Strengthening and Repair of Building Structures
Underpinning and Strengthening of Building Foundations
Methods for Underpinning Building Foundations
Pile and Beam Underpinning
Piles are installed beside the walls, and beams are inserted, both within the walls and through them, to join the tops of the piles. 
`Pynford’ Stool Method of Underpinning
This method can be used where the existing foundations are in a poor condition, and it enables the wall to be underpinned in a continuous run without the need for needles or shoring.
Underpinning by Cantilever Needle Beam Method
is an extension of the pit method. If the foundation has to be extended only to one side and the plan possesses a stronger interior column, this method is suitable.
Mass Concrete Underpinning Method
This method involves extending the old foundation until it reaches a stable stratum. The soil below the existing foundation is excavated in a controlled manner. In order to transfer the load from the old foundation to the new one, a new pin is provided by means of placing a dry sand-cement pack.
What Underpinning Means
Underpinning is a method for repairing and strengthening building foundations. This method helps to strengthen the foundation of an existing building or any other infrastructure. These involve the installation of permanent or temporary support to an already held foundation so that additional depth and bearing capacity are achieved. The main objective of most underpinning work is to transfer the load carried by a foundation from its existing bearing level to a new level at a lower depth.
What Are Mini-Piles/Root Piles? How to Use for Underpinning?
Mini-piles/root piles can be implemented where the loads from the foundation have to be transferred to strata located at a distance greater than 5m. This method is adaptable for soil that has a variable nature. Mini-piles are also used in some methods such as needle and pile underpinning and root pile or angle piling.
- Piles of diameter between 150 to 300mm in diameter are driven, which may be either augured or driven steel-cased ones.
- Small, vertical mini-piles can also be installed through pre-drilled holes.
Methods for Widening and Strengthening of Existing Strip Foundations
Widening and strengthening of existing strip foundations by section enlargement. 
Strengthening of Load-Bearing Timber Structures
Methods for Strengthening of Load-Bearing Timber Structures
Steel Bars, Plates, or Profiles
Steel bars, plates, or profiles can be added or inserted in any side of a timber beam in order to increase its carrying capacity. They can be applied to the tension zone, the compression zone, or both. In the case of plates or profiles bonded to the surface of the timber element, the connection is done with steel bolts, studs, or dowels, while insertions of bars are bonded with the use of epoxy resin. 
Fibre Reinforced Polymers (FRPs)
Just like steel reinforcements, FRPs can be bonded anywhere around or inside cut slots inside beams (see Figure below). While reinforcements working in tension together with timber have good behavior bonded externally or internally, as long as the connection is appropriately accomplished, reinforcements subjected to compression are better used inside slots opened in the timber beams in order to avoid element instability. Hence, the bonding surface is doubled in this way. 
Concrete Slab
The cast of a concrete slab on a timber floor, while assuring a proper connection between concrete and timber beams, creates a composite system (see Figure below) that takes the best profit from both materials. The concrete slab is stressed to compression while the timber beams work in tension.
Timber Elements
The strengthening of sections using timber elements is done by adding new timber elements to the original beam, connecting both members using traditional timber carpentry. It is probably the most recommendable option, always that it is possible to apply given all the referred advantages of using timber as strengthening material and mechanical connections to achieve the bond between original strengthening members.
Is It Possible to Use Concrete for Strengthening of Load-Bearing Timber Structures?
Concrete slab: cast on timber floors creates a composite system, utilizing the strengths of both materials.
Widely used to enhance building behavior under horizontal actions, but unsuitable for historical buildings due to invasiveness. For less significant structures, it offers benefits: triples load-bearing capacity, increases bending stiffness, improves comfort with enhanced damping and sound insulation, and enhances fire protection by acting as a barrier to propagation.
Is It Possible to Use Fibre Reinforced Polymers (FRPs) for Strengthening of Load-Bearing Timber Structures?
FRPs can strengthen load-bearing timber structures by bonding externally or internally in cut slots within beams. External bonding is suitable for timber in tension, while internal bonding doubles the bonding surface and prevents instability in compression reinforcement. Thin FRP sheets enhance the strength, stiffness, and ductility of wood members. FRPs are beneficial in stress-concentrated regions like bolted joints and with tensile and flexural members.
Methods for Strengthening of Timber Floor Joists
Strengthening or Repair of Timber Floor Joists by the Addition of Steel or Wooden Sections
As in the case of frames, the strengthening of wooden floors by the addition of steel sections is a suitable solution to reinforce damaged structures (rotten or worm-eaten) and withstand increased loads. 
What “Prosthesisation” Means
Prosthesisation refers to interventions in which a part of a single member is cut out and substituted by an element that substitutes its function.
Where It Can Be Applied
Prosthesisation can be applied on the extremities of elements, close to the supports of floor beams and roof frames, where conditions are met for advanced states of degradation.
What Materials Are Used
The decayed part of the element may be replaced by timber, with the connection between new and original parts done with several available materials: timber, steel, FRPs, and epoxy adhesives.
Schemes Showing Repair of Decayed Rafter Extremity
Repair of Cracks
Epoxy resins with low viscosity can be injected on cracks, conferring to the section a mechanical behavior identical to the sane situation. The introduction of reinforcement bars of stainless steel or FRPs bonded with epoxy can as well be an appropriate solution for both longitudinal and transversal cracks.