SEISMIC STRENGTHENING AND RETROFITTING
Needs/Purpose of Retrofitting
•To resist a minor level of earthquake ground motion without damage;
•To resist a moderate level of earthquake ground motion without structural damage, but possibly experience some non-structural
• To resist a major level of earthquake ground motion having an intensity equal to the strongest either experienced or forecast for the building site, without collapse, but possibly with some structural as well as non-structural damage.
- The need to improve the ability of an existing structure to withstand seismic forces arises usually from the evidence of damage and poor behaviour during recent earthquakes. To restore the damaged structures after an earthquake
What to retrofit?
It is required to retrofit the structures, damaged due to earthquake or strengthen the existing structures. Cost-benefit analysis.
Seismic Strengthening Considerations
- Economic value of the structure and remaining years of service life.
- Occupancy of the structure including the number of persons at risk within the structure.
- The potential for structural failure to result in release of hazardous substances and injuries outside the structure.
- Function of the structure and the economic or societal cost which would result from loss of service due to earthquake induced damage.
- Historic significance of the structure and the effects of seismic upgrades on the cultural resource.
- The site-specific seismic hazard.
The relative cost of achieving upgrades to various criteria.
When to retrofit?
- Just after a damaging earthquake, temporary supports & emergency repairs are to be carried out so that precariously standing buildings may not collapse during aftershocks & the less damaged ones could be quickly brought back into use.
- Once things settle down after the earthquake, distinction is to be made in the type of action required i.e. repairs, restoration & strengthening, since the cost, time & skill required in the three may be quite different.
- An investigation of existing conditions is intended to determine the state of the building’s health, to establish a diagnosis and to arrive at a suitable remedial measure.
Steps of Retrofitting
Steps necessary to arrive at an appropriate retrofitting measure are:
a. Reviewing existing construction documents
b. Field Investigations
c. Probing and exploratory demolition
d. Testing of materials
e. Analysis of existing framing
f. Making an evaluation
g. Preparing a report of condition assessment
Material testing methods that involve removal & destruction of a portion of the member to determine its properties are called destructive testing.
Non-destructive testing does not alter the members’ properties or affect the service of the structure.
Retrofitting of Rubble Masonry Buildings
- Stone is a very common construction material in hilly & rocky areas. Most of the buildings in stone have rubble masonry with mud or cement-sand mortar. The stone walls are usually made in two wythes giving smooth finish along the two faces of wall and the space, in between, is filled with smaller stone pieces. If the two wythes are not interconnected by sufficient number of ‘through stones’ these split during earthquake shaking and result in collapse of wall.
- To avoid this splitting, sufficient number of ‘Headers’ or through elements are to be added in existing rubble masonry walls. These Header elements can be of stones, RC or wood.The easiest option is to provide RC elements. For this purpose, a hole in the wall is to be made at selected location by gently removing the stones from the two sides of the wall. Care has to be taken in removing the stones, so that the wall is not damaged. The space created by removal of stones, is filled with concrete and a steel rod bent at two ends.
- In addition to the above strengthening of individual walls, unsupported length , height of walls & size and placing of openings is also to be controlled as per codal requirements. The integral box action of the building is to be ensured by providing seismic belts at lintel & roof level, vertical reinforcement at corners & junctions is to be provided.
Retrofitting of Buildings Made of Rectangular Masonry Units
Retrofitting of these buildings includes strengthening of
•individual walls and
•ensuring of integral box action.
Strengthening of Walls
- Individual walls in a building are to be checked for strength, quality of construction, unsupported length & height, size and placing of openings. In ordinary buildings, the strength & quality of construction can be judged by visual inspection of masonry units and mortar. If the walls have cracks, voids, loose pockets or degradation of mortar, these need to strengthen by grouting. Grouting can be done using shrinkage compensated cement slurry or polymer grouts or epoxy. Polymer/epoxy grouting is costly and is normally not used in masonry. Shrinkage compensated cement slurry grouting is considered satisfactory in ordinary masonry buildings. Larger voids can be directly filled by shrinkage compensated cement-sand mortar or polymer modified mortar.
- Masonry walls can also be strengthened by epoxy glueing the FRP strips .
- Another method of strengthening masonry walls has been employed in some historical buildings. In this method, vertical cores are drilled in the thick masonry walls and filled with reinforced concrete. The vertical RC members so created, act as columns and support the masonry wall, without affecting
its external appearance.
The bearing area of the strip footing is increased by providing RC beams on both sides of the wall.These beams are interconnected at several locations through gaps created in the wall. This results in effective transfer of load from the wall to the added RC beams.
Strengthening of Foundations
- In addition to this, the drainage condition around the building is to be improved to avoid saturation of soil. A concrete apron around the building is helpful in avoiding direct soaking of the soil in the vicinity of the foundation.
RETROFITTING OF ROOFS AND FLOORS
- Pitched roofs are the most vulnerable during earthquakes. Usually these roofs are cladded with heavy, brittle & loose stone pieces or tiles. These tiles fall during shaking & cause injury. These should be replaced by lightweight corrugated GI sheets, properly connected tiles. Sloping roofs have tendency to open up during earthquakes & relative motion between eaves & crown, which results in loosening & falling of cladding.
- For seismic safety, the roof should move as a whole & there should not be any relative motion of different members. This can be achieved by proper bracing in horizontal, vertical & inclined plane.
- The roofs also need to be connected properly with the walls. Anchor bolts are to be grouted into roof band/wall to hold the rafters. In case of seismic belts, the bolts may be anchored to the roof belt.