Structural Strengthening Services for Long-Lasting Building Performance

1 Introduction
2 What are structural strengthening services?
3 Why buildings require structural strengthening
4 Assessment and signs of structural problems
5 Structural strengthening methods
6 Benefits and limitations of strengthening
7 Design, construction and quality control
8 Importance of timely intervention
9 Related articles on Designing Buildings

[edit] Introduction

Buildings and other structures naturally experience deterioration as a result of ageing, environmental exposure, repeated loading, changes in use and inadequate maintenance. Cracking, reinforcement corrosion, material degradation, foundation movement and overstressing can progressively affect structural safety, serviceability and durability. Structural strengthening is an engineering intervention used to restore or increase the capacity or performance of existing structural elements, often allowing buildings to remain in use rather than requiring demolition and reconstruction.

Structural strengthening should be distinguished from general repair. Repair may restore a damaged element to its previous condition, whereas strengthening increases its capacity, stiffness, stability or other performance characteristics where required. The appropriate approach depends on the condition of the existing structure, the causes of deterioration, the required performance and the proposed future use.

[edit] What are structural strengthening services?

Structural strengthening services comprise the investigation, design and construction of measures intended to improve the load-carrying capacity, stability, stiffness, ductility or durability of existing structures. They may be required following deterioration, structural damage, changes in loading, alterations to a building, changes of use or deficiencies identified during a structural assessment.

Strengthening can be applied to residential, commercial and industrial buildings as well as bridges and other infrastructure. The objective is not simply to add material to an existing element, but to establish an effective load path between the existing and new components and to ensure that the strengthened structure performs as intended.

[edit] Why buildings require structural strengthening

Buildings may require strengthening because of reinforcement corrosion, concrete deterioration, foundation movement, accidental or environmental damage, inadequate original design or construction, increased imposed loads, or changes to the building's configuration or use. Alterations such as removing walls, creating openings, installing heavy plant or adding storeys can change the distribution of loads and may require existing structural elements to be strengthened.

In some circumstances, strengthening may provide a more practical alternative to partial or complete reconstruction. However, it is not automatically the most appropriate or most economical solution. The decision should follow an assessment of the existing structure, its condition, the cause of any defects and the requirements for its future use.

[edit] Assessment and signs of structural problems

A strengthening project should normally begin with a structural investigation and condition assessment. This may include visual inspection, dimensional surveys, review of available drawings and records, material testing, investigation of reinforcement, monitoring of defects and structural analysis. Where necessary, intrusive investigations can be undertaken to establish the construction and condition of concealed elements.

Visible signs that may warrant further investigation include significant or progressive cracking, concrete spalling, exposed or corroded reinforcement, excessive deflection, distortion, settlement, water ingress and unusual vibration. Cracking does not necessarily indicate a structural failure: its significance depends on its location, width, pattern, cause and whether it is changing over time. Similarly, rust staining or spalling may indicate reinforcement corrosion but requires investigation to establish its extent and structural consequences.

The assessment should seek to identify and address the underlying cause of deterioration rather than simply treating visible symptoms. For example, repairing concrete affected by reinforcement corrosion without addressing the cause of corrosion may result in further deterioration.

[edit] Structural strengthening methods

The strengthening technique selected depends on the structural material, existing condition, required increase in capacity, available space, environmental exposure, fire requirements, construction access and other project constraints. Common approaches include the following.

Concrete jacketing: Additional reinforced concrete or cementitious material is installed around an existing member, increasing its dimensions and potentially its strength and stiffness. The existing and new reinforcement must be appropriately detailed to achieve effective structural interaction.
Steel jacketing and steel strengthening: Steel plates, sections or jackets can be connected to existing structural members to increase their capacity or stiffness. Connection and load-transfer details, corrosion protection and fire performance are important design considerations.
Fibre-reinforced polymer systems: Carbon, glass or other fibre-reinforced polymer systems can be bonded or mechanically fixed to structural elements. They can provide additional tensile capacity or confinement with relatively little increase in structural weight. Their performance is affected by factors including substrate condition, bond, temperature and fire exposure.
Beam, slab and column strengthening: Individual structural members can be strengthened using combinations of steelwork, concrete, additional reinforcement, bonded plates or fibre-reinforced materials. The appropriate method depends on the governing structural deficiency.
Foundation strengthening and underpinning: Where inadequate foundation capacity or ground movement is responsible for structural problems, measures may include underpinning, ground improvement, additional foundations or changes to drainage and water management. The underlying ground conditions and cause of movement should be established before selecting a solution.
Crack repair and injection: Resin injection, cementitious grouting, surface sealing or other repair methods may be appropriate for particular types of cracking. Injection does not automatically strengthen a structure, and the method must be selected according to the cause, movement characteristics and structural significance of the crack.
Shotcrete and sprayed concrete: Sprayed concrete or mortar can be used for repair, protection and, where appropriately designed and reinforced, structural strengthening. It is used in applications including the rehabilitation of concrete and masonry structures.

Strengthening methods may also be combined with concrete repair, corrosion mitigation, waterproofing and protective treatments. The selection and design of the system should take account of the existing structure and the required service life rather than relying on a generic strengthening solution.

[edit] Benefits and limitations of strengthening

Structural strengthening can increase load-carrying capacity, improve serviceability and, where appropriately designed, enhance resistance to particular forms of structural action such as seismic loading. It can also extend the useful service life of existing structures and enable refurbishment or changes of use that might otherwise require more extensive reconstruction.

Retaining an existing structure can reduce the quantity of demolition and new construction required, potentially reducing waste and the environmental impacts associated with reconstruction. However, strengthening works themselves require materials, energy and construction activity, so their environmental benefits should be assessed on a project-specific basis.

Strengthening is not necessarily faster or less expensive than reconstruction. Existing structures can be difficult to investigate and work around, and temporary works, access restrictions, decanting, protection of occupants and complex load-transfer arrangements can add cost and programme constraints. These factors should be considered when comparing strengthening with alternative forms of intervention.

[edit] Design, construction and quality control

Structural strengthening should be designed by an appropriately qualified structural engineer on the basis of an adequate investigation of the existing structure. The design should consider existing and proposed loads, material properties, structural behaviour, load paths, connections, compatibility between existing and new materials, durability, fire performance and construction sequencing.

Construction can affect the final performance of a strengthening system as significantly as the design. Existing elements may require temporary propping or load relief before strengthening work begins, while surfaces may need preparation to achieve adequate bonding or load transfer. Installation should follow the design requirements and relevant standards, with appropriate inspection and testing.

Particular attention may be required where strengthening is intended to address deterioration. Damaged or contaminated concrete may need to be removed and repaired, reinforcement may require treatment or replacement, and sources of moisture or other deterioration should be controlled. Strengthening a deteriorated element without addressing the mechanism causing the deterioration may provide only a temporary improvement.

[edit] Importance of timely intervention

Structural deterioration can become progressively more difficult and expensive to address if its causes are left untreated. Early investigation can establish whether a defect is cosmetic, related to durability, or indicative of a structural deficiency. Where strengthening is required, timely intervention may reduce the extent of subsequent deterioration and minimise disruption to the building's use.

The presence of cracks, corrosion, spalling, settlement or other defects should not, however, be taken as evidence that strengthening is necessarily required. The appropriate response may instead be monitoring, repair, maintenance, replacement of a component or remediation of an underlying cause. A structural assessment is therefore essential before selecting an intervention.