Emerging concepts in structural design of tall buildings...

Emerging concepts in structural design of tall buildings:

Long Beach Civic Center

Presentation for the Los Angeles Tall Buildings Structural Design Council

May 6, 2016

Project Team

Structural Engineer of RecordM. Sarkisian, E. Long & A. Krebs

Structural Engineer of RecordN. Youssef, O. Hata & S. Stewart

Peer ReviewI. Almufi

Business Continuity ConsultantC. Haselton

Navigating Resiliency by Design

Presentation for the Los Angeles Tall Buildings Structural Design

Council

This presentation is an attempt to accomplish resilience through explicit performance targets and measureable parameters with a certain confidence level (50%).

While there is research being performed, data collected & software developed …

There isn’t yet a unified standard or streamlined process …

We have to navigate through the best available “ART”


Council

Project Overview

Civic Center Redevelopment


Council

Civic Center Overall Plan


Council

Podium

Council Chambers

Resilience Objectives


Council

Key discussion at project onset: what are the owner’s resilience objectives?

Educated the owner on code-expected performance (in terms of downtime – owners intuitively understand this as a measure of resilience)

Owner was not content with expected downtime of code buildings

Elected to pursue REDi Gold objectives and use of REDiresilience – based design framework for achieving the objectives

Resilience Requirements Embedded in RFP


Council

CONTINUITY OF CITY OPERATIONS

The Civic Center is not required to be an essential facility, but should be designed to a 50% confidence level that after a design earthquake occurring, the City should be able to:

(i) experience few or no injuries (ii) reoccupy the new facility within a week (iii) have full functionality within 30 days, and (iv) experience less than 5% financial loss (as compared to the replacement value, or achieve a REDi Gold certification or equivalent.

See http://www.arup.com/Publications/REDi_Rating_System.aspx for more information.

http://www.arup.com/Publications/REDi_Rating_System.aspx

REDi Rating System


Council

A framework to implement “resilience-based design” for achieving ‘beyond-code’ resilience objectives.

Performance of Code Based Design


Council

Performance of Code Based Design• Prescriptive• Provides Life Safety• Does not ensure re-occupancy or economical repair

Performance of nonstructural components not adequately addressed by current building code procedures - account for more than 70% of earthquake related economic losses.

National standards do not incorporate resiliency goals.

No established engineering design criteria for resiliency.

Seismic Performance Assessment of Buildings (FEMA P-58)


Council

Project performance goals similar to FEMA P-58 performance measures

• Casualties• Repair Cost • Repair Time

Use FEMA P-58 methodology:

• Assess building performance• Establish supplemental structural and

nonstructural design criteria

REDi Downtime Assessment Methodology


Council

FEMA P-58 provides estimates of repair time, but not downtime.

Limitations of FEMA P-58 repair time assessment:

• Unrealistic labor allocation and repair sequence logic.

• Repair time associated with full recovery, not re-occupancy and/or functionality.

• Does not account for delays that prevent initiation of repairs (impeding factors) and disruption of utilities.

The Seismic Performance Prediction Program (SP3)


Council

FEMA P-58 was released in 2012, but a large barrier to widespread adoption has been related to software and ease-of-use (high cost of entry).

The SP3 software was created with the specific goal of making the use of FEMA P-58 feasible at the rapid pace of an engineering office.

The SP3 Software: • Commercial platform and runs on the cloud • Runs analyses for FEMA P-58, REDi, and U.S. Resiliency Council Ratings • Streamlined package including embedded USGS soil/hazard, simplified structural

response estimates, pre-population algorithms for building contents, etc. (streamlined to meet the above goal of usability).

• Currently being used by many companies for new design, retrofit, due-diligence analysis, and insurance risk assessments.


Council

General Building Description

Typical Tower Floor Plan


Council

General Information

City Hall Total GSF: 270,800Port Headquarters Total GSF: 237,100

Typical Tower Floor GSF: 23,375 +/-Number of Tower Floors: 11

Typical Tower Framing Plan


Council

Structural System Elevation


Council

Structural System Overview


Council

Seismic Performance


Council

Positive Attributes of Core Shear Wall System

Robust system providing strength and stiffness.

Walls to be designed to remain essentially elastic, i.e. limit residual drift.

Limiting inter-story drift is beneficial to drift sensitive non-structural components (partitions and curtain walls).

However, limiting inter-story drift results in higher response floor accelerations – higher anchorage/bracing forces and acceleration demand equipment.


Council

Design Approach

Code-Designed Building Resiliency Assessment


Council

As mentioned previously, code-based design is:• Focused on Life Safety • Does not ensure re-occupancy or economical

repair

Using a simplified SP3 analysis for this building (for a DE level), we expect this from a code-based design:

• Safety: Expect good collapse safety, but may have some falling hazards.

• Economic Loss: 10-15% loss (walls are better than most building systems for office occupancies).

• Functional Recovery Time: 4-12 months, depending heavily on impeding factors and pre-EQ planning.

Contrast: The goals on this project are <=5% loss and <=7 days to reoccupancy (much better than code!)

Design Approach


Council

Performance Objectives

Target Criteria (EDP)- Inter-story Drift

- Story Acceleration- Wall Rotation- Residual Drift

Design- Displacement Based

- Structural Configuration

Analysis(NLRHA to validate structural response)

SP3/FEMA P-58 Evaluation(Validate Performance)

ETABS Analysis Model


Council

Uncoupled Wall System – Performance Assessment


Council

Feasibility of using coupling beams was assessed with displacement based design approach:

In order to satisfy re-occupancy requirements:

limit coupling beam rotation to be in DS0 => drift ratio ~ 0.3%-0.4%.

EDP E-W N-S

Max. Drift 1.3% 1.0%

Max. Wall Rotation 0.005rad 0.008rad

0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

Tim

e (d

ays)

Breakdown: Median Re-Occupancy

Assessment Re-Occupancy (days)

Functional Recovery (days)

Incl. MEP* 20 116

Excl. MEP 18 19

* Assume code design anchorage for MEPFunctional recovery governed by long lead time to replace equipment

Partitions, concrete walls and stairs governed downtime

Results from NLRHA

Performance Assessment Results

Description of Structural System


Council

Typical Tower Floor PlanLevels 4, 6, 8 & 10

Typical Tower Floor PlanLevels 5, 7, 9 & 11

Structural Performance Criteria


CouncilApril 27 2016

Concrete Shear Walls → plastic rotation complies with ASCE 41 Immediate Occupancy acceptance criteria << median rotation for DS1 (0.0084 radian)

Average maximum inter-story drift ratio < 1.0%

Average residual inter-story drift ratio < 0.5%

Nonstructural Component Performance Criteria


Council


Anchorage and bracing design based on averaged maximum story accelerations from analysis

Limit Rp (response modification factor) ≤ 2.0 for all mechanical and electrical components

Ip = 1.5 for selected equipment and components (critical and long-lead time):

• Cooling Generating Systems – Chillers, Cooling Towers, Pumps and Compressors;

• HVAC Distribution – AHU

• Electrical Service and Distribution – Control Panels and Switchgear (including low voltage)

Comparison of Code and project anchorage forces (flexible equipment)



Council


Require shake-table testing of:

• mission-critical equipment and components needed to maintain functionality; and

• long-lead time equipment and components

Equipment must remain operable after design level earthquake.



Council

Curtain Wall:• Service level (1%) – connections remain elastic, envelope remains effective in

preventing air and water intrusion.• Strength level (2%) – no catastrophic failure

Stairs: designed to accommodate 2% relative displacement.

Elevators and Lifts (one elevator per building servicing all floors):• Designed with Rp = 1.0• Guide rail support and retainer plates designed to CBC 2010 DSA requirements.

Mitigation of impeding factors to be coordinated.

Summary of Resilience-Based Design


Council

“True resilience” can only be achieved by:

• Targeting higher functionality objectives• Adopting enhanced design and planning measures• Verification of performance through loss assessment

Downtime assessment is useful…but it is not a substitute for good design:

• Component strain demands vs. global demands like drift• Systems and components fragilities are still under development• Methodology has not been fully validated

Big Project Conclusions and Take-Aways


Council

The City and Port of Long Beach got a much better performing building.

• The design objectives include explicit targets for repair cost and recovery time.

• The performance is checked by FEMA P-58/REDi/SP3 rather than prescriptive code.

Resilience-based design is feasible and beneficial

Emerging concepts in structural design of tall buildings...

Documents

Transcript of Emerging concepts in structural design of tall buildings...