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Jul 03, 2023

City Council to Vote on Structural Retrofit Approach For Central Library

The City Council is scheduled to vote on the structural retrofit approach to be used for the Central Library. Designed by Myron Hunt in 1924, the Pasadena Central Library, which is a designated

The City Council is scheduled to vote on the structural retrofit approach to be used for the Central Library.

Designed by Myron Hunt in 1924, the Pasadena Central Library, which is a designated Pasadena historic resource, closed in 2021 due to seismic safety issues.

Over the past six months, the design team has vetted multiple retrofit alternatives with the Technical Oversight Committee and the City’s structural peer review advisory team comprised of Nabih Youssef and Associates (NYA) and Krakower & Associates.

According to a staff report contained in Monday’s Council Agenda, the number of alternatives has been narrowed to three based on the evaluation criteria set forth by the project’s established goals and objectives.

The approaches vary in expected earthquake performance ranging from meeting minimum code requirements for historic buildings to exceeding code requirements for new buildings.

The costs for the approaches also vary with Baseline and Concrete Shear Walls estimated to have similar costs while Base Isolation is estimated to be much more expensive.

Based on all the factors considered by the design and project team, staff recommends that City Council select the Concrete Shear Wall earthquake retrofit approach and direct staff to proceed with detailed design of the project.

The three alternatives include:Baseline Approach: Characteristics removes limited amounts of inner brick “wythe” (a vertical section of masonry wall comprised of several layers of brick) and replaces it with concrete columns to support remaining brick and support floors and roof; Permanent formwork to be installed and remain in place to support the backing of the columns.

This approach does not meet the basis for design (BOD) for new building structural performance in the event of an earthquake; and Minimum retrofit needed to meet Pasadena Unreinforced Masonry (URM) Ordinance and California Historic Building Code.

Although this approach costs the least and it would provide a shorter construction duration, this approach comes with the lowest structural performance level of the three approaches resulting in potential for increased damage during moderate and major earthquakes; and constructability concerns for forming columns in relationship to the demolition of the URM walls.

Concrete Shear Wall Approach. Under this approach, the entire inner-brick “wythe” would be removed and replaced with a concrete shear wall which supports the outer “wythe” of brick and also acts as support for the floors and roof; Permanent formwork to be installed and remain in place to support the backing of the shear walls; and Meets BOD for new building structural performance in the event of an earthquake.

The approach is the lowest cost approach that meets BOD, and a shorter construction duration. It is also the most common and proven engineering and construction approach.

City staff is recommending the City Council adopt this approach.

This approach contains greater potential for damage and longer recovery time after major earthquakes compared to Base Isolation Approach III.

The Base Isolation Approach utilizes base isolators under the ground floor around the perimeter and at every column to reduce the effects of an earthquake; removes limited amounts of existing inner-brick “wythe” and replaces it with concrete columns to support remaining brick and support floors and roof; Permanent formwork to be installed and remain in place to support the backing of the columns; and Exceeds BOD for new building structural performance in the event of an earthquake. This approach would have the shortest recovery time after major earthquake compared to Baseline and Concrete Shear Wall approaches; reduces magnitude of ground motion impacts and, therefore, reduces potential for damage in moderate and major earthquakes.

But it costs the most and would take the longest to design and construct, delaying project completion up to a year; reduces amount of basement’s usable square footage compared to the Baseline and Concrete Shear Wall approaches; and constructability concerns for forming columns in relationship to the demolition of the URM walls.

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