D ESIGN S PECIFICATIONS FOR B RIDGES

Design of seismic isolations systems for bridges often follow the AASHTO Guide Specifications, published by the American Association of State Highway and Transportation Officials. The initial specifications were published in 1991, with a major revision in 1999.

These bridge design specifications have in some ways followed the evolution of the UBC code revisions. The original 1991 edition was relatively straightforward and simple to apply but the 1999 revision added layers of complexity. Additionally, the 1999 revision changed the calculations of the seismic limit state to severely restrict the use of elastomeric type isolators under high seismic demands.

4.2.1 THE 1991 AASHTO GUIDE SPECIFICATIONS

The 1991 AASHTO seismic isolation provisions permitted isolated structures to be designed for the same ductility factors (as implemented through the R factor) as for non-isolated bridges. This differed from buildings where the UBC at this time recommended an R value for isolated structures of one-half the value for non-isolated structures. However, AASHTO recommended a value of R = 1.5 for essentially elastic response as a damage avoidance design strategy.

AASHTO defined two response spectrum analysis procedures, the single-mode and multi-mode methods. The former was similar to a static procedure and the latter to a conventional response spectrum analysis. Time history analysis was permitted for all isolated bridges and required for systems without a self-centering capability (sliding systems).

Prototype tests were required for all isolation systems, following generally similar requirements to the UBC both for test procedures and system adequacy criteria.

In addition to the seismic design provisions, the 1991 AASHTO specifications provided additions to the existing AASHTO design provisions for Elastomeric Bearings when these types of bearing were used in implementing seismic isolation design. This section provided procedures for designing elastomeric bearings using a limiting strain criterion. As this code was the only source providing elastomeric design conditions for seismic isolation the formulations provided here were also used in design of this type of isolator for buildings (see Chapter 9 of these Guidelines).

4.2.2 THE 1999 AASHTO GUIDE SPECIFICATIONS

The 1999 revision to the AASHTO Guide Specifications implemented major changes. The main differences between the 1991 and 1999 Guide Specifications were:

• Limitations on R factors. The R factor was limited to one-half the value specified for non- isolated bridges but not less than 1.5. For bridges, this provided a narrow range of R from 1.5 to 2.5, implying very limited ductility.

• An additional analysis procedure, the Uniform Load Method. This is essentially a static load procedure that takes account of sub-structure flexibility.

• Guidelines are provided for analyzing bridges with added viscous damping devices.

• Design must account for lower and upper bounds on displacements, using multipliers to account for temperature, aging, wear contamination and scragging. These factors are device-specific and values are provided for sliding systems, low-damping rubber systems and high-damping rubber systems. In general the multipliers tend to have the greatest effect in increasing displacements in sliding systems. This is similar to UBC that requires a displacement multiplier of 3.0 for sliders. • More extensive testing requirements, including system characterization tests. There are

requirements for vertical load stability design and testing using multipliers that are a function of seismic zone.

• Additional design requirements for specific types of device such as elastomeric bearings and sliding bearings.

The 1999 AASHTO specifications introduce a number of new factors and equations but a commentary is provided and the procedures are straight-forward to apply. The HCG spreadsheet Bridge.xls incorporates the 1999 AASHTO provisions and performs analysis based on (1) the uniform load method and (2) the time history analysis method.

Figure 4-3 is an example of the Control sheet of the Bridge workbook. The procedure for a bridge isolation system design is as follows:

• Enter design information on the Design worksheet. Data includes bent and superstructure weights, bent types and dimensions and span lengths.

• Enter isolator data on the Control worksheet. This includes number and type of bearings per bent, plan size, layers etc. Use the detailed isolator assessment on the Isolators sheet to select plan size. The layers and lead core sizes are selected by trial and error.

• Activate the Solve Displacement macro from the button on the Control sheet. This solves for the isolation performance using the uniform load method.

• Activate the Nonlinear Analysis macro from the button on the Control sheet. This solves for the isolation performance using the time history method. This macro assembles longitudinal and transverse models and analyzes them for seven spectrum compatible acceleration records using a version of the DRAIN-2D program. This will run in a window. You need to wait until this is complete (20 to 30 seconds usually) and then activate the Import Results macro.

The results of each step are summarized on the Control sheet. The comparison between the analysis and design results should be checked. Usually the longitudinal analysis will produce results between 10% to 20% lower than the design procedure, which is a function of the more accurate damping model. The transverse analysis will often provide a different load distribution from the design procedure, especially if the deck if flexible. This is because the effects of torsion and deck flexibility are more accurately modeled in the time history type of analysis.

The Control sheet lists the status of each isolator is terms of the 1999 AASHTO equations at either OK or NG. The Isolators sheet provides detailed calculations for the seismic and non-seismic load combinations.

FIGURE 4-3 BRIDGE BEARING DESIGN PROCESS