Caltrans, quoted from


Past earthquakes in California have shown the vulnerability of some older structures, designed with non-ductile design standards to earthquake-induced forces and deformations.  Caltrans has embarked on an extensive seismic retrofit program to strengthen our inventory of bridges to insure satisfactory performance during anticipated future earthquakes.  As part of the effort to assure public safety during seismic events, Caltrans has funded an extensive research program as well as developed design procedures that have furthered the state of practice of earthquake bridge engineering.  The Seismic Design Criteria (SDC) are an encyclopedia of new and currently practiced seismic design and analysis methodologies for the design of new bridges in California.  The SDC adopts a performance-based approach specifying minimum levels of structural system performance, component performance, analysis, and design practices for ordinary standard bridges.  Bridges with non-standard features or operational requirements above and beyond the ordinary standard bridge may require a greater degree of attention than specified by the SDC.  Many of the methodologies contained in the SDC have evolved from the seismic retrofit program.  Some of the procedures are major departures from previous practice while others are slight modifications to current practice.  The most significant change in design philosophy for new bridges is a shift from a force-based assessment of seismic demand to a displacement-based assessment of demand and capacity.  The former force approach was based on generating design level earthquake demands by reducing ultimate elastic response spectra forces by a reduction factor.  The reduction factor was selected based on structure geometry, anticipated ductility, and acceptable risk.  The newly adopted displacement approach is based on comparing the elastic displacement demand to the inelastic displacement capacity of the primary structural components while insuring a minimum level of inelastic capacity at all potential plastic hinge locations. 


The SDC has been developed with input from the Caltrans Offices of Structure Design, Earthquake Engineering and Design Support, and Materials and Foundations.  Some refinement of the SDC concepts and procedures is anticipated as the criteria are applied to a wide spectrum of bridge projects. 



Memo 20-1 outlines the bridge category and classification, seismic performance criteria, seismic design philosophy and approach, seismic demands and capacities on structural components and seismic design practices that collectively make up Caltrans’ seismic design methodology. 

 How bridges respond during earthquakes is complex. Insights into bridge behavior and methods for improving their performance are constantly being developed. This continuous evolution requires that Caltrans periodically reviews and updates its seismic design methodology and criteria.  Designers need to be conscious of emerging technology and research results and are encouraged to bring new ideas to the attention of the Office of Structures Design (OSD) management for review and approval. The process for submitting design methodology revisions to OSD management is outlined in Memo to Designers 20-11. 

The Caltrans seismic design methodology applies to all highway bridges designed in California.   Bridges are categorized as either Important or Ordinary depending on the desired level of seismic performance. The Ordinary category is divided into two classifications Standard and Non-standard.  A bridge’s category and classification will determine its seismic performance level and which methods are used for estimating the seismic demands and structural capacities.  The seismic design criteria for Ordinary Standard bridges are contained in the Caltrans Seismic Design Criteria (SDC). The seismic design criteria for Important bridges and Ordinary  Non-standard bridges shall be developed by the project design team on a case-by-case basis, and approved by OSD management. The project specific criteria must establish the design parameters required to meet the level of performance outlined in Table 1. See Memo to Designers 20-11 for the project specific criteria approval process. An index to seismic related memos is contained in Attachment 1.

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 The latest version of the Seismic Design Citieria can be downloaded from the links below.
Seismic Design Criteria Version 1.1, July 1999 (PDF), [or here]
Seismic Design Criteria Memo To Designers  20-1 (PDF 208Kb), [or here]
Seismic Design Criteria MTD 20-1 Attachment 1 (PDF 10Kb), [or here]