Earthquake Codes History

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Preface

On this anniversary of the 1906 San Francisco earthquake, it is important to acknowledge the gift of intellectand time that so many engineers have given over the last 100 years in the effort to develop and improve theearthquake provisions of building codes. Frank McClure was one such engineer who worked tirelessly andselflessly to advance the relevancy of building codes with regard to earthquake design.

Frank was a native San Franciscan, and attended the University of California where he received a B.S. degree

in Civil Engineering. In 1955, he established his own engineering firm, specializing in earthquake resistantdesign and research, and later partnered with David Messinger, with whom he worked with until 1975. In1976, he accepted a position with his alma mater, where he remained until his retirement in 1991.

Throughout his career, Frank was constantly involved with numerous regional, state, national and internationalearthquake engineering committees and code writing bodies. His participation was always valued due to thepassion and knowledge he brought, much of it gained from his travels around the globe to personally inspectand document the damage caused by earthquakes. Among many other accomplishments, he served on the

Field Act Advisory Board, the Joint Legislative Committee on Seismic Safety, the Seismology Code DevelopmentCommittee of the International Conference of Building Officials, the Committee on Earthquake Engineeringof the National Research Council, and was President of the Earthquake Engineering Research Institute.

I first met Frank when he hired me straight out of graduate school. He was very instrumental in defining thedirection of my career with the keen interest he had in computer applications for the automation of earthquakeengineering. An engineer with a vision ahead of his time, Frank was one of my most ardent supporters whenI started Computers and Structures, Inc. 30 years ago. Over the years he closely followed and critiqued many

of our developments and proved to be one of our greatest cheerleaders.

In honor of Frank, Computers and Structures, Inc. is proud to reprint the following article he originally wrotein January of 1968. In his own words, Frank describes the early evolution of earthquake codes, an importanthistorical review from a man who was an eyewitness to many of the code developments of the last half of the20th century.

Finally, we are grateful to Augusta McClure and the McClure Family for granting us the permission to

reprint this article.

A Publication of Computers & Structures, Inc.

MODERN EARTHQUAKE CODES

History & Development

By Frank E. McClure, Structural Engineer

ASHRAF HABIBULLAH - CEO | Computers & Structures, Inc. April 18, 2006


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MODERN EARTHQUAKE CODES | History & DevelopmentFrank E. McClure

Consulting Structural Engineer

Outline of Presentation

1. Introduction

2. Before San Francisco 1906

3. After San Francisco 1906

4. Japan 1923

5. Santa Barbara 1925

6. Palo Alto 1927

7. State Chamber of Commerce Committee 1928

8. Long Beach 19339. El Centro 1940

10. SEAOC Seismology Committee 1957-1960

11. SEAOC Seismology Committee 1960-1967

12. Conclusion

13. Acknowledgement

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Introduction

It might be well to review briefly the stated purposes of building codes. The purpose, as stated in Section 102of the Uniform Building Code, is "to provide minimum standards to safeguard life or limb, health, propertyand public welfare." This stipulation of "minimum standards" is in keeping with constitutional curbs onpolice powers. Structures meeting these "minimum standards" should be capable of resisting anticipated loadsand forces without serious structural damage but would not be expected to experience a major earthquake

without sustaining non-structural damage such as plaster-cracking, glass breakage, etc. The latter damagecan only be reduced when the owner is willing to pay for additional precautions beyond that required byminimum standards.

Building codes can find justification only in the responsibility of government to protect public healthand safety and major property damage. It is important that code requirements accomplish this and nothingmore. Discussions on designs in excess of the code responsibility should be left to the owner on the adviceof his structural engineer.

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Before San Francisco 1906

Buildings in San Francisco were designed by eastern firms. A review of the drawings of some buildingsthat resisted the 1906 earthquake indicated that provisions were made for wind forces as high as 30 lbs. persquare foot. There were no formal building regulations that included any earthquake provisions prior to 1906.In fact the word "earthquake" was not mentioned in any code until 1927, in the Palo Alto Code. Althoughin 1903 it did have a few general requirements written that probably were never read or enforced.

After San Francisco 1906

A report prepared by the San Francisco Section of the American Society of Civil Engineers in 1907entitled The Effects of the San Francisco Earthquake of April 18, 1906 on Engineering Construction statedthat "Sufficient evidence is at hand to warrant the statement that a building designed with a proper systemof bracing to withstand wind at a pressure of 30 lbs. per square foot will resist safely the stresses caused by ashock of an intensity equal to that of the recent earthquake." There were perhaps 15 or 16 tall major steelframe, fire resistant structures in existence. Reinforced concrete was in its infancy. Records at the City Hallindicate that San Francisco had no real building code until July 5, 1906. The 1906 Code required that anybuilding over 100 feet high or a height of over three times its least horizontal dimension, that the steel frame

should be designed for a wind force of 30 lbs. per square foot, acting in any direction upon the entire exposedsurface. In 1906 the wind factor was changed to 15 lbs. per square foot. In 1910, the concrete frame wasincluded with the steel frame and the wind factor raised to 20 lbs. Then in 1926 the wind factor was againreduced to 15 lbs., where it remained until the complete revision of the building code was made in 1947.This 15 lb. provision applied to buildings over 102 feet or where the height exceeds three times the leasthorizontal dimension, but there was no provision for wind- even earthquake- for buildings less than102 feet in height.

Japan 1923Several engineers from San Francisco went to Japan after the 1923 earthquake there, where three major

buildings, statically designed for lateral forces of 10 percent of gravity, showed marked resistant behavior.

The Board of Fire Underwriters of the Pacific, when their earthquake department was established in1926, was greatly influenced by the observations in Japan 1923 and Santa Barbara 1925 and advocated astatic design, using a 10 percent lateral force factor.

Santa Barbara 1925

The Santa Barbara earthquake proved to be more of a turning point than most people realize. Theearthquake there caused heavy building damage and resulted in a sudden and wide-spread demand forearthquake insurance. This in turn was followed immediately by an increase in earthquake insurance ratesand in the amount of such insurance required by the State Corporation Commissioner before he would approvebond issues on certain types of buildings. The result of this two-fold handicap was a sharp recession of

building in the state, and in the emergency, the aid of the State Chamber was urgently sought by businessinterests generally. It soon became evident that the fundamental need was for a statewide building code

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which would require adequate standards of building construction and a reasonable resistance of such

construction against earth movements. This was the genesis of the State Chamber's Code Committee.At the same time in San Francisco, there was a group of structural engineers who were in San Francisco

soon after 1906 and were concerned about the poor construction practices developing in the 1920s and thereduction of the 1906 30 lb. wind requirement. This lead to the establishment of the San Francisco Section ofthe American Society of Civil Engineers code committee that met every week for seven years. This local codecommittee, later in 1928, was made a part of the State Chamber of Commerce Code Committee which alsoincluded the AIA, the AGC and the then Pacific Coast Building Officials Conference - now the InternationalConference of Building Officials.

The year 1925, because of the Santa Barbara earthquake, may be considered as marking the realbeginning of earthquake studies and research in the United States. It was in this year that, by direction of theUnited States Congress, the United States Coast and Geodetic Survey was given the responsibility to makeinvestigations and reports on seismology. The work of the Coast and Geodetic Survey, in particular thepublishing of strong-motion earthquake records, was to have significant influence on the development of codes.

Palo Alto 1927

The "Palo Alto" Code and the 1927 Uniform Building Code were two of the first codes in the UnitedStates to incorporate lateral force requirements which were based on the Newtonian concept that the seismicforce was a product of the mass times the acceleration. The coefficient of acceleration varies depending onthe bearing capacity of the ground but was approximately 10% of the dead load. The Palo Alto Code isunderstood to have been developed with the advice of Professors Bailey, Willis, and Marx of StanfordUniversity. The Uniform Building Code provisions were in the appendix of that code and were planned foroptional use. Cities such as San Bernardino, Sacramento, Santa Barbara, Klamath and Alhambra are said to

have incorporated them in the body of their codes.

State Chamber of Commerce Committee 1928

The State Chamber of Commerce recognized the need for a building code which was "dedicated to thesafeguarding of buildings against earthquake disaster." Studies under this sponsorship included work by manyof the leading structural engineers of the State and resulted in a report which, while not adopted, formed thefoundation of codes that followed. Many circumstances prevented an early conclusion of the committee's

work, at times the work was entirely suspended. On May 27, 1938, the code was presented to the StateChamber which ordered it printed.

Long Beach 1933

The first mandatory seismic codes used to any extent in the United States were published in 1933following the March 10, 1933, Long Beach earthquake. Two California State Laws were passed.

The Field Act was passed because of the large portion of school buildings that were extensively damaged.

This Act assigned to the Division of Architecture of the State Department of Public Works, the authority and

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responsibility, under the police power of the State, to pass upon and approve or reject plans and specifications

and to supervise the construction of all public school buildings. "Appendix A" of the Rules and Regulationswas then adopted by the Division of Architecture which were practically in toto the earthquake provisionsunder consideration by the State Chamber of Commerce Code Committee at that time.

"Appendix A" required masonry buildings, without frames, to be designed for a lateral force of 10% ofthe dead, plus a portion of the live load. Other buildings were to be designed for 2% to 5%. The lowercoefficients were related to higher allowable foundation loads. In 1937, the requirements were revised to makethe coefficients 6% to 10% for buildings 3 stories or less in height, or buildings without a moment resistant

frame. Buildings more than 3 stories in height, with a complete moment-resistive frame, had coefficients of2% to 6%, provided the frame could resist 2% of the load. In 1941, the coefficients were 6% to 10%, dependingonly upon the type of foundation materials. Since 1953, the coefficient used is based upon the equation:

based on dead load only.

In 1933, the other California State Law that was passed was the Riley Act which required that all

buildings, except certain type dwellings and farm buildings, to be designed for a lateral force of 2% of the totalvertical design load or 15 lbs. per square foot wind for heights less than 60 feet. In 1953, this requirement wasrevised to require 3% for buildings less than 40 feet and 2% for those over 40 feet. The wind loading was notchanged. In 1965, the Riley Act provisions were changed to the provisions in Title 24 which, for the lateralforce requirement, are the provisions of the 1961 Uniform Building Code. The wind requirement varies withintwo areas in California and the height zones above ground.

In 1933, the Los Angeles Building Ordinance adopted a lateral force code with a coefficient of 8% of

dead load plus half live load. This was also required in the Uniform Building Code of 1935 on soils good for2000 psf or more in areas of highest seismicity (Zone 3), with double this value for weaker soils.

El Centro 1940

The El Centro earthquake in 1940 provided strong-motion accelerograms from the instruments installedby the Coast and Geodetic Survey after 1933, and started a new era in seismic codes, tending toward a more"dynamic" approach. In 1943, the City of Los Angeles recognized indirectly the influence of flexibility on the

earthquake design coefficients, making the coefficients a function of the number of stories in the buildingby the formula:

C being a percent of the dead load. This same provision was incorporated in the Uniform Building Code in1949. In 1959, when the height limit of 150 feet was removed from the Los Angeles Code, this formula wasmodified to read:

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C =.60

N + 4.5

C =.60

N = 4.5

C = 4.6SN+ 0.9(S-8)


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S = 13, for buildings 13 stories or less.

It was not until 1947 that San Francisco had anything more stringent than the Riley Act in its Code. Atable of variable coefficients was adopted, with maximum value for one story buildings of 8% and minimum

value for 30 stories of 3.7% with variations for soil conditions. These were applied to design vertical loads.This prompted the formation, in 1948, of a Joint Committee on Lateral Forces of the San Francisco

Section of ASCE and the Structural Engineers Association of Northern California. This committee, afterseveral years' study, issued a report wherein it was recommended that lateral force coefficients be used whichwere related to the estimated, or calculated fundamental period of vibration of the structure. This report,published in 1951, has received worldwide acclaim and is the basis for many earthquake codes. It was being

used as a guide in 1967 in Colombia, South America. San Francisco, in 1956, adopted a variation of therecommendations of the Joint Committee.

SEAOC Seismology Committee 1957-1960

In the fall of 1957, charged with the responsibility of developing a uniform seismic code acceptable tothe structural engineers throughout the State of California, SEAOC assigned the problem to a newly appointed

Seismology Committee. Detailed studies and analyses and recommendations were considered for over twoyears and the final report was adopted by the Board of Directors of SEAOC in December 1959.

The important provisions of the 1960 SEAOC Recommendations are:

1. Recommendations that only one zone be used for California.

2. Formula for period of building or provisions for determination of T. based onsubstantial technical data.

3. The coefficient "C" is a function of the cube root of "T".

4. The distribution of the base shear was given by a formula.5. A formula for overturning was given.

6. The total lateral force or base shear was given by the formula: V equals KCW, which introduced a newfactor "K" which took into account the type or arrangement of the resisting elements.

7. The "now famous" Section (j) was introduced which required that buildings more than 13 stories or onehundred and sixty feet (160') in height shall have a complete moment resisting space frame capable ofresisting not less than 25 percent of the required seismic load for the structure as a whole. The frameshall be made of ductile material or a ductile combination of materials. The necessary ductility shall beconsidered to be provided by a steel frame with moment resistant connections or by other systemsproven by tests and studies to provide equivalent energy absorption. Until the code was changed in 1967,this was interpreted to be a structural steel complete moment resisting framing system.

8. A Commentary was prepared to complement the recommendations in the code, present items of detailto supplement and explain its broad considerations. It was decided, however, for the building codes toapply to all classes of buildings, that it would be unreasonable and economically unjustified to attempt toimpose a design planned to achieve structures which would survive even the strongest earthquake,

without any damage. One of the more important statements in the 1960 Commentary is "TheRecommended Lateral Force Requirements are intended to provide this protection in the event of an


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earthquake of intensity or severity of the strongest of those which California has recorded." This level ofprotection was defined as the primary function of a building code is to provide "minimum standards" toassure public safety. Requirements contained in such codes are intended to safeguard against majorstructural failures and to provide protection against loss of life and personal injury. The code does notassure protection against non-structural damage, such as cracked plaster, broken glass, broken lightfixtures, cracked ornamentation, cracked filler walls, or overturned equipment. Some owner-sponsoredcodes go further than this, an example, the Field Act, and have as their purpose also, the protection ofproperty; however, this is not the purpose of building codes generally.

The 1960 SEAOC Code was adopted by the City and County of Los Angeles in 1960, andin the Uniform Building Code in 1961 with certain exceptions, such as retaining the Zone provisions, etc.;the City of San Francisco adopted the of SEAOC Code in 1962. As stated before, Title 24, the "California StateBuilding Regulations" for state agencies having jurisdiction over building construction, adopted the 1961Uniform Building Code lateral force provisions in 1963.


Earthquake resistant design is undergoing continuous review all the time. Any code should beconsidered as an interim code and so the SEAOC Committee continued to study the problem of the wordingof "Section (j)". In 1967 the Board of Directors of SEAOC, approved the Seismology Committee's revisedrecommended lateral forces. The major revisions were as follows:

1. Detailed requirements for reinforced concrete to qualify a moment-resisting frame to provide thenecessary ductility to permit this construction in buildings over 160 feet in height.

2. Requirements for the use of shear walls and braced frames.

3. Special requirements for design, construction and inspection.4. Minor modification of the factor "3" having to do with overturning.5. Concerning "Section (j)" and the use of "ductile reinforced concrete", the major change was not only the

acceptance of reinforced concrete for high rise-construction, but a change in philosophy from equivalencyof energy absorption to an adequacy criterion. The two materials-structural steel and "ductile reinforcedconcrete"-are different and equivalency in performance in earthquake response is not expected.

6. There was an expansion of the section in the Commentary describing the extent of damage that wasexpected under these revised requirements.

From the Commentary "...structures designed in conformance with the provisions and principles setforth therein should be able to:

1. Resist minor earthquake without damage.2. Resist moderate earthquakes without structural damage, but with some non-structural damage.3. Resist major earthquakes of the intensity or severity of the strongest experienced in California, without

collapse, but with some structural, as well as non-structural, damage."


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The 1967 SEAOC recommended lateral force requirements were adopted into the 1967 Uniform BuildingCode and are under consideration for adoption into Title 24 and the San Francisco Building Code.

Conclusion

The building code by itself cannot be a guarantee of good construction. Competent design by engineerswith experience in earthquake engineering, coupled with adequate inspection by qualified persons andconscientious workmanship by skilled contractors, are the primary means of obtaining earthquakeresistant structures.

Acknowledgement

I wish to acknowledge my indebtedness to Henry Degenkolb and Karl V. Steinbrugge for allowing meto use their slides and the personal contribution by Harold Hammill and Harold Engle, who provided someof the history from 1906 to 1933.


Frank E. McClure

The McClure and Messinger Gang ~ Circa 1997

Frank McClure with David Messinger, Ashraf Habibullah and Arthur Anderson, Jr.

Copyright 2006 Computers and Structures, Inc.


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1960, and in the Uniform Building Code in 1961 with certain exceptions, such as retaining

the Zone provisions, etc.; the City of San Francisco adopted the SEAOC Code in 1962. Asstated before, Title 24, the "California State Building Regulations" for state agencies having

jurisdiction over building construction, adopted the 1961 Uniform Building Code lateral

force provisions in 1963.


Earthquake resistant design is undergoing continuous review all the time. Any codeshould be considered as an interim code and so the SEAOC Committee continued to study

the problem of the wording of "Section (j)". In 1967 the Board of Directors SEAOC, approved

the Seismology Committee's revised recommended lateral forces. The major revisions were

as follows:

1. Detailed requirements for reinforced concrete to qualify a moment-resisting frame to

provide the necessary ductility to permit this construction in buildings over 160 feetin height.

2. Requirements for the use of shear walls and braced frames.

3. Special requirements for design, construction and inspection.

4. Minor modification of the factor "3" having to do with overturning.

5. Concerning "Section (j)" and the use of "ductile reinforced concrete," the major change

was not only the acceptance of reinforced concrete for high rise-construction, but achange in philosophy from equivalency of energy absorption to an adequacy criterion.The two materials-structural steel and "ductile reinforced concrete"-are different and

equivalency in performance in earthquake response is not expected.

6. There was an expansion of the section in the Commentary describing the extent ofdamage that was expected under these revised requirements.

From the Commentary "...structures designed in conformance with the provisions and

principles set forth therein should be able to:

1. Resist minor earthquake without damage.

2. Resist moderate earthquakes without structural damage, but with some non-structural

damage.

3. Resist major earthquakes of the intensity or severity of the strongest experienced inCalifornia, without collapse, but with some structural, as well as non-structural, damage."

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The 1967 SEAOC recommended lateral force requirements were adopted into the

1967 Uniform Building Code and are under consideration for adoption into Title 24 andthe San Francisco Building Code.

Conclusion

The building code by itself cannot be a guarantee of good construction. Competent

design by engineers with experience in earthquake engineering, coupled with adequate

inspection by qualified persons and conscientious workmanship by skilled contractors,are the primary means of obtaining earthquake resistant structures.

Acknowledgement

I wish to acknowledge my indebtedness to Henry Degenkolb and Karl V. Steinbrugge

for allowing me to use their slides and the personal contribution by Haro1d Hammill and

Harold Engle, who provided some of the history from 1906 to 1933.

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NOTES

Documents

Earthquake Codes History