Repair and Strengthening Methods for Existing Masonry Buildings

6.1  Basic Concept of Repair, Rehabilitation and Retrofitting

6.1.1  The three terms

6.1.2  Principles of seismic safety of masonry buildings

Every retrofitting intervention is aimed at restoring one of these five things — state them before you list any technique:

1. Integral box action — the whole building must respond as one closed box, not as a set of independent walls.
2. Integrity of the various components.
3. Roof-to-wall connection.
4. Wall-to-wall connection at the corners and junctions.
5. Wall-to-foundation connection.
6. Limit on openings — size, number and position.

6.1.3  Why retrofitting is needed

A building may require retrofitting for any of the following reasons:

1. Codal requirement — to meet the prescribed requirements of NBC 105 and other relevant codes, which have been revised since the building was built.
2. Change in use — for example converting a residential building into a school, which raises the importance factor I.
3. Functional change — a large conference room is wanted, or additional floors are to be added that were not in the original design.
4. Upgrading the performance of the building generally.
5. After damage — the building has been shaken and must be made safe against the next event.

6.1.4  Steps in the retrofitting strategy

1. Realisation of the retrofit requirement. Establish why the building is to be retrofitted (the five reasons in §6.1.3). This governs everything that follows.
2. Selection of the engineer / architect. Retrofitting needs competence beyond ordinary structural practice, so a competitive selection process should be followed; otherwise consult the municipality.
3. Notification to the municipality or concerned authority. The owner gives preliminary notice stating the purpose and obtains a go-ahead.
4. Establish the performance objectives. In consultation with the owner and the relevant code, fix the target — typically: no damage under a minor (frequent) earthquake, no significant structural damage under the design basis earthquake (DBE), and no collapse under the maximum considered earthquake (MCE).
5. Review the existing condition of the building. Age, design code (if any), present condition, change of use, neighbouring buildings and facilities, availability of drawings, geotechnical report, and the intended future use.
6. Carry out the vulnerability evaluation — from preliminary visual screening through to detailed evaluation and material testing (§6.2). This identifies the deficiencies.
7. Preliminary design with cost estimate, and a feasibility decision. Decide whether to retrofit at all, or to demolish and rebuild. Economics alone is not the only criterion — a heritage building may be retrofitted even when the economics do not justify it.
8. Detailed design according to the retrofitting guideline appropriate to that building type.
9. Prepare construction documents and a quality assurance plan, including the construction sequence. These form the basis for selecting the contractor.
10. Submit to the authority for approval; the authority may send the design for peer review.
11. Monitor construction and implement quality control. Any change dictated by ground conditions must be made by the engineer and approved by the authority before proceeding.
12. Post-retrofit evaluation to confirm the adequacy and effectiveness of the intervention.

6.1.5  Three generic categories of intervention

For earthquake-damaged concrete and masonry wall buildings the whole field reduces to three categories:

6.2  Vulnerability Assessment and Methods

Assessment is done twice: before the retrofitting, to identify the deficiency; and after the retrofitting, to estimate the adequacy and effectiveness of what was done.

6.2.1  What a seismic evaluation considers

• Earthquake intensity expected at the site
• Structural configuration and components
• Structural condition (existing cracks, decay, past interventions)
• Geological condition
• Foundation capability
• Non-structural components

6.2.2  The three phases of seismic evaluation

6.2.3  Common earthquake vulnerability assessment tools

The 11 GNDT parameters — worth listing in full

6.2.4  Detailed investigation — NDT and partially destructive testing

Once weak zones have been identified, the in-situ quality of the material must be measured.

6.3  Retrofitting Materials

6.4  Retrofitting Techniques Based on Failure Mechanism

6.4.1  The two strategies

There is also a third, managerial, layer of strategies that is not structural at all but is often the right answer: occupancy change, demolition, temporary retrofit, phased retrofit, retrofit during occupancy vs retrofit of a vacant building, and interior vs exterior retrofit.

6.4.2  Classification of retrofitting techniques

6.4.3  Strengthening existing walls

The lateral strength of a building can be improved by increasing the strength and stiffness of the individual walls, whether they are cracked or uncracked. There are four methods:

(A) Grouting

1. A number of holes are drilled in the wall — roughly one hole per 2 to 4 m².
2. Water is injected first, to wash the inside of the wall and improve the cohesion between the grout and the wall elements.
3. A cement–water mixture (1:1) is then grouted at low pressure (0.1–0.25 MPa), starting from the lowest holes and working upwards. Polymeric mortars may be used instead.

The increase in shear strength achieved this way is considerable. However, grouting cannot be relied on to improve the connection between orthogonal walls — that needs a different technique. The pressure can be obtained by gravity flow from a super-elevated tank.

(B) Wire-mesh jacketing (ferrocement / RC covering on both faces)

(C) Connection between existing stone walls — “sewing”

In stone buildings of historic importance, consisting of fully dressed stone masonry in good mortar, effective sewing of perpendicular walls can be done by drilling inclined holes through them, inserting steel rods and injecting cement grout. This restores the wall-to-wall connection without disturbing the appearance — important for heritage structures.

(D) Pre-stressing the walls

• A horizontal compression state induced by horizontal tendons increases the shear strength of the walls (recall τ_per = 0.1 + f_d/6 — precompression directly buys shear capacity).
• It also considerably improves the connection of orthogonal walls.
• The easiest way is to place two steel rods on the two sides of the wall and tension them with turnbuckles, against anchor plates.
• Only a slight pre-stress is needed — about 0.1 MPa on the vertical section of the wall gives good effects.
• Pre-stressing is also useful to strengthen the spandrel between two rows of openings where no rigid slab exists.

6.4.4  Restoring integral box action

Splint and bandage — the Nepali standard

This is the single most widely used masonry retrofitting technique in Nepal. The wall is stripped of plaster, wire mesh strips are nailed on in the pattern of vertical splints and horizontal bandages, they are overlapped and tied at every junction, and the whole is rendered in cement mortar. It reproduces, on an existing building, exactly the bands and vertical bars that IS 4326 requires in a new one.

Horizontal and vertical seismic belts

Containment reinforcement

Connection of floor with wall, and crack stitching

Buttresses, jacketing and foundation strengthening

Other techniques covered in the course: jacketing of brick masonry posts and columns (an added RC section with new longitudinal bars and new tie bars around the existing section); RC jacketing of beams and beam–column joints with added stirrups, added reinforcement and welded connecting bars; strengthening of foundations; and vertical bars at inside corners.

6.4.5  Technique keyed to the failure mechanism — the table that answers the exam

6.5–6.6  Existing Retrofitting Practice in Nepal

6.5.1  The scale of the problem

• About 500,000 houses were damaged in the 2015 Gorkha earthquake.
• 85–90% of them were masonry houses — stone masonry, block masonry and brick masonry.
• Masonry houses are very vulnerable to earthquakes, and the great majority of Nepal's building stock is non-engineered masonry built without any code.
• The practical question is therefore “where do we start?” — and the honest answer is that there is no standardised procedure and no single code; the treatment differs from structure to structure.

6.5.2  What is actually done in Nepal

6.5.3  The institutional framework

1. NBC 105: 2020 (seismic design), NBC 202 (mandatory rules of thumb for load-bearing masonry) and NBC 203 (low-strength masonry) set the target that a retrofitted building must reach.
2. IS 4326–1993 (earthquake-resistant design and construction) and IS 13935 (repair and seismic strengthening of buildings) supply the detailing.
3. The DUDBC retrofitting design guidelines and catalogues, prepared after 2015, give building-type-specific solutions.
4. NSET and UN-HABITAT programmes have driven the school-retrofitting and public-awareness efforts, and produced the vulnerability assessment toolkits used in practice.
5. Retrofitting design must be submitted to the municipality, which may require a peer review before approval — the same route as a new building permit.

Quick Revision

Chapter Summary

1. §6.1 — The three terms. Repair restores appearance and serviceability but adds no strength; restoration recovers the original strength; retrofitting upgrades the building beyond its original strength and is applied to undamaged buildings too. Draw the H–Δ curve (Fig. 6.1). Retrofitting aims at restoring integral box action and the roof-to-wall, wall-to-wall and wall-to-foundation connections. It follows a twelve-step process from realising the requirement, through vulnerability evaluation, feasibility, detailed design and municipal approval, to monitored construction and post-retrofit evaluation. There is no standard procedure — each building needs a unique solution.
2. §6.2 — Vulnerability assessment. Done before retrofitting to find the deficiency and after it to confirm effectiveness. Three phases: screening, evaluation, detailed evaluation. Three standard tools: the UN-HABITAT toolkit (multi-hazard, Compliance Index < 0.75 triggers detailed assessment), the GNDT sheet (11 parameters, classes A–D, vulnerability index to damage grade) and FEMA 154 (rapid visual sidewalk survey). Detailed investigation uses NDT (rebound hammer, UPV, profometer) and partially destructive tests (core, penetration, pull-off, sampling).
3. §6.3 — Materials. Cement grout, cement mortar and micro-concrete, galvanised welded wire mesh, reinforcing bars, reinforced concrete, steel sections and turnbuckles, through-stones, epoxy resins, timber, and FRP composites — light, non-corroding, applicable to any profile, adding strength and ductility with no increase in member size or dead load.
4. §6.4 — Techniques. Two strategies: increase capacity or reduce demand; two classes: global or local. Walls are strengthened by grouting (1:1 at 0.1–0.25 MPa, bottom upwards, but no help for orthogonal connections), wire-mesh jacketing (50 × 50 mm mesh both faces, ties at 500–750 mm, 20–40 mm render — which does fix orthogonal connections), sewing with inclined grouted bars, and pre-stressing to about 0.1 MPa. Box action is restored by splint and bandage, horizontal and vertical seismic belts and containment reinforcement. Each failure mechanism has its own matching technique — learn the table in §6.4.5.
5. §6.5–6.6 — Nepal. About 500,000 houses damaged in 2015, 85–90% of them masonry. The default practice is splint and bandage with galvanised wire mesh and cement plaster, supported by seismic belts, containment reinforcement, through-stones and grouting for stone-in-mud houses, and RC jacketing for engineered buildings. The framework is NBC 105 / 202 / 203, IS 4326 and IS 13935, DUDBC guidelines and NSET programmes, with municipal approval and peer review.

All Past Questions on This Chapter

Chapter 6 — Repair and Strengthening Methods for Existing Masonry Buildings · 4 hours · 6 marks · ENCE 367 Seismic Resistant Design of Masonry Structures