Rehabilitation Of Rc Concrete Buildings

Learning from Earthquakes to Improve Rehabilitation of Reinforced Concrete Buildings James O. Jirsa The 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 concrete construction • Waffle slab systems • Column failures • Reinforcement details • Infill walls

Experience Reconnaissance studies Codes & Standards

Documentation

Design guidelines

Analytical studies Experimental studies

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

Mode of Failure Observed

% of cases

Shear, compression, or other failure of columns

43

Shear in beams

9

Shear in waffle slabs

9

Flexure in beams

2

Beam-column joints

8

Shear walls, shear or bending

1.5

Other modes

7

Could not be identified

25

Damage statistics 120 Severe Collapse

100 80 No. of 60 Buildings 40 20 0 RC Frames

Steel Frames

Waffle Slabs

Bldgs Masonry w/Shear walls

Damage/height of buildings 120 Severe Collapse

100 80 No. of 60 Buildings 40 20 0 <5

6 to 10

11 to 15

No. of Stories

>15

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 BUILDING PLAN

Stairway

Stairway A

C2 C4

C7

C7

C6

C6

C6

C6

C6

C6

C6

C6 C7

C7

8.00 B 3.75

C9

C3

C8

C8

C5

C5

C5

C5

C5

C5

C5

C5

C5 C5

C4 C2

C3

C1 30 for h=100 cm. 25 for h=85 cm.

1 2

3

(meters)

4

5

6

7

8

9

10

11

12

13

14

15 16

ELEVATION LINE A TYPE 1

TYPE 2

TYPE 4

TYPE 5

TYPE 3

13 @ 7.20 m. 3.50

3.50 ELEVATION LINE B

MAIN BARS ADDITIONAL #4 BARS

TYPE 6

Cable braces

NEW STEEL BEAMS

CABLE BRACING

NEW STEEL BEAMS

Details EXISTING COLUMN CABLE

CABLE

A

A

′

WAFFLE SLAB (SOLID ZONE AROUND COLUMN) SLAB ZONE TO BE DEMOLISHED

DETAIL A EXISTING COLUMN REINFORCEMENT

INTERIOR CABLES

CABLE

ANCHOR MECHANISM

NEW CONCRETE

fc′ = 5000 psi STEEL PLATE 6-IN. X 6-IN. X 12-IN.

DETAIL A EXTERIOR CABLES

SECTION A-A

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