The Need For and the Development of Seismic Codes in the United States:
Part I
by:
Mustafa Mahamid, Ph.D., P.E.
The purpose of building codes in general, as stated in the International Building Code is “to establish the minimum requirements to safeguard the public health, safety and general welfare through structural strength, means of egress facilities, stability…..”.
Structures meeting minimum requirements should be capable of resisting applied earthquake loads without serious structural damage that lead to collapse that may cause loss of human lives. Where non-structural and architectural damages would be expected as a result of a major earthquake, these damages can be reduced if the owner of the building/structure is willing to pay for additional precautions beyond what is required by the minimum standards.
Earthquake design requirements were not in any building code before 1906 San Francisco earthquake. Review of design drawings for buildings in San Francisco showed that buildings were designed for 30 psf wind load not for earthquake load. In fact, the word “Earthquake” was mentioned for the first time in Palo Alto Code in 1927.
The records showed that there was no building code in San Francisco until 1906.
As a result of the 1906 Earthquake and the investigations of buildings damages, the American Society of Civil Engineers (ASCE) San Francisco Section prepared a report entitled “the effect of San Francisco Earthquake of April 18 1906 on Engineering Construction” stated that the buildings designed for 30 psf wind load will resist safely the forces produced as a result of an earthquake with a magnitude similar to the San Francisco earthquake. Therefore the 1906 San Francisco code required any building with a height equal to or greater than 100 feet to be designed for 30 psf lateral load. This was changed to 15 psf later in 1906 and to 20 psf in 1910 and back to 15 psf in 1926. This value was maintained until 1947 and it was applied to buildings with height over 102 ft or if its height exceeds three times its least horizontal dimension, but there was no wind or earthquake provisions for buildings with height less than 102 ft.
As a result of the 1923 earthquake in Japan, several structural engineers from San Francisco went to Japan and investigated several buildings. They noticed that three buildings that were deigned for a lateral force equal to 10% of gravity showed good resistance behavior. As a result the Board of Fire Underwriters of the Pacific adopted the static design using 10% of gravity for lateral forces.
Santa Barbara earthquake in 1925 was the turning point in the seismic codes in the United States as a result of the heavy damage observed in buildings. This resulted in requirement for earthquake insurance that lead to the urgent need for adequate standards of building construction and reasonable resistance of such construction against earth movements.
At the same time, a group of structural engineers from San Francisco who observed the 1906 earthquake were concerned about the reduction of the 1906 30 psf design lateral load and about the poor construction practices developing in the 1920’s. This lead to the establishment of the ASCE San Francisco code committee that later became part of the State Chamber of Commerce Code Committee.
Santa Barbara earthquake was the beginning of the earthquake studies in the United States. In 1925 the United States Congress gave the United States Coast and Geodetic Survey the responsibility to investigate and provide reports on seismology which had a significant effect on the seismic code development. As a result the 1927 Uniform Building Code and the Palo Alto Code were the first two codes in the United States to incorporate seismic provisions which were based on the Newtonian concept “mass times acceleration.” The coefficient of acceleration varies depending on the soil bearing capacity but it is approximately 10% of the dead load.
In 1928 the State Chamber of Commerce realized the need for a building code that provides guidelines to design buildings to resist earthquakes and prevent collapse and loss of human lives. Significant amount of work and studies performed by structural engineers across the state of California resulted in a report that formed the foundation of the codes that followed.
The first Mandatory seismic codes used in the United States were published in 1933 following the March 10, 1933 Long Beach earthquake when two California State Laws were passed due to the extensive damage occurred in school buildings.
In this code, masonry buildings were required to be designed for a lateral load equals to 10% dead load plus portion of the live load. Other types of building were required to be designed for 2% to 5% dead load plus portion of live load.
In 1937 the coefficient were revised to 6% to 10% for buildings 3 stories or less in height, or buildings without a moment resistant frame. Buildings with height more than 3 stories with a complete moment resisting frame had coefficients of 2% to 6%, provided the frame could resist 2% of the load. In 1941, the coefficients were to 6% to 10%, depending upon the type of foundation materials.