ASCE Structural Engineering Conference November 10, 2014

Transcription

1 ASCE Structural Engineering Conference November 10, 2014 Seismic Retrofit of Steel Frame Buildings in Low Seismicity Zone Applications Jay Shen Ph.D. P.E., S.E. Iowa State University

2 Outline 1. Introduction 2. Seismic Hazard in Low Seismic Zone 3. Performance Objectives for Existing Steel Buildings 4. Evaluation and Retrofit of Steel Buildings in Low Seismic Zone A Case Study 2

3 Introduction: Current Practice National standards for retrofit (such as ASCE 41) were developed primarily from provisions for new buildings, and mainly for high seismic zone applications; Low seismic zone application has distinctive features in both seismic hazard and structural behavior that have not been properly reflected for cost-effective retrofit. Research and education in seismic retrofitting is inadequate in any seismic zones, and particularly in low seismic zone application. 3

4 Overview: Seismic design of a new building Lateral seismic force, V Elastic Strength, VE Anticipated lateral forcedisplacement relation to damages V Actual Strength, Vy Seismic Design force, Vs Design Displacement (design limit on linear deformation) Successive damages Collapse Roof displacement 4 (1) The design force, Vs, is only a fraction of expected strength, VE, if the structure remains elastic ; (2) The expected actual strength, Vy, depends on as built structures.

5 Overview: Seismic retrofit of an existing building V Expected lateral forcedisplacement relation of the retrofit building V Collapse Actual lateral force-displacement relation of an existing steel building in low seismic zone Roof displacement (1) Conduct seismic evaluation to reveal actual performance of the existing building (the actual lateral force displacement relation); (2) Retrofit the building if the actual performance is below the expected performance (the blue curve).

6 Features of Steel Buildings in Low Seismic Zone The buildings were designed for wind load as lateral force, and therefore not detailed to tolerate any yielding and buckling. Gravity frames have an inherently as-built lateralload resisting capacity that might be significant for the objective of collapse prevention, without any retrofit. 6

7 Seismic Retrofit in Low Seismic Zone Procedure 1. Determine Seismic hazard based on ASCE 7 & ASCE Define performance objectives of the existing building (POEB) 3. Evaluate seismic performance of the existing building YES Meet POED? NO 4. Conduct performance-based seismic retrofit so as to meet the performance objectives END

8 Seismic Hazard of Steel Building in Low Seismic Zones Spectral response acceleration parameter Sx1 for Basic Safety Earthquake -2 SD NE MN IA WI IL IN MI OH KS MO KY New Madrid Fault 1. In Very Low seismic zone: the steel frames that are properly designed for wind loads should remain primarily elastic with possible very minor, nonstructural damage. 2. In Low seismic zone:non-seismic steel frames might suffer significant damage to structural and non-structural components. OK AR MS TN

9 Lateral Strength of Existing Building - Example For an existing steel building in Low seismic zone, say SX1 = 0.15 (g): Example: The 6-story (78 ft in height and 150 ft x 150 ft in size) steel braced frame building was designed for 30 psf, 80 psf dead load, and 50 psf live load. The total wind load: VW = 350 kips. The total seismic load: VS = (SX1)W/RT, where T = period of the building (about sec. for a 6-story building), and R is in a range of 2 to 3, depending on the structural details. Let T = 1.0 sec., The total seismic load: VS = 540 kips (for R = 3) = 810 kips (for R = 2) In comparison of VS with VW, the building will most likely suffer heavy structural damage, and more rigorous evaluation would be needed to determine the levels of damage before any retrofit is proposed.

10 Seismic Hazard in Low Seismic Zones Seismic hazard for existing buildings is given in two levels (ASCE 41-13): BSE-1E: Basic Safety Earthquake-1, taken as a seismic hazard with 20% probability of exceedance in 50 years (20% PE/50); BSE-2E: Basic Safety Earthquake-2, taken as a seismic hazard with 5% probability of exceedance in 50 years (5%PE/50)

11 Performance Objectives of Existing Steel Buildings in Low Seismic Zone Performance Levels of Braced Steel Frame Buildings 1. Collapse Prevention (CP) 2. Life Safety (LS) 3. Fully Functional (FF) 11

12 Performance Objectives Existing Steel Buildings in Low Seismic Zone 1. Collapse Prevention (CP) - Near partial or total collapse; - Significant degradation in stiffness and strength in steel braced frames; - Gravity-load-carrying system remains stable; - The steel braced frame would have: 2% inter-story drift (transient or permanent); Extensive yielding and buckling of braces; Many braces and gusset plates may fail (complete fracture).

13 Performance Objectives Existing Steel Buildings in Low Seismic Zone 2. Life Safety (LS) - Structures suffer significant damage; - Some margin against partial or total collapse remains; - Overall risk of life-threatening injury due to structural or nonstructural damages is low. - The steel braced frame would have: 1.5% transient and 0.5% permanent inter-story drift. Many braces yield or buckle, but unlikely fail; Many connections may fracture (mainly due to brace buckling), but unlikely totally fail.

14 Performance Objectives Existing Steel Buildings in Low Seismic Zone 3. Fully Functional (FF) - Structure retains original strength and stiffness; - Minor yielding or buckling of some braces. - Minor cracking of facades, partitions, and ceilings. All systems for building operation remain fully functional with occational minor repairs. - The steel braced frames would have: 0.5% transient and negligible permanent inter-story drift. Some braces show minor yielding or buckling.

15 Basic Performance Objective for Existing Buildings (BPOE) All buildings are divided into Risk Category (RC): Building with RC IV Essential facilities (hospitals, fire stations, etc.); Building with RC III Contains a large of number of people (schools, etc.); Building with RC I or II Those that are not in RC III or RC IV (commercial, office, etc.)

16 Basic Performance Objective for Existing Buildings (BPOE) Risk Category (RC) I &II BSE-1E (20%PE/50) Life Safety (LS) BSE-2E (5%PE/50) Collapse Prevention (CP) III Fully Functional (FF) Life Safety (LS) IV Fully Functional (FF) Life Safety (LS)

17 Basic Performance Objective for Existing Buildings (BPOE) Higher performance RC I & II Increasing risk category RC III RC IV BSE 1E (20%) BSE 2E (5%)

18 Evaluation and Retrofit of Steel Buildings In Low Seismic Zone Case Study - Seismic evaluation and retrofit of a 9-story hospital building with non-seismic steel braced frames. The fundamental period of the structure, T = 2.0 sec. Location: Low-seismic zone in the Midwest. Seismic hazard at the building site: BSE-2E: Sxs = 0.20g; Sx1 = 0.15g; ST = 0.08 g BSE-1E: Sxs = 0.08g; Sx1 = 0.05g; ST = 0.025g Risk category: RC IV (This building is for hospital) Thus, the Basic Performance Objectives for this building are: (1) Fully Functional under BSE-1E; and (2) Life Safety under BSE-2E.

19 Evaluation and Retrofit of Steel Buildings In Low Seismic Zone ft 30 ft 30 ft 5 30 ft BF BF Building Plan (Concrete floor is not shown) A 30 ft B 30 ft 30 ft C D 30 ft E 30 ft F Typical Shear Tab Connection Lateral force Braced Frame (brittle failure Gravity Frame Roof Displacement Lateral Force Resisting Capacity Braced Frame (BF) (Line 1 & 5) Gravity Frame (Line B E)

20 Inelastic Modelling (Essential step leading to cost effective retrofit) F In Gravity Frame F

21 Inelastic Dynamic Analysis BSE-2E BSE-1E Response Spectra ground motions for low seismic zone

22 Evaluation Using Inelastic Dynamic Analysis Incremental inelastic dynamic analysis is able to identify damage levels subjected to earthquake grounds of BSE-1E and BSE-2E in a set of analytical simulations. Three damage levels were used to relate the analysis to performance objectives: Minor structural/minor non-structures Damage (MMD) Fully Functional Significant Damage (SD) Life Safety Near Collapse (NC) Collapse Prevention

23 Evaluation by Inelastic Dynamic Analysis SmCT,MMD is the mean spectral intensities to cause Minor yielding/minor Cracking Damage (MMD) SmCT,SD is the mean spectral intensities to cause Severe Damage (SD) BSE-2E ST = (g) BSE-1E ST = (g) Damage Levels Levels under Mean Spectral Intensities Sct (g) Sct (g) Inter-Story Drift Ratio

24 Evaluation by Inelastic Dynamic Analysis 1. Fully-Functional performance under BSE-1E is adequate since SmCT, MMD = 0.052g > ST = 0.025g of BSE-1E. No retrofit is needed for this objective. 2. Life Safety performance under BSE-2E is somewhat inadequate since SmCT, SD = 0.068g < ST = 0.08g of BSE-2E Many braces buckled severely to cause concerns about the margin against partial or total collapse for life safety. Retrofit is needed for Life Safety performance objective.

25 Proposed Retrofit for Life Safety Objective The brace buckling caused non-seismic brace to fracture, and led to many fractured connections. It is costly to replace braces and their connections. Cost-effective Approach: Use of built-up section (with channels, plates) to control buckling of existing braces Simple process to be fabricated on the building site with minimal interruption to the building function..

26 Proposed Retrofit for Life Safety Objective Cost-effective Approach: Inner tube (existing brace) Protective outer tube (a built-up section for existing braces

27 Proposed Retrofit for Life Safety Objective Gap for the existing brace to have controlled buckling to accommodate existing frame Inner tube (existing brace) Bucklingcontrolled brace (BCB) Brace in new frame Protective outer tube (a built-up section for existing braces Existing brace

28 Proposed Retrofit for Life Safety Objective

29 Proposed Retrofit for Life Safety Objective Conventional Brace (existing) BCB Outer tube Bulking leads to fracture of brace and its connection. Inner tube (existing brace) Brace (inner tube) buckling is controlled by the outside tube. Outer tube Inner tube

30 Proposed Retrofit for Life Safety Objective BCBF Conventional CBF CCBF BCBF 050 BCBF 075 BCBF 100 Lateral load versus displacement Curves

31 Summary of Retrofit of Steel Buildings No retrofit needed Some minor brace buckling BCB is capable of preventing buckling induced failure with low cost for Life Safety performance objective; and may results in higher performance level, Fully Functional without additional cost. Fully Functional Life Safety Fully Functional BCB can also be a costeffective system for new buildings in high-risk seismic region. LS/CP CP High seismic zone Very low seismic hazard

32 Acknowledgement Supports from American Institute of Steel Construction (AISC) and Steel Fabricators in the Midwest made the study possible Graduate students: Narathip S., W. Rou, and O. Seker. Faculty members Dr. Fanous and Dr. Rouse.