rehabilitation of rc concrete buildings

Learning from Earthquakes

to ImproveRehabilitation of Reinforced

Concrete Buildings

James O. JirsaThe University of Texas at Austin

Objectives of NATO SfP977231

• Seismic evaluation and retrofitting of

existing buildings in Turkey and

Greece.

• Transfer, adapt, and implement

and/or develop innovative

technologies and methodologies for

both countries.

Requirements for rehabilitation to

be implemented

• Demand/need for rehabilitation

• Availability of techniques that are

–Constructible

–Cost-effective

–Convincing

Buildings in Turkey

Buildings in Mexico City

Approaches for reaching objective

• Reconnaissance studies after recent earthquakes– Little documentation of performance of

rehabilitated buildings

• Field experience—applicable to typical buildings in region– Mexico City after 1984

• Experimental studies– NATO Project

– Other reports in this Workshop

• Demonstration projects

Widespread

damage to

concreteconstruction

• Waffle slab systems

• Column failures

• Reinforcement details

• Infill walls

Reconnaissance studies

Documentation

Analytical studies

Experimental studies

Designguidelines

Codes & Standards

Experience

Shortcut

Demonstration projects

• New approaches must be “sold” to potential users

• Implementation depends on the user’s perception of technique

• Seeing is “believing”

• Field application and demonstration projects may be most convincing

• Education of owners and engineers

Mexico City after 1985

• Characteristics of damage– Lake bed zone—foundation limitations

– Construction types

• Rehabilitation activities– Affected zone was a “laboratory” for rehab

• Documentation– NSF/CONACyT Workshop

• Case study– Cable-bracing techniques

25Could not be identified

7Other modes

1.5Shear walls, shear or bending

8Beam-column joints

2Flexure in beams

9Shear in waffle slabs

9Shear in beams

43Shear, compression, or other failure

of columns

% of casesMode of Failure Observed

Damage statistics

0

20

40

60

80

100

120

No. of

Buildings

RC

Frames

Steel

Frames

Waffle

Slabs

Bldgs

w/Shear

walls

Masonry

Severe

Collapse

Damage/height of buildings

0

20

40

60

80

100

120

No. of

Buildings

<5 6 to 10 11 to 15 >15

No. of Stories

Severe

Collapse

Following the earthquake

• Owners and occupants were concerned about potential hazards in future events

• Buildings needed by users

• Repair and strengthening proceeded with very few design guidelines or standards for construction in place

• Engineers met challenge with creative solutions

Rehab prior to 1985

• Some buildings repaired following

earthquakes in 1957 and 1979

• Almost no information available about

those buildings

• Exception--Two buildings strengthened

before 1985 performed well and were

extensively studied after the earthquake

Building braced

pre-1985

Foundation

effects

1989 Workshop

• Site visits to buildings under rehab

• Discussions with engineers in charge

of rehabilitation design and

construction

• Review of approval process for rehab

projects

• Participants defined the need to

document rehab work

Beam and column jacketing

New bracing

systems

New walls

Mixed systems

Removal of top stories

Rehabilitation of Existing Reinforced Concrete Buildings in Mexico City: Case Studies

Case study: Layout of building

Stairway

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

A

B

8.00

3.75

Stairway

C1

C2

C3

C4

C5

C6C7

C8C9 C8

C7 C6 C6 C6 C6 C6 C6 C6

C5 C5 C5 C5 C5 C5 C5 C5 C5

C7 C7C2

C3

C4

3.50

ELEVATION LINE B

3.50

13 @ 7.20 m.

(meters) ELEVATION LINE A

BUILDING PLAN

TYPE 4 TYPE 5 TYPE 6

30 for h=100 cm.

TYPE 2TYPE 1 TYPE 3

25 for h=85 cm.

MAIN BARS

ADDITIONAL #4 BARS

Cable braces

NEW STEEL BEAMSNEW STEEL BEAMS CABLE BRACING

Details

EXTERIOR CABLES

NEW CONCRETE

STEEL PLATE6-IN. X 6-IN. X 12-IN.

ANCHOR MECHANISMCABLE

DETAIL A

SLAB ZONE TO BEDEMOLISHED

EXISTING COLUMNREINFORCEMENT

INTERIOR CABLES

CABLE

EXISTING COLUMN

DETAIL A

WAFFLE SLAB(SOLID ZONE AROUNDCOLUMN)

CABLE

A A

SECTION A-A

′

′ =fc psi5000

Additional

Modifications

Column Compression

Continuity of horizontal elements

Computed response

Cable

bracing for

2-story

school

Anchorage and cable details

Cable bracing for 12-story steel frame

Concluding remarks

• Future actions to improve “learning from earthquakes”–Documentation of rehabilitation

projects for evaluation of performance in future earthquakes.

–Instrumentation of buildings to enable more detailed evaluation of performance.

• Challenges–Focus efforts on areas where need is

greatest• Marginal residential construction• Determination and enforcement of

minimum requirements

–Maintain and create interest in earthquake mitigation• Competition with other political and

social exigencies • No well-defined industry to pressure

policy makers