Retrofitting Design of Building Structures

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The retrofitting of shotcrete will be improved if the substrate surface is particularly rough after removal of loose or deteriorate portions [ 26 ]. The ferrocement consists of closely spaced multiple layers of hardware embedded in a high strength MPa cement mortar layer mm thickness [ 27 ], shown in Fig. The mechanical properties of ferrocement depend on mesh properties as the mesh improves the in-plane inelastic deformation capacity by confining the masonry units after cracking. In general, the surface treatment method can significantly improve the strength and stiffness of masonry structure.

Retrofitting Design of Building Structures

Furthermore, they both increase the wall height-to-ratio, the in-plane lateral resistance, out-of-plane stability and arching action are increased accordingly [ 30 ]. Obviously, this technique is suitable for the vertical masonry element and it will be harmful if being implemented on the horizontal masonry elements like arches.

Nevertheless, the shortcomings of this method are the much time consumption in the application and it destroys the original aesthetics. Therefore, this technique is not suitable for the retrofitting of masonry heritage,. Sometimes, the masonry units in the buildings are still of good quality but the mortar is poor or it was not fully filled. Therefore, the mortar could be replaced or refilled by new bonding material with higher strength. Grout injection and re-pointing are the most often used techniques. Grout injection is implemented by filling the voids and cracks [ 31 ] developed different types of grouts for filling spaces ranging in size from very narrow cracks to large voids and empty joints.

This technique has been found to be effective at restoring the initial stiffness and strength of masonry, but no significant improvement in the initial stiffness or strength. However, the effectiveness of this technique can be improved if it is used in combination with other techniques [ 33 ] conducted a study by combining FRP rods and re-pointing technique on masonry structure.

The results displayed that using re-pointing technique combined with FRP laminates is the most effective retrofitting technique. It should be noted that this approach will work efficiently only if the mechanical property of the mix and its physical and chemical compatibility with the masonry to be retrofitted has been achieved [ 34 ]. In the retrofitting of masonry heritages, the preservation of the original aesthetics and the compatibility in terms of physicochemical and mechanical characteristics are the most important concerns [ 35 ].

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The former means that the authenticity of masonry heritages need to be preversed after retrofitting while the latter means that masonry and the retrofitting material should have a good compatibility in terms of physicochemical and mechanical performance. The use of incompatible retrofitting materials may initiate decay mechanisms or even lead to catastrophic results [ 36 ]. The application of grout injection and re-pointing can preserve the original appearance of masonry heritage. As previously stated, the physical and chemical compatibility between masonry heritage and retrofitting materials is critical, while the interaction between retrofitting material and masonry is still not known clearly.

Therefore, recent research regarding the design and selection or restoration mortar is interlinked with compatibility assessment to ensure the long-term durability of masonry heritage [ 35 ] presented a methodological approach for the selection of restoration mortars regarding fragility analysis. In the paper, selection of the optimum mortar, complying with the set compatibility and performance requirement can be accomplished by setting requirements during the characterization of the retrofitting materials and the investigation of masonry heritage. This technique is suitable for most masonry buildings, especially for the masonry heritage as the authenticity can be preversed after retrofitting.

The prerequisite is that the retrofitting mortar does not have a detrimental effect on the initial masonry. This method becomes popular and practical because of its minimal cost and ease of implementation, and most importantly, its sustainability. The application of this technique is to install steel elements next to the original masonry element, which might be tied together or not. During an earthquake, small cracks are expected to occur and they will develop and propagate if external loading exceeds its load carrying capacity.

Seismic retrofit of masonry structures

However, the new steel system has a considerably large stiffness and will stop the cracking on the masonry wall from propagating [ 37 , 38 ]. In such cases, the external load will be carried by the stronger steel system while the initial masonry system may work as a structural element instead of carrying loads. This technique is very effective in improving the load resistance of a structure as steel is a strong retrofitting material.

Therefore, this approach is applicable for the weak masonry structures or the structures that need to be improved remarkably. However, as the appearance of steel will change the aesthetics of the original masonry structure, thus, it is not a suitable retrofitting approach for masonry heritage. Furthermore, the high cost is another concern of its implementation in developing countries. In the post-tensioning strengthening method, the pre-stressed reinforcements are placed along the vertical elements for the sake of improving the strength and ductility of the lateral load resisting frame of the structure [ 37 ].

In detail, this method is carried out by drilling a hole through the masonry wallets, and vertically placing pre-stressed reinforcement in the drilled hole. The compressive force will be provided by the pre-stressed reinforcement, which can counteract the tensile force occurred in masonry wall, thus improving its load carrying capacity. Experiments illustrated that the lateral load resistance of masonry walls can be doubled [ 40 , 41 ] implemented this approach on masonry walls in out-of-plane direction to analyse the flexural behaviour.

Though the results presented that the ductility of the reinforced masonry panel was not improved, the strength and stiffness were increased remarkably. However, in the results of experiments conducted by [ 42 ], some difference regarding the ductility was found. The maximum strength can be increased by a factor of 2. Similarly, [ 43 ] found that the shear capacity and ductility can be improved significantly, while the ability of energy dissipation is also increased remarkably.

Besides the application on bare masonry panels, this technique can be used to improve the seismic performance of RC frame infilled with masonry wall [ 44 ] conducted such a study to find that the engagement between RC frames and masonry infill has been improved through this retrofitting technique, thus leading to a postponed failure mechanism. It should be noted that the axial force provided by the pre-stressing bar and the vertical load should be smaller than a certain limit. Exceeding the limit, the ductility will decrease [ 43 ]. The center core is similar to the post-tensioning technique to some extent, which involves of installing a reinforced core in the vertical direction of the masonry walls.

The differences between this technique and post-tensioning are that the steel bar is not pre-stressed and the drilled hole is much bigger. This method has successfully doubled the strength of a masonry panel in a static cyclic test [ 28 , 46 ] has successfully applied this technique on more than 60 projects to mitigate the earthquake hazard.

Some of the shortcomings in the above approaches, adding mass, for instance, can be overcome by using mesh reinforcement. FRP composite was first used to retrofit or strengthen the existing concrete structures. It has been extended and applied on other masonry, timber structures and extensively studied as well [ 47 ]. The improvement might be much more dependent on the to be retrofitted structure.

In the study of [ 49 ], FRP has been found to improve the shear resistance of the masonry buildings by 3. The study concluded that if the economy and mechanical behaviour are both concerned, it is better to choose unidirectional FRP laminates or fabric strips instead of using two-dimensional fabrics [ 50 ] carried out research to investigate the effectiveness of the FRP with different configurations: grid arrangement and diagonal strips. It is noted that the asymmetrical application of the reinforcing is not effective in improving the shear resistance of masonry walls.

Moreover, it is found that the grid strips provide a better stress redistribution, which results in a less brittle failure, while the diagonal strips are more effective in enhancing the shear capacity.

Retrofitting of Heritage Structures

In terms of the failure type of masonry panels retrofitted with FRP, the detachment of FRP from masonry surface plays an important role [ 51 ]. The effectiveness of retrofitting will be lost if the FRP starts debonding from the masonry surface [ 52 ] found that the failure modes are masonry crushing, FRP rupture and debonding. The reinforcing of masonry panels using FRP possesses the merits of little added mass, low disturbance and relatively high improvement in strength.

The initial cost of FRP material is about 5 to 10 times more expensive than steel [ 53 ], which is a big concern in choosing the retrofitting approaches.

In addition, the property and performance, especially their long-term behaviour, of FRP materials have not been thoroughly understood [ 54 ]. Moreover, the FRP is normally applied by externally attaching the strips or sheets on the surface of masonry wall, and a water-proof barrier and prevention of the natural transpiration of the masonry structure might be created. Finally, this type of strengthened structures will be particularly weak if the epoxy-based bonding material is used in the strengthening with FRP composites [ 30 ].

If FRP is too expensive to afford in the developing countries, the Polypropylene PP band and bamboo meshes can be alternatives. PP band is a universal cheap packing material having considerable elongation capacity, which has been introduced as a cheap reinforcing approach in Japan.

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This strengthening approach is often used in reinforcing adobe masonry structure. This retrofitting approach has found to be helpful in preventing the loss of material and maintaining wall integrity. With the application on high-strength masonry structure, the effectiveness will be much less significant. With the bamboo meshes. The retrofitted adobe house could withstand over twice larger input energy than the non-retrofitted specimen.

The advantages of pp-band and bamboo meshes are their low cost and easy availability.

The mesh reinforcement is not only effective in reinforcing the vertical masonry structures but also significant in the strengthening of horizontal elements like vaults and arches, especially the composite materials made with textile fibers both with polymeric and cementitious matrices [ 58 ] strengthened masonry arches and vaults with different composites TRM, SRG and FRP , all the experimental results showed that all the reinforcement systems were very efficient in increasing the maximum load [ 59 ] used the post-strengthening method with C-FRP on masonry vaults, and a similar result was found.

Though the polymer-reinforces fibers are the most commonly used to strengthen vaults and arches, they still possess the disadvantages of lack of water vapor permeability [ 62 ] proposed an alternative method by embedding long steel fibers and basalt textiles in the mortar to provide a steel-basalt reinforce mortar-based composite. The retrofitting results are compared with the polymer composites and both cases are effective in strengthening masonry vaults in terms of increasing of load and deformation capacity [ 6 ] agrees with the work of [ 62 ] that basalt on a mortar matrix provides both higher capacity and better ductility compared with polymer matrix.

This system is carried out by inserting a continuous mesh of high strength reinforcement in the mortar joints, which are striped off about mm. Then, the mesh of reinforcement will be anchored to the masonry panel with transverse metal bars with the number of per m 2. After that, the reinforcement and anchoring bars will be covered by re-pointing mortar back to the joints. The dimension of the reinforcement mesh normally ranges from mm, and it must be smaller than the thickness of the panel [ 64 ].

The detailed configuration of a typical reticulatus system is displayed in Fig. In the study of [ 65 ], the reticulatus system has been applied on historic masonry to investigate the flexural strengthening. The results have proved the improvement and the potential application of this technique. However, the improvement of bending force and the initial stiffness was realized only under appropriate pre-tension. In the work of [ 66 ], the reticulatus system made of fibre-reinforcement was used to retrofit an ancient building to provide a cross-interlock to resist against tensile strength produced by lateral forces.

Though the compression, shear and flexural strength of stone or rubble masonry wall reinforced by the reticulatus system can be increased, the effectiveness of this reinforcing technique relies on the reinforcement mesh embedded in the mortar joints [ 63 ]. Besides the difference of improvement on retrofitting materials, the improvement of same retrofitting material on different masonry structures varies as well.

Retrofitting Design of Building Structures

As this reinforcing system can keep the original aesthetic of the building, it is therefore suitable in reinforcing the fair-face masonry or the masonry heritages. Furthermore, this technique fits the masonry panels of both regular and irregular shape. Similar with FRP, the reticulatus system does not put on much extra load. Further investigation of the application of this technique on other masonry structures, such as brick masonry, should be conducted.

This technique involves constructional columns confining the masonry walls at all corners and wall intersections as well as the vertical borders of door and windows openings [ 27 ]. The integrity will be improved much more remarkably if the constructional columns relate to ring beams at floors levels. Both the constructional columns and ring beams confine the masonry structure at the same storey.