Seismic Upgrading

Seismic upgrading of structures includes:

  • LOCALISED STRENGTHENING OF BEAM/PILLAR HINGE ZONES
  • STRENGTHENING FOR MASONRY ARCHES AND VAULTS
  • LINTEL BANDS
  • ANCHORING ROPES
  • INTERVENTIONS ON NON LOAD-BEARING STRUCTURES
  • INTERVENTIONS ON DAMAGED OR DEFICIENT VERTICAL STRUCTURAL MEMBERS IN INDUSTRIAL BUILDINGS

 

Properties of strengthening systems for the seismic upgrading of structures

The aim of seismic upgrading is to eliminate the fragile collapse mechanisms of load-bearing members, the collapse mechanisms of floors along their plane and to improve the overall deformation capacity of structures.

Composite materials from the FRP System and FRG System lines are used to achieve these objectives and, thanks to their strength, low weight and ease of application, they are used for installations on critical areas of structures.

These objectives are generally reached by increasing the ductility of the hinge zones in reinforced concrete structures and restoring the box-like behaviour of load-bearing wall structures. This makes them more resistant to horizontal loads by eliminating the orthogonal forces acting on the masonry panels and by connecting the perpendicular load-bearing members together.

 

Localised strengthening of column beam junction

Improving the mechanical performance characteristics of existing reinforced concrete structures, designed when anti-seismic requirements and prescriptions were not mandatory and so built to support vertical loads only, represents a problem that has heavy social and economic repercussions in the Mediterranean area structures. The overall behaviour of concrete frame is often unsatisfactory, in that they lack the required ductility and a dependable hierarchy of structural and/or mechanical resistance, thus inducing global collapse mechanisms. Recent seismic events have highlighted the numerous problems that occur in column/beam junction due to the formation of hinge points with a plastic behaviour at the top or base of pillars. The low amount of confinement of pillars, due to the presence of too few stirrups or stirrups that have splayed out of position, creates a flexural crisis at the top or on base of the pillar resulting in the non-confined concrete being compressed and crushed, instability in the reinforcing bars under compression and slipping of those in tension. The absence of stirrups in the column/beam junction in particular, and especially those positioned externally, may give rise to a more localised crisis due to shear failure of the panel. Therefore, in order to guarantee more adequate behaviour from this type of system in the event of seismic activity and to increase its overall ductility, in compliance with national guidelines (ReLuis Guidelines), the shear strength of the beams and pillars where they intersect at the junction is increased and the ends of the pillars, where more ductility under compressive/flexural loads is required, are confined.

The types of installation that improve the performance characteristics of column/beam junction in compliance with ReLuis Guidelines (Chapter 3) are as follows:

1) The load-bearing capacity of the junction panel and the resistance of the top portion of the pillar to shear forces from buffer walls are increased by applying strips of uniaxial metal fibre fabric (WRAP S FABRIC) diagonally around the hinge point (Figure 7.1). This phase also includes the application of quadriaxial carbon fibre fabric (WRAP C QUADRIAX) in an “L” formation at the intersection between the beam and pillar (Figure 7.2).

 

 Figure 7.1 – Diagrams from chapter 3 of the ReLuis Guidelines

 

 Figure 7.2 – Diagrams from chapter 3 of the ReLuis Guidelines

 

2) The shear strength of the junction panel is increased by applying balanced quadriaxial carbon fibre fabric (WRAP C QUADRI-AX) to the junction zone (Figure 7.3).

 

 Figure 7.3 – Diagrams from chapter 3 of the ReLuis Guidelines

 

3) Confinement of the ends of the pillars is carried out by wrapping them with uniaxial carbon fibre fabric (WRAP C UNI-AX), which significantly increases their shear strength and deformation capacity. For the upper end of the pillar, the increase in shear strength offered by the confinement is also beneficial against the added shear force from the strut that forms in the buffer wall (Figure 7.4).

 

 Figure 7.4 – Diagrams from chapter 3 of the ReLuis Guidelines

 

4) The shear strength of the ends of the beams is increased by binding them with uniaxial carbon fibre fabric (WRAP C UNI-AX) in a “U” formation (Figure 7.5).

 

 Figure 7.5 – Diagrams from chapter 3 of the ReLuis Guidelines

 

APPLICATION TECHNIQUE FOR “WRAP” FABRICS

Procedure

1) Prepare the substrate (as per procedure on page 26). Prepare all surfaces to be repaired by completely removing all the weak concrete with a hand or power chisel or with other suitable means, such as hydro-scarifying, to obtain a solid, sufficiently rough substrate with no detached portions. If the weak concrete has been removed with a hand or power chisel, clean all exposed reinforcing bars with a brush or by hydro-sandblasting to remove the rust and bring the reinforcing bars back to a bare metal finish. Hydro-sandblasting is not required if the surface has been prepared by hydroscarifying, but you must wait quite a long time after this operation before treating the reinforcing bars due to on-site logistics constraints. After removing all the rust, treat the reinforcing bars by brush-applying two coats of FER or FER 1K one-component, anti-corrosion cementitious mortar. The specific function of both these products, made from cementitious binders, powdered polymers and corrosion inhibitors, is to prevent the formation of rust. Clean all surfaces to be repaired and saturate the substrate leaving a dry surface (s.s.d.) by hydro-cleaning. Reintegrate the concrete using a product from the GROUT range.

2) Carefully smooth all sharp corners to form a rounded edge with a radius of at least 20 mm.

3) Apply a coat of WRAP PRIMER 1 two-component epoxy primer with a brush or roller to consolidate the surface.

4) Skim the primed surfaces with WRAP 11/12.

5) Apply strips of uniaxial, high-strength metal fibre fabric (WRAP S FABRIC) in a criss-cross formation (Figures 7.6 and 7.7).

 

Figures 7.6 and 7.7 – Shear strengthening of a column/beam junction using WRAP S FABRIC

 

6) Apply extra corner pieces made from quadriaxial, high-strength carbon fibre fabric (WRAP C QUADRI-AX) at the intersection between the column and beams.

7) Apply balanced quadriaxial carbon fibre fabric (WRAP C QUADRI-AX) to the central panel at the hinge point.

8) Confine the top end of the pillar with uniaxial carbon fibre fabric (WRAP C UNI-AX).

9) Apply open stirrups of uniaxial carbon fibre fabric (WRAP C UNI-AX) at the ends of the beams in a “U” formation.

10) All the WRAP C fabric must be sufficiently impregnated with WRAP 31.

11) Dust the resin (WRAP 31) while it is still wet with dry quartz sand to form a good bonding surface for the successive finishing layer.

(ref. “Design Guide” procedure G.1.7 and relative technical specifications)*.

 

Figures 7.8, 7.9 and 7.10 – Seismic upgrading of beam-column hinge zones using WRAP fabrics

 

Strengthening of masonry arches and vaults

The main aim of installations on arched and vaulted structures is to reduce the loads they impart, so they may be strengthened by plating them using FRP System or FRG System technology. Thanks to the low weight of composites, the use of this type of technology allows strengthening to be applied without increasing the overall mass of the structure.

Asymmetric and dynamic loads, typical of seismic activity, may induce cracking in vaulted structures due to the formation of plastic hinge points. It is widely known that arched structures collapse due to the formation of at least four plastic zone. One possible collapse mechanism may be due to the formation of three hinge points and a double pendulum effect which cause shear slip of one part of the arch with respect to the other. To prevent this type of mechanism, vaults may be protected by applying carbon fibre fabric (WRAP C UNI-AX, WRAP C BI-AX or WRAP C QUADRI-AX), glass fibre fabric (WRAP G UNI-AX or WRAP G QUADRI-AX) or basalt fibre fabric (WRAP B UNI-AX) along the external generatrix of the vaults. FRP strips represent a targeted intervention designed to withstand the tensile forces in the direction of the stresses acting most heavily on each single masonry member (typically flexion, compression/flexion or shear). The inorganic matrix consolidating and strengthening system (FRG System) is of benefit by distributing the structural capacity of the masonry member so that the improved tensile strength characteristics of the masonry are more or less widespread. Therefore, a global strengthening intervention on vaulted structures is carried out on the internal intrados or extrados by using pre-primed, alkaliresistant (A.R.) glass fibre mesh (GRID G 220) or pre-primed basalt fibre mesh (GRID B 250) applied with two-component, high ductility, readymixed hydraulic lime (NHL) and Eco-pozzolan based mortar (PLANITOP HDM RESTAURO) or two-component, ready-mixed, high ductility, fibre reinforced cementitious mortar (PLANITOP HDM / PLANITOP HDM MAXI). Thanks to their high synthetic resin content, the latter products offer a high level of adhesion and, once hardened, form a layer which is compact, waterproof, impermeable to the aggressive gases contained in the atmosphere and highly permeable to water vapour. What is more, PLANITOP HDM RESTAURO mortar does not contain cementitious materials and, once hardened, it is highly permeable which allows masonry to transpire naturally.

By using these systems, and thanks to the special weave of the meshes, masonry is stronger and more ductile and stresses are distributed more uniformly. It follows, therefore, that if the structure were to move, the system would have the capacity to distribute the forces over the entire surface of the members, so that the inevitable cracking forms in the construction joints and in the stone, brick and tuff substrate at the same time. The system adheres perfectly to the substrate, and its mechanical properties are such that localised stresses always provoke a crisis in the substrate rather than at the substrate/ strengthening system interface.

Experimental testing, carried out by the Department of Structural Engineering University of Naples “Federico II”, has demonstrated that the strengthening system is highly beneficial in terms of improved shear strength and ductility, and that it distributes stresses more evenly to limit the highly fragile post-peak behaviour of the strengthened member, particularly advantageous in the event of seismic activity.

  • Considerable increase in strength (+100%);
  • Increase in cracking load (approximately 80% peak load);
  • Increase of ductility;
  • Not significant increase in stiffness;
  • More uniform crack formation due to the excellent compatibility between the composites and the substrate;
  • No debonding.

 

APPLICATION PROCEDURE

Procedure

Substrate preparation

Prepare all surfaces by removing any supports from the extrados or intrados of the vault along with any loose or detached areas to form a substrate that is sound, compact and strong so that the materials and products applied do not detach. Remove all loose material from the surfaces to be repaired with a vacuum cleaner. Open any cracks (both surface cracks and through cracks) and clean with a vacuum cleaner to remove all traces of dust.

Consolidate surface and through cracks by injecting them with lime-based bonding slurry made from a product from the ANTIQUE range such as ANTIQUE I or ANTIQUE F21, or with a cementitious slurry such as STABILCEM.

(ref. “Design Guide” procedure H.4.1 and technical specifications H.4.1.1, H.4.1.2 and H.4.1.3)*.

 

Installation of a strengthening package for complete protection of vaults

The first step in applying the system is to smooth over the external surface of the vault with a 5-6 mm thick layer of PLANITOP HDM MAXI two-component, high ductility, pozzolanic-reaction binder based mortar (or PLANITOP HDM RESTAURO) to form a sufficiently flat surface.

Place pre-primed basalt fibre mesh (GRID B250) or pre-primed, alkaliresistant A.R. glass fibre mesh (GRID G 220) cut to suit the curved shape of the vault so that it protects the main ribs. Place the mesh so that it follows the form of the vault, hem it over the support areas for the vault and run it down along the existing masonry for 400 mm. Lay the strips of mesh alongside each other and overlap them by around 200 mm. Apply a second layer of PLANITOP HDM MAXI (or PLANITOP HDM RESTAURO) around 5-6 mm thick while the first layer is still wet to completely cover the mesh.

(ref. “Design Guide” procedures G.2.8 and G.2.9 and relative technical specifications)*.

 

Figures 7.11, 7.12, 7.13, 7.14, 7.15 and 7.16 – Strengthening the outer face of masonry vaults using PLANITOP HDM and GRID G 220 – Palazzo Sforza, Milan (Italy)

 

Installation of a strengthening package to protect the ribs of a vault structure

Another way of strengthening a vault is to protect the ribs with fabrics from the WRAP range. The first step before applying the fabric is to prepare the substrate with a layer of mortar (PLANITOP HDM / PLANITOP HDM MAXI / PLANITOP HDM RESTAURO) to even out the surfaces on which the fabric is to be placed. Once the mortar has cured, the epoxy cycle is carried out by applying a coat of WRAP PRIMER 1 (specific epoxy primer for the WRAP system) with a brush or roller on the surfaces in correspondence with the ribs. Then skim the primed surfaces with WRAP 11 (twocomponent, epoxy grout for levelling off surfaces). Apply strips of WRAP fabric impregnated with WRAP 31 (two-component, epoxy resin used to impregnate fabrics from the WRAP line). Massage the impregnated fabric with a special WRAP ROLLER to eliminate any air bubbles from the mortar-strengthening interface. Dust the resin (WRAP 31) while it is still wet with dry quartz sand to form a good bonding surface for the successive finishing layer. Once the mortar and resin have cured, replace the supports that were previously removed if required.

(ref. “Design Guide” procedures G.2.3 and G.2.10 and relative technical specifications)*.

 

Figures 7.17, 7.18, 7.19, 7.20, 7.21, 7.22, 7.23, 7.24 and 7.25 – Protection applied around reinforcement ribs using fabrics from the WRAP line

 

Tie area strips

For masonry structures, it is vital to guarantee structural regularity and box-type, monolithic behaviour of the entire structure. Tie area strips make it possible for both adjacent walls (in the case of walls that are not anchored together or in which the anchoring is inefficient) and opposing walls, as well as walls with elements pressing down on them (such as vaults that are not evenly supported or sufficiently balanced by the walls) to interact with each other. In so doing, they provide the most complete solution against horizontal forces (seismic activity), allowing movements and rotation of the walls themselves to reduce their vulnerability to kinematic mechanisms being triggered, such as the walls tilting over due to rotation.

In view of the new technology now available the use of “traditional” tie area strips is not recommended, in that their stiffness and additional weight to the overall seismic mass, incompatible with walls and wooden members, may worsen the behaviour and response of the structure in both the elastic and plastic phases.

Therefore, to prevent tipping mechanisms moving walls out of plane, the trend nowadays is to opt for installations with composite materials when carrying out restoration work on churches or buildings of particular architectural or historical interest, probably due to the fact that the installation is classified as “reversible”, especially by national heritage bodies. The technical and operational feasibility must always be carefully assessed and must always take into consideration the actual state of the building and where it is located. Tie area strips may be created using uniaxial carbon fibre fabric (WRAP C UNI-AX), glass fibre fabric (WRAP G UNI-AX) or basalt fibre fabric (WRAP B UNI-AX). Tie area strips may also be created by applying inorganic matrix composites from the FRG SYSTEM.

 

Figure 7.26 – Position of uniaxial fabric strengthening designed to prevent simple tilting mechanisms

 

APPLICATION PROCEDURE

Procedure

In the case of strengthening for lintel bands correct preparation of the substrate is vital, and is carried out by removing any render and any other loose portions to form a substrate that is sound, compact and mechanically strong so that the materials and products applied do not detach. In the areas of the wall where the strengthening is to be applied, a layer of two-component, fibre reinforced, highly ductile, pozzolanic-reaction mortar (PLANITOP HDM / PLANITOP HDM MAXI / PLANITOP HDM RESTAURO) is required to create a sufficiently flat substrate. Once the mortar has cured, apply a coat of two-component, epoxy primer (WRAP PRIMER 1, specifically formulated for the WRAP system). While it is still wet, immediately apply two-component epoxy grout for structural bonds (WRAP 11) and then WRAP fabric impregnated with medium-viscosity epoxy resin (WRAP 31, specifically formulated for WRAP FABRIC). Finally, dust the resin while it is still wet with dry quartz sand to form a good bonding surface for the successive finishing layer.

(ref. “Design Guide” procedures G.2.3 and G.2.12 and relative technical specifications)*.

Figure 7.27 – Application of WRAP PRIMER 1

Figure 7.28 – Application of WRAP 11

Figure 7.29 – Application of WRAP C UNI AX 300 and impregnation of the fabric with WRAP 31

 

 

Anchoring ropes

To protect structural strengthening made using the FRP System and FRG System on concrete, stone, brick and wooden structures damaged by weather and natural causes that require structural and functional restoration, structural connections can be made using WRAP FIOCCO. WRAP FIOCCO is a “structural connection” system made from monodirectional carbon fibres (WRAP C FIOCCO), glass fibres (WRAP G FIOCCO) and steel fibres (WRAP S FIOCCO) wrapped in gauze to form cord. WRAP C and G are then impregnated in situ with WRAP 21, and are available in various diameters to meet a wide range of site requirements. The “ribbons” are used to make structural connections in general between substrates and strengthening systems. The cord may be used in combination with fabric from the FRP System line, with CARBOPLATE plates and in strengthening systems made using GRID mesh to help them anchor more strongly, especially when employed for flexural and shear strengthening interventions.

 

APPLICATION EXAMPLES

1) Anchoring structural strengthening made using the GRID mesh strengthening system on vaults and brick, stone and tuff facing walls.

 

2) Connections between existing perimeter facing walls and pultruded carbon fibre plates (CARBOPLATE) and fabrics from the WRAP line used for the structural strengthening of beams, floor slabs, etc.

 

ADVANTAGES

The advantages include a considerable increase in the connection between strengthening applied on structural members and existing substrates and higher durability of materials used for building or repairing civil and industrial structures in aggressive environments where “reinforced connections” need to be applied. WRAP FIOCCO eliminates any risk of corrosion in the strengthening applied when steel is used.

 

APPLICATION

The system is applied as follows:

1. Drill the holes for the WRAP FIOCCO
2. Prepare the WRAP FIOCCO
3. Insert the WRAP FIOCCO

(ref. “Design Guide” procedures G.1.1 and G.2.4 and relative technical specifications)*.

 

1. Drilling the holes for the WRAP FIOCCO

WRAP FIOCCO is available in external diameters 6, 8, 10 and 12 mm. The holes drilled in the member must be between 120 and 200 mm in diameter and at least 200 mm deep (the depth of the holes depends on the thickness of the masonry). If the above guidelines are followed correctly, the material injected into the holes will completely embed the WRAP FIOCCO and provide sufficient anchorage with the substrate. After drilling the hole, remove all dust and loose material with compressed air.

 

2. Preparation of the WRAP FIOCCO

Cut pieces of WRAP FIOCCO at least 400 mm long (the length depends on the thickness of the masonry). Roll back the protective gauze to the same length as the depth of the hole and impregnate this part with WRAP 21. In order to make the impregnated part of the cord adhere better when it is placed inside the hole, dust it with dry quartz sand to give it a rougher surface. Once hardened, the ropes formed as described above is ready to be applied.

 

3. Inserting the WRAP FIOCCO

Apply a coat of WRAP PRIMER 1 in the holes and then completely fill the holes with epoxy while the primer is still wet. The choice of which product to use must be made according to the type of hole to be filled. For horizontal holes, holes in ceilings or holes in particularly porous substrates, it is better to use WRAP 11 or WRAP 12 epoxy resin or FIX EP epoxy chemical anchor for structural loads, while for holes in floors, holes at a slight angle or holes in compact substrates without internal cracks (e.g. concrete), it is better to use WRAP 31 medium viscosity epoxy resin. Fill the holes with WRAP 11 and WRAP 12 using an empty silicone sealant tube and an extrusion gun, while WRAP 31 can be simply poured into the holes. After filling the holes insert the pieces of WRAP FIOCCO prepared previously as above. Splay open the portion of the ropes which have not been inserted into the holes into a fan shape, place them on the part of the structure to be connected and impregnate them with WRAP 31. A coat of WRAP 31 must also be applied on the substrate before applying the splayed ropes.

 

Seismic upgrading of non load-bearing structures

D.M. (Ministerial Decree) 14.01.2008 § 7.3.6.3

For construction elements with no particular structural function, measures must be taken to prevent their brittle, premature collapse and the possibility of them being displaced by the action of Fa (design seismic force, § 7.2.3) corresponding to the SLV (…omissis…).

 

Circular No. 617/2009 §C7.3.6.3

The prevention of fragile, premature collapse of buffer walls and the possibility of them being pushed out due to the action of the Fa (ed. design seismic force) may be achieved by inserting rendering mesh on both sides of the wall and connecting them to each other and to the surrounding structures …(omissis), that is, by inserting horizontal reinforcement elements in the bed of mortar … (omissis). The term “non-structural” indicates all those elements that do not have to absorb workloads such as partition walls (dividing walls in internal areas), buffer walls (walls that “close” the building by separating the internal space from the outside), decorative features, plant equipment, etc.

The non-structural parts are represented by buffer walls and partitions. Because of their weight and position, the potential danger of the latter types must never be underestimated when considering the safety of people using a building, including in those cases in which the structure is not particularly badly damaged.

In compliance with national guidelines, to guarantee the connections between the concrete frames and buffer wall panels, anti-overturning interventions are contemplated using inorganic matrix, fibre reinforced materials from the FRG System line.

Figure 22 – Diagrams from chapter 4 of the ReLuis Guidelines

 

Figure 23 – Diagrams from chapter 4 of the ReLuis Guidelines

 

Figure 24 – Diagrams from chapter 4 of the ReLuis Guidelines

 

Anti-overturning system for buffer walls

In order to prevent walls over turning in the event of seismic activity, the sequence involves creating a reinforcing band of bonding at the interface between the buffer wall and the concrete frame to prevent rotational movements at the base of the wall.

(ref. “ReLuis Guidelines” par. 4.1).

 

APPLICATION TECHNIQUE

1) Remove a strip of the existing render along the perimeter anchoring bands around 500 mm wide (250 + 250 mm);

2) drill a hole in the buffer wall and temporarily cover the hole until the cord is inserted;

3) apply a first layer of PLANITOP HDM MAXI and, at the same time, straddle a 450 mm wide strip around the buffer wall of pre-primed, alkali resistant glass fibre mesh (EGRID G 120), used for the localised protection of cracked facing walls, to cover the 500 mm strip of wall from where the render was previously removed;

4) apply a second layer of PLANITOP HDM MAXI so that it completely covers the glass fibre reinforcement mesh;

5) insert the WRAP S FIOCCO/10 mm high strength, steel fibre cord through the hole in the wall and splay the ends along the two sides of the strengthening using WRAP 11 epoxy grout for structural bonds.

(ref. “Design Guide” procedure G.3.1 and relative technical specifications)*.

Figures 7.35, 7.36, 7.37, 7.38, 7.39 and 7.40 – Perimeter connection of a buffer wall

Seismic protection system for non-structural partition walls

An innovative protection system against seismic activity called WRAP EQ SYSTEM is used in this sector to protect non-structural members. It acts like “seismic wallpaper” and gives people more time to evacuate a building in the event of an earthquake.

One of the most critical features of a building hit by an earthquake is the fact that it becomes very difficult for people to evacuate rooms because of the damage caused to structural and non structural members and elements.

WRAP EQ SYSTEM improves the distribution of stresses induced by dynamic loads on the structure and reduces the vulnerability of secondary partition walls in the event of seismic activity. The system also improves the performance characteristics of concrete and masonry floors and reduces their risk of collapse.

 

WRAP EQ SYSTEM acts like an “air-bag” for internal and external secondary partition walls (e.g. buffer walls), and stops walls collapsing or tipping out of plane during seismic activity. In so doing, people may evacuate buildings without any particular risk.

 

WRAP EQ SYSTEM comprises the following:

  • WRAP EQ ADHESIVE
    One-component, ready-to-use, polyurethane dispersion-based water adhesive with very low emission of volatile organic compounds (VOC) for impregnating WRAP EQ NET biaxial, primed glass fibre fabric.
  • WRAP EQ NET
    Biaxial, primed, glass fibre fabric to protect secondary partitions in buildings from seismic activity.

 

The reinforcing system also adheres perfectly to rendered surfaces, as long as they are solid and compact, and increases ductility so that dynamic stresses are distributed more evenly. WRAP EQ ADHESIVE is formulated so that it is easy and safe to apply both indoors and outdoors without harming the environment.

 

Laboratory tests carried out on a vibrating test table

It carried out a series of experimental tests in collaboration with the Karlsruhe Institute of Technology (KIT) in Germany on full-scale buffer panels 2.5 metres wide and 3 metres high fastened securely to a steel frame.

 

TEST RESULTS

Strengthened buffer panels and buffer panels without strengthening were tested on a vibrating table to evaluate how the risk to non load-bearing structures may be reduced in the event of seismic activity, the aim being to prevent collapse and safeguard human life.

 

Figure 7.42 – Panel without strengthening (left) and a panel strengthened with wrap EQ System (right)

Figure 7.43 – Collapse of a partition wall without wrap EQ System

 

CONCLUSIONS

1) The brickwork panel was stressed at a frequency typically found during seismic activity (f1 = 6.5 Hz);

2) At higher frequencies (f2 = 9.0 Hz) both flexural and shear damage was found along the wall without strengthening.

Figure 7.44 – Flexural failure at the top of a panel

Figure 7.45 – Shear failure at the bottom of a panel

 

3) At these frequencies, the maximum acceleration and deformation occurred at 2.4g. It was also found that the frequency reduced rapidly when structural cracks started to open.

4) The frequencies measured on the wall strengthened with WRAP EQ SYSTEM were decidedly higher, a clear sign that the opening of the cracks occurred later.

5) The frequency levels in the sample strengthened with WRAP EQ SYSTEM were increased to almost the level of resonance and collapse after approximately 3,500 cycles of sinusoidal load, with a response amplitude of 3.5g.

6) There was a difference of 80% in the maximum acceleration and deformation due to the bending moment between the “as-built” panel and the panel protected by WRAP EQ SYSTEM.

 

 

APPLICATION TECHNIQUE

Before applying WRAP EQ SYSTEM remove all paint from the substrate. If necessary, prime the area where the system is to be applied with a coat of WRAP EQ ADHESIVE diluted 1:0.5 with water using a roller to consolidate the surface at the interface between the substrate and the protection system. Then apply WRAP EQ ADHESIVE with a brush or roller and place WRAP EQ NET fabric carefully on the adhesive so that there are no creases. To guarantee an efficient, evenly distributed effect from the system, overlap the sheets of fabric lengthways by at least 150 mm and overlap the ends of the sheets by at least 100 mm. After flattening out the fabric, apply a second coat of WRAP EQ ADHESIVE. When the adhesive is completely dry, apply a skim coat (PLANITOP 200).

(ref. “Design Guide” procedure G.3.2 and technical specification G.3.2.1)*.

 

Figures 7.48, 7.49, 7.50, 7.51 and 7.52 – WRAP EQ SYSTEM

 

ADVANTAGES OF THE SYSTEM

The main characteristics of WRAP EQ SYSTEM are:

  • Light and compact (< 2 mm);
  • May be applied directly over existing render;
  • Odourless;
  • For indoor and outdoor use;
  • Very low emission of volatile organic compounds (VOC);
  • Classified EC1 Plus.

 

WRAP EQ SYSTEM is covered by a worldwide patent and is just one of exclusive solutions dedicated to the sector of Structural Engineering.

Installations on damaged or deficient vertical structural members in industrial buildings in compliance with the “Guidelines for localised and global installations on single-storey industrial buildings not designed according to anti-seismic criteria”

In single storey, prefabricated structures the vertical strengthening elements, or pillars, are normally connected solidly at their base with a pocket footing which forms a socket-type constraint, while the tops of the pillars are connected to the beams with hinged or roller constraints. The force diagram of the pillar, therefore, is that of a cantilever held at the outer face of the pocket.

When there are high stresses, such as those generated by an earthquake, the pillar may move from the vertical plane due to rigid rotation at the foot of the pillar. This movement is just as likely to be caused by a rotary movement of the entire foundation as by damage to the reinforced cement elements (pockets, plinths, etc.). It is very difficult to ascertain which of the probable causes the damage should be attributed to by means of a simple visual inspection, and usually a much more thorough assessment of the ground and foundations is required along with a more invasive inspection.

It is quite clear that in many pillars there had been an initial formation of a plastic hinge at the base which, in certain cases, had led to the formation of cracks, while in other cases the concrete around the reinforcing bars had been displaced and the reinforcing bars had become unstable where there was a lack of transversal reinforcement.

On-site visual inspections have also led to the discovery that, in numerous cases, the pillars had been damaged by the impact of horizontal members such as beams and tiles that had fallen or collapsed because they no longer had any support. (ref. Guidelines par. 1.3).

Amongst the categories of installations covered by the “Guidelines for localised and global installations on single-storey industrial buildings not designed according to anti-seismic criteria”, issued following the earthquake in May 2012 in Italy, there is the strengthening of pillars by binding them with various types of fabric to increase their resistance and ductility. Amongst the weak points highlighted in such structures are insufficient compressive-flexural capacity at the base of the pillars of height H and insufficient shear capacity. Below are examples of interventions using technology developed in compliance with Chapter 4 “Data sheets for selecting the dimensions, execution and site organisation of interventions”, which contains guidelines and suggestions that may be adapted to suit typical site conditions and situations.

Figures 7.53 and 7.54 – Damage to industrial buildings caused by the earthquake in Emilia (Italy) in 2012

 

Confinement and strengthening at the base of pillars by forming a sleeve in HPFRC

(In compliance with data sheet N.ID.RP-4)

Amongst the various installations which may be carried out, one strengthening technique is to form a sleeve around the pillar using high performance fibre reinforced concrete (HPFRC). This type of installation, which involves adding slim sections and slightly modifies the geometry and mass of the pillar, increases its resistance to bending stresses. Confining the critical zone at the base of the pillar, therefore, increases the ductility and load-bearing capacity (axial forces, bending moment and shear) of the main section of the pillar itself.

In compliance with the Italian Guidelines, the strengthening system proposed to form a sleeve around the pillar made from PLANITOP HPC freeflowing, high strength, fibre reinforced, high ductility, shrinkage-compensated cementitious mortar in combination with FIBRE HPC stiff steel fibres for repairing and strengthening concrete. PLANITOP HPC complies with the principles defined in EN 1504-9 (“Products and systems for protecting and repairing concrete structures: definitions, requirements, quality control and conformity assessment. General principles for the use and application of systems”), and the minimum requirements of EN 1504-3 (“structural and non-structural repairs”) for R4-class structural mortars.

 

APPLICATION TECHNIQUE

Procedure

1) Before making holes in any of the structural components, use a cover meter to identify the areas without reinforcement.

2) Carefully drill holes in the floor to embed the bars that will be connected to the sleeve. An alternative method is to make the connection out of electro-welded reinforcement mesh after creating a seat in the floor to position the mesh.

3) Hydro-sandblast or scarify the surface of the pillar to remove any weak concrete and create a sufficiently rough surface to guarantee good adhesion between the base concrete and fibre reinforced mortar without having to resort to the use of epoxy adhesive.

4) Apply formwork around the pillar and cast the sleeve by pouring in PLANITOP HPC.

(ref. “Design Guide” procedure G.1.10 and technical specification G.1.10.1)*.

 

Figure 55 – Diagram from “Guidelines for localised and global interventions on single-storey industrial buildings not designed according to anti-seismic criteria”

 

Combined bending and axial load strengthening around the base of pillars using fibre reinforced composites with anchoring ropes

(In compliance with data sheet N.ID.RP-7)

 

APPLICATION TECHNIQUE

Procedure

1) Break out a strip of the floor approximately 500 mm wide around the pillar and remove all the material under the pillar down to the upper part of the plinth.

2) Clean the surface of the pillar where the composite materials will be attached and round off the corners of the pillar to form a bending be not less than 25 mm.

3) Drill holes around 20 mm in diameter in the filler mortar between the pillar and the pocket to a depth of around 300 mm.

4) Apply the strengthening system, comprising uniaxial, high-strength, high modulus carbon fibre fabric (WRAP C UNI-AX), or alternatively pultruded carbon fibre plates (CARBOPLATE) impregnated with epoxy resin or uniaxial steel fibre fabric (WRAP S FABRIC) on the surface of the pillar up to at least 1/3 of the overall height of the pillar above the pocket. Place the fabrics (or pultruded plates) so that the fibres (or filaments) run parallel with the axis of the pillar. The strips of fabric must be placed so they also run in the gap between the pillar and the pocket for the entire length of the anchoring pocket. The width of the strips of uniaxial composite fabric applied on each side of the pillar depends on how much the compressive/flexural strength needs to be increased.

5) Place uniaxial, high strength steel fibre cord (WRAP S FIOCCO) in the holes drilled previously and embed them in two-component epoxy resin (WRAP 11/12) or an epoxy chemical anchor (FIX EP). Make sure the ends of the cord are splayed open and placed on the surface of the uniaxial composite applied previously (point 4). The steel fibre cord must go right down to the bottom of the hole and run along the side of the pillar for at least 700 mm. WRAP S FABRIC may also be used to create the ropes.

6) Completely cover the metallic rope with two-component epoxy resin (WRAP 11/12) to preventing it coming into contact with the carbon fibre fabric to be applied later.

7) Confine the pillar with a series of closed rings of uniaxial, high strength, high modulus carbon fibre fabric (WRAP C UNI-AX) running continuously up the pillar from the top of the pocket to the upper part of the uniaxial fabric. Place the confinement rings of fabric so that the fibres (or filaments) are perpendicular to the axis of the pillar.

8) Dust the entire surface of the strengthening system with dry, fine quartz sand so that the successive layer of protective finish adheres better.

(ref. “Design Guide” procedure G.1.9 and relative technical specifications)*.

 

Figures 7.56 and 7.57 – Diagrams from “Guidelines for localised and global interventions on single-storey industrial buildings not designed according to anti-seismic criteria”

 

Figure 7.59 – Drilling holes in the filler mortar in the gap between the pillar and the pocket

Figure 7.60 – Priming the surfaces of the pillar with WRAP PRIMER 1

Figure 7.61 – Levelling off the surface with WRAP 11

Figure 7.62 – Compression-flexural strengthening using WRAP S FABRIC

Figure 7.63 – Applying and embedding WRAP S FABRIC in the holes

Figure 7.64 – Application of WRAP 31 impregnating resin

7.65 – Confinement using uniaxial carbon fibre fabric 7.59 WRAP C UNI-AX