seismic forces have increased by as much as a factor of 2 to 6.

Is your former associate who practiced from 1924 to 1985 still living? If so, he would be an excellent person to talk to on this subject.

In the beginning of my structural engineering career, I too worked in a firm with an office practice that viewed the codes as exactly what they are: MINIMUM design standards. It was very common for the principals to select office-wide design criteria that exceeded the code required minimums (for live loads) by a factor of 60 to 100 percent.

Is that level of conservatism really needed considering the degree to which the codes have continued to increase both live and lateral loads?

We continue to learn more and more about the behavior of buildings and other structures as we learn from our mistakes. We engineers can learn from those mistakes, if and only if, there is institutional memory that lasts longer than one or two generations; hence the importance of engineering history.

 

[JAE]

The 1924 engineer is not living. The ones who started in the 40's are, but retired. I would guess, that if you are doing a research project or article, that many firms in your area could point you to older, retired engineers who came from that era.

The old buildings still stand, perhaps, due to a number of reasons,
1. The general conservatism in the designs
2. The buildings most probably have not experienced the full, code load.
3. Redundancy in the design from walls, secondary moment strength, stairs, etc.

 

[Qshake]

First, let me say that I agree with JAE's three points on why buildings designed to older codes are still standing.

With respect to seismic loading let me also state that many buildings (single story unreinforced masonry or otherwise) have failed during recent earthquakes. It is our good fortune that no one was inside or that any great number have been killed in them. This also extends to other countries as well, where in many cases there have been many unfortunate deaths. You say "those countries don't use our codes" and that is true to some extent. However, our codes ofter serve as a model code for development in those countries. Moreover, following many of the world's devastating earthquakes international organizations thoroughtly investigate those codes with respect to the failures. These organizations then make the appropriate recommendations. This is an important part of the development of the codes as JAE noted, some of the buildings have not seen the full code forces. Well, couple that idea with the number of differing factors which when combined make a seismic event a diaster and you can see why it is necessary to look beyond the US for information on the behavior of structures in seismic events.

Lastly, while Northridge moment connection failure points to details, details, details it also points to another part of seismic design that was not necessarily addressed by the codes, especially the early ones. That point is that buildings and their components are subject to cyclic loading not static loading as implied by the codes in the lateral force methods.

By the way, I really like this forum and hope that others find the subject matter interesting as well. As you've both pointed out, there is much to learn from the older engineers and their plans and calculations.

 

[TowerEngineer]

Qshake,

I largely agree with your comments with repsect to cyclic loading and strength losses in the moment connections. Most failures I can think of have been the result of a failure of one connection 'detail' or other, or as you say, "...details, details, details". It is my thesis that the static seismic force design procedures and static force levels are, and have been, adequate for purposes of resisting the maximum probable earthquake for more than twenty years. In my view, the problem has always been one of devising a method of providing sufficient strength and ductility in the lateral force-resisting system of the structure. That is to say, I don't think this is the proper place to put the energy dissipating mechanism -- especially in rigid welded moment connections (structural yielding + permanent offsets = structural damage).

The recent proliferation of the use of base-isolated structures has good promise of limiting structural damage to buildings for regions with relatively moderate ground shaking (around magnitude 7.0 or less). However, I think this method of limiting structural damage of buildings located in regions of very high seismicity (magnitude 8 or greater) will prove largely ineffective. The problem is generally one of total displacement. In the 1906 San Francisco earthquake, for instance, total lateral ground displacements were as much as 20 feet or more and extended over a distance of approximately 250 miles (from north of Santa Rosa to south of Hollister, CA). Only time will tell when we finally get a large earthquake that tests the base-isolated structures. By the way, there were many structures located in SF that survived the 1906 event without significant damage that were destroyed in the subsequent fire. It would probably have been very useful to have documentation on the structures that did survive the initial earthquake. Many of these structures may have been unreinforced or lightly reinforced one and two story masonry buildings. I know, for instance, that the Fairmont Hotel (a relatively large masonry structure) survived the initial earthquake and its aftershocks but fell victim to the fire that later ensued.

I think a much more promising method of dissipating the inertial force is by means of utilizing energy dissipation devices (EDDs) as described in the 1999 SEAOC Blue Book. I recently attended a seminar on the design of a PS/PT concrete parking structure built in Eugene, OR that used metallic-yielding devices for limiting relative lateral displacement of the structure and which devices also served as the primary means of energy dissipation. The cost of the small U-shaped pieces of steel was extremly small in relation to the cost of base-isolation devices. I think the use of EDDs in building design is the wave of the present and future -- especially metaillic-yielding and viscoelastic EDDs. As someone (I can't remeber who) noted at one of the technical sessions of a recent SEI Structures Congress, mechanical engineers have used a system of springs and shock absorbers as a means of limiting damage to the automobile frame and for means of energy dissipation for decades and he didn't see why the building industry couldn't do the same thing. I agree wholeheartedly with this viewpoint. We just need to develop the engineering know-how in order to create structures that will sustain little or no "structural damage" during cyclic lateral loading events that are coupled with large total horizontal displacements. As an analogy, the structural frame can be viewed as the part that is to be protected from damage by a sub-system of "springs and shock absorbers" in the form of lateral bracing and EDDs. This system is applicable to steel, masonry, and concrete structures. Wood structures are, of course, the one exception to the rule. The amount of internal lateral displacement that may generally be permitted before relatively significant amounts of architectural and structural damage occurs is very small. Base isolation is promising for the largest of these structures, but is not an economically viable solution for most ordinary wood frame construction -- nor would it appear necessary from an historical perspective in relation to structural failures.

Therefrom springs my interest in historic building structures and their design. It is not only important to know what doesn't work and why (engineering failures), but it is also important to know what does work and why (engineering success). I think we have had a pretty good handle on the forces that are involved in environmental loads for more than 40 years. The problem seems to be how best to deal with resisting the lateral loads generated by extreme environmental events. I think we engineers will have a better handle on where we need to go if we can better understand where we have been and what led us to where we are in the art and science of structural engineering.

In the near future, I hope to have some relevant historic lateral load and design factor of safety information used in the construction of the Empire State Building and the Brooklyn Bridge. These two structures have been standing a very long time and will serve as good examples for purposes of engineering history discussion. I will share that information in this forum as soon as possible.

I also hope to obtain information with respect to the design of the Golden Gate and Bay Bridge construction in the San Francisco bay. Both of these structures were designed and built in the early 1930's and have been subjected to very extreme environmental loads -- in the form of both wind and seismic loading. It may also prove interesting to know exactly what lateral load was used in the design of the Washington Monument and the Eiffel Tower -- each of which were the tallest structures erected subsequent to the construction of the Pyramids of Egypt.

Thanks to all for your contributions to this forum.

 

[JAE]

Great dialog, TE....

Picking up on your third paragraph, it will be interesting to see what develops in the use of these active systems to reduce seismic forces in buildings. Especially where the "line" is drawn, both code-wise and economics-wise, with respect to where it becomes a proper choice to use such a device. Would only a zone 3 or 4 initiate the economic choice to use such a device in a building? When you get into the lower seismic regions, do you fall back to the current philosophy where the design provides for only public safety, and not necessarily building survival?

I suppose if the cost, standardization, and familiarity of these devices became as common as the steel joist, perhaps it would make sense to use them everywhere, everytime.

 

[TowerEngineer]

JAE,

I think the place we're headed now is toward what is referred to as 'performance-based' engineering wherein we try to involve the building owner in the design process. I think the essential idea is that the owner become more knowledgeable with respect to what can be expected of the structure under a particular environmental load. In the ideal engineering world, and as a minimum, life safety would be protected in every circumstance. Then beyond that minimum design standard, the owner of the building would to a large degree determine what level of building damage and repair would be acceptable to him.

Of course, this has the potential of becoming a real nightmare for the insurance underwriters. The underwriters would need the services of a structural engineer in order to review the design of the particular structure of interest before determining the insurability of the building and the cost of the insurance.

With regard to metallic yielding and other EDDs, it looked to me as though there is widespread application for their use because of the very low cost of the device (I think somewhat less than $10/unit for the metallic yielding devices). Surprisingly, there weren't very many structural engineers at the seminar I attended in Portland, OR wherein the design philosophy and experimental testing was presented. I really expected to see more interest in the subject. The cost of the EDDs was inconsequential and the result was that no structural damage would occur whatsoever -- even under strong ground motion. I can't remember off-hand, but I think the scale of the model was somewhere on the order of 1:4. The model went through a very long series of time-history steps and never lost lateral stability and sustained no structural damage. Very impressive!

It will take some re-education and changing of minds with respect to permissible lateral displacement before it will become accepted by the code-writing bodies. I think the building structural design was approved largely because of alot of arm-twisting and the prestige of the design firm involved in the project. As it was, the structural design was approved more or less as an exception to the norm, and only then because the structure was formerly situated in a region of low seismicity (zone 2), but has since been redefined as zone 3.

 

[JAE]

There's that dreadful word that structural engineers seem to hate: re-education.

It takes so long to bring a new "invention" into the structural engineering marketplace. Engineers are hesitent to use "devices" in lieu of tried-and-true methods that "have worked for years". Its tough being a guinea pig. The code writing bodies should take the lead in developing design standards for the devices as well as approvals for their use.

 

[TowerEngineer]

JAE,

In terms of recent code administration and code writing, it has generally been the practice of code writing bodies and code enforcement officials to require proven test results before permitting the use of a new technology or design rationale.

I remember attending one of the very first seminars held on base isolation in Emeryville, California sometime in early 1984. This was a largely untested method of reducing forces within the building structure. The use of base isolation in seismic design has proliferated in the last 15 years, however, and it is of some significant relevance that the methodology has the backing (in forms of both research and promotion) of Dr. Igor Popov at UC Berkeley. It has taken awhile, but the code-writing bodies have reconized that one potential short-coming of base isolation is that total displacement must be limited.

Unfortunately, there must be some willingness on the part of the building official to accept the use of a new technology before a substantial investment will be made in the research and development of that new technology.

Looking at this from a historical perspective, it is very unfortunate for we engineers who practice structural engineering because the Eiffel Tower and the Brooklyn Bridge probably would never have been built if they had the same institutional barriers to their design and construction that we live with today. There were great political hurdles to overcome in their time, but Eiffel and Roebling were both scientific geniuses that were far ahead of their contemporaries in the design and use of the new steels (both wrought iron and bessemer steel).

To a large degree our problem is one of social attitude toward risk taking. Almost noone thinks twice about getting in their car to drive in heavy traffic to and from work every day. Yet, close to 50,000 people die on our streets and highways every year. Then, too, there is the airline industry that has its own safety related problems. Contrast this with the loss of life because of a structural failure of one sort or another. It seems to me that there really isn't any logic in the whole life-safety system. Just think how much time is spent in the home, and the fact that most homes have very little scrutiny with respect to their ability to resist wind and seismic loads. Yet, what are the odds that many people are likely to be injured or die even if there is a total collapse of a major structure such as a bridge or tall building?

As I see it, the problem is generally one of liability. With automobiles and aircraft it is very difficult to place the blame on a single engineer or small group of engineers for a particular failure. Whereas, with a building structure, the blame is a very easily placed on the engineer of record if anything untoward happens. A little tort reform would go a long way in fixing what ails this country in terms of new engineering products and design rationale.

What of the engineers who first used slope-deflection and moment-distribution methods in their designs? Do you think they had much trouble convincing their contemporaries that these forms of analysis were correct?

 

[Qshake]

TE,

I agree that displacement is the concern of seismic analysis and design. In fact, it now becoming more and more prevalent in our codes and analysis. We are getting away from force-based design and moving toward displacement based design.

As far as areas with twenty feet of movement, there is little to do. Proper mitigation protocol, in my opinion, mandates that we don't build in these areas. However, there are many companies out there willing to risk lives, building, product etc. on the "big" one.

About those buildings in the 1906 earthquake, there are probably a lot which survived only to burn down. The real performance of a building relies both on structural design but also on fire safety etc. In other words the whole design process. Also, one can conclude that certain buildings are more vulnerable in specific earthquakes due largely to the dynamic characteristics. This has been documented time and time again.

Seismic isolation, as you've noted, is an idea whose application to buildings is long overdue. On the other hand I can see why it might have been a hard sell!

 

[TowerEngineer]

Qshake,

Unfortunately, nearly half the state of California is subject to very high seismicity (zone 4). The San Andreas fault runs right through the heart of San Francisco and Los Angeles. There will continue to be huge earthquakes in those cities for thousands of years. One of the questions is: 'On what frequency?' Some say that Los Angeles is long overdue.

Is it our job as structural engineers and government agents to mandate that people limit their risk and force them to live in one region or another? What about the freedom to take personal risk? What would we do with the more than 7.5 million and 15 million people that already live in the San Francisco and Los Angeles areas and in every community between?

I also understand that the greatest earthquake of recent history occurred someplace in the midwest -- Missouri or there abouts. It is supposed to have measured somewhere around a Magnitude 9.5 to 10.0 and is responsible for burying a whole city below the great Mississippi River -- which was re-routed during the EQ. If my memory serves me, this EQ happened somewhere around 1850? Here in my own state (Oregon) a great volcanic eruption -- possibly one of the greatest explosions ever -- occurred when Mount Mazama blew its top creating Crater Lake. This happened as little as 6,000 - 7,000 years ago ang blew hot ash as far distant as Alberta, Canada and Wyoming. In geologic terms, 6,000 years is the blink of an eye (or maybe two or three).

On the point of the fire that ensued after the 1906 SF earthquake, the firefighters were helpless to fight the fire because the water supply infrastructure (underground pipeline system) was completely destroyed by the earthquake. They had no water to fight the fire as it swept from one neighborhood to the next, and could do nothing but watch helplessly from the hills of Oakland as their once great city burned to the ground.

I fear the same fate is in store for Los Angeles if and when the next great earthquake hits that city, and there is nothing that can be done about it. Don't get me wrong, I'm the world's last and greatest optimist. However, I believe Murphy was also an optimist in that his famous statement: "If anything can go wrong, it will" is a positive statement! The people of SF are optimists also showed their resilience by re-building their city.

People will always make unwise choices in life, and living in an area of high seismic activity is one of those choices. It's kind of interesting... Californians look at earthquakes much the same as Texans and Kansans look at tornadoes, i.e., with disdain until one crops up in their neighborhood and destroys their life's work, if not their lives.

The bottom line is that there is no place on earth that is absent of one form of environmental risk or another. Though I have to say, I think we Americans are extremely blessed when you compare our environs to those of nearly any alternative. And with the freedoms we enjoy, we are at liberty to choose to relocate to almost any place we want -- including those with relatively high wind and seismic risks.

 

[Qshake]

Yes, I'm well aware of the many faults that lie above and beneath the surface in the California region. And I am also aware of the circumstances behind the fire which consumed San Fransisco following the earthquake.

I would hope that the water system is now far superior to that of the early 1900s and will perform as expected in the next large earthquake. Mitigation in those heavily populated areas requires planning, planning, planning. As long as there are man-made structures be they utilities, structures etc there will be hazards and subsequent risks which must be considered when building in these areas. No I don't advocate moving any large population of that area just because of those hazards. I assume that the people there find the risk acceptable. I can only hope they would also find it necessary to be fiscally responsible for those decsisions. That is, I'm afraid, hardly the case.

In areas that have yet to be developed I hope that some thought is given to what happens in these areas be it flood, earthquake, wildfires etc. Many affluent Californians build homes on cliffs and bluffs which are prone to failure in hard, long periods of rain. Yet we build homes there because we can (fiscally and constructability) and yet those homes still cascade down the bluff and some will do it all over again. I just hope they aren't burdening the rest of us who chose not turn a cheek to the forces of nature time and time again.

Not only is the mid-west the location for a story-book earthquake (1811-1812) but also phenomenal flooding ('92-'93). Thankfully, there wasn't much to destroy in Missouri, Illinois, Tennessee and Arkansas at that time. During the flooding period though much was destroyed from agriculture to whole towns. In some cases, those town rebuilt on higher ground. I am glad for their foresight to move and not rebuild in the same location. I think they will also be comforted on many a stormy night knowing that the ordeal they went through won't plague them again.

My point throughout the dialogue is one of community and personal responsibilty. Yes, we can move our house to straddle the San Andreas fault, but how practical is this. The fault itself moves several centimeters a year. So not only would one have to worry about the EQ but also accomondate the movement as well. Not too many house out there with expansion joints, but I guess if someone is willing to pay for it some firm is willing to design it. Likewise the community, if it desires to build in an area of known hazard, must bear the cost of mitigation.