Seismic Retrofit of Steel Frame Buildings in Low Seismicity Zone Applications

Jay ShenPh.D. P.E., S.E.

Iowa State University

ASCE Structural Engineering Conference November 10, 2014

Outline

1. Introduction2. Seismic Hazard in Low Seismic Zone3. Performance Objectives for Existing

Steel Buildings4. Evaluation and Retrofit of Steel

Buildings in Low Seismic Zone –A Case Study

2

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

Introduction: Current Practice

4

Lateral seismic force, V

V

CollapseSeismic Design force, Vs

to damages

Actual Strength, Vy

Roof displacement

“Elastic Strength”, VE

Design Displacement (design limit on linear deformation)

Successive damages

Anticipatedlateral force-displacement relation

(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.

Overview: Seismic design of a new building

(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).

Overview: Seismic retrofit of an existing building

V

V

Actual lateral force-displacement relation of an existing steel building in low seismic zone

Expected lateral force-displacement relation of the retrofit building

Collapse

Roof displacement

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 lateral-load resisting capacity that might be significant for the objective of “collapse prevention”, without any retrofit.

6

Features of Steel Buildings in Low Seismic Zone

Seismic Retrofit in Low Seismic Zone – Procedure

1. Determine Seismic hazard based on

ASCE 7 & ASCE 41

2. Define performance objectives of the existing

building (POEB)

3. Evaluate seismic performance of the existing building

4. Conduct performance-based seismic retrofit so as to meet the performance objectives

END

Meet POED?YES NO

Seismic Hazard of Steel Building in Low Seismic Zones

1. In “Very Low” seismic zone： the steel frames that are properly designed for wind loads should remain primarily elastic with possible very minor, non-structural damage.

2. In “Low” seismic zone：non-seismic steel frames might suffer significant damage to structural and non-structural components.

New Madrid Fault

IA

WI

IL

MO

IN

KY

TNAR

MS

OH

KS

OK

MI

NE

SD MNSpectral response acceleration parameter Sx1for Basic Safety Earthquake -2

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 0.8 – 1.2 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)

Lateral Strength of Existing Building - Example

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.

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)

Seismic Hazard in Low Seismic Zones

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)

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).

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 non-

structural 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.

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 occationalminor repairs.

- The steel braced frames would have:

0.5% transient and negligible permanent inter-story drift.

Some braces show minor yielding or buckling.

Performance Objectives Existing Steel Buildings in Low Seismic Zone

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.)

RiskCategory

(RC)

BSE-1E(20%PE/50)

BSE-2E(5%PE/50)

I &IILife Safety

(LS)Collapse

Prevention (CP)

IIIFully Functional

(FF)Life Safety

(LS)

IVFully Functional

(FF)Life Safety

(LS)

Basic Performance Objective for Existing Buildings (BPOE)

Basic Performance Objective for Existing Buildings (BPOE)

Increasing risk category

BSE‐1E (20%)  

Higher performance 

BSE‐2E  (5%) RC I & II

RC IV

RC III

Evaluation and Retrofit of Steel BuildingsIn 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.

Evaluation and Retrofit of Steel BuildingsIn Low Seismic Zone

Building Plan(Concrete floor is not 

shown)

Braced Frame (BF) (Line 1 & 5) 

Typical Shear Tab Connection

Lateral Force Resisting Capacity

Roof Displacement

Lateral force

30 ft

30 ft5

A

B

C

D

E

F

30 ft 30 ft 30 ft2 3 41

BFBF

30 ft

30 ft

30 ft

30 ft

Gravity Frame (Line B‐E) 

Braced Frame(brittle failure

Gravity Frame

Inelastic Modelling(Essential step leading to cost effective retrofit)

In Gravity Frame F

F

Inelastic Dynamic Analysis

Response Spectra ground motions for low seismic zone

BSE-2EBSE-1E

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

Evaluation by Inelastic Dynamic Analysis

Damage Levels Levels under Mean Spectral Intensities

Inter-Story Drift Ratio Sc

t(g)

Sct(

g)

BSE-1E ST = 0.025 (g)

BSE-2E ST = 0.080 (g)

SmCT,MMD is the meanspectral intensities to cause“Minor yielding/Minor CrackingDamage” (MMD)SmCT,SD is the mean spectralintensities to cause “SevereDamage” (SD)

Evaluation by Inelastic Dynamic Analysis

1. Fully-Functional performance under BSE-1E isadequate 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 aboutthe margin against partial or total collapse for life safety.

Retrofit is needed for Life Safety performance objective.

Proposed Retrofit for Life Safety Objective

The brace buckling caused non-seismic brace tofracture, 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..

Proposed Retrofit for Life Safety Objective

Cost-effective Approach:Inner tube (existing brace)

Protective outer tube (a built-up section for existing braces

Proposed Retrofit for Life Safety ObjectiveGap for the existing brace to have controlled buckling to accommodate existing frame

Inner tube (existing brace)

Protective outer tube (a built-up section for existing braces

Brace in new frame

Existingbrace

Buckling-controlled

brace (BCB)

Proposed Retrofit for Life Safety Objective

Proposed Retrofit for Life Safety Objective

Bulking leads to fracture of brace and its connection.

Brace (inner tube) buckling is controlled by the outside tube.

Inner tubeOuter tube

Outer tube

Inner tube(existing brace)

Conventional Brace(existing)

BCB

Proposed Retrofit for Life Safety Objective

CCBFBCBF – 050 BCBF – 075 BCBF – 100

BCBF

ConventionalCBF

Lateral load versus displacement Curves

Summary of Retrofit of Steel Buildings

Highseismic zone

LS/CP

Fully Functional

• No retrofit needed• Some minor brace buckling

Very low seismic hazard

Life Safety

CP

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.

• BCB can also be a cost-effective system for new buildings in high-risk seismic region.

Fully Functional

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.