Flexural Strengthening

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Flexural strengthening is required for structural members subjected to a design bending moment higher than their corresponding nominal flexural capacity. Flexural strengthening with composites may be carried out by applying one or more layers of fabric to the strained area of the member to be strengthened (ref. CNR DT 200/2004 par. 4.2 and ReLuis Guidelines par. 3.3.1.1).

Flexural design at ULS of FRP strengthened members that both flexural capacity Mrd, and factored ultimate moment, Msd, satisfy the following equation:

Msd ≤ Mrd

 

Flexural strengthening of concrete beams

 

This type of strengthening may be achieved by applying one or more plates of CARBOPLATE or one or more layers of uniaxial fabric, such as WRAP C UNI-AX, WRAP C UNI-AX HM, WRAP G UNI-AX, WRAP B UNI-AX or WRAP S FABRIC, to the strained areas of the beam.

FRP FORMULA – DESIGN OF FLEXURAL STRENGTHENING FOR A RECTANGULAR BEAM

The objective of the installation is to increase the load-bearing capacity of the structural members to be strengthened so that the stresses within the beams are reduced to the limits specified.

Characteristics of the materials used in the calculation

Height: H = 45 cm;

Base: b = 30 cm;

Area of steel in tension zone: RL = n°2 ø24 mm ≈ 904 mm²

Area of steel in compression zone: RU = n°2 ø24 mm ≈ 904 mm²

FeB38k grade steel: fyk ≥ 375 N/mm²

Concrete – C16/20: Rck 20 N/mm²

 

Design bending moment acting on the section:

Msd = 117.59 kN*m

 

DESIGN/CALCULATION PROCEDURE

The analysis is carried out using the semi-probabilistic “Limit State Design” approach, taking into consideration the combined loads and then multiplying them by a safety factor.

 

 

ACTIONS/STRESSES

 

 

The technique proposed, using products from the WRAP SYSTEM line, is applied “in parallel” to the existing structure. It acts in conjunction with the structure rather than replaces it, so that by working together the strength and load-bearing capacity of the beams are increased without modifying the distribution or stiffness of the masses.

 

TYPE OF STRENGTHENING ➠ No. 2 butted plates of CARBOPLATE E 170/100

Flexural capacity of the strengthened member in the section after strengthening:

Mrd, ULS post-strengthening = 161.79 kN*m/beam ➠ Msd < Mrd

 

APPLICATION TECHNIQUE FOR “CARBOPLATE” PLATES

Procedure

1) Substrate preparation.

Prepare all surfaces to be repaired by completely removing all weak concrete with a hand or power chisel or 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. Hydrosandblasting 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. Reinstate the concrete using a product from the GROUT range.

2) Apply an even coat of WRAP PRIMER 1 by brush or roller. If the substrate is particularly absorbent, apply a second coat of primer once the previous one has been completely absorbed.

3) Level off the surface where the CARBOPLATE is to be placed with a 1 to 2 mm thick layer of WRAP 11 or ADESILEX PG1 applied with a notched trowel while the WRAP PRIMER 1 is still wet. If epoxy adhesive with a longer workability time is required, WRAP 12 or ADESILEX PG2 may be used.

4) Cut the CARBOPLATE to the length required with a diamond-tipped grinding disk.

5) Remove the protective backing (peel-ply) from the CARBOPLATE. Skim the bonding surface of the CARBOPLATE with WRAP 11 or ADESILEX PG1. If epoxy adhesive with a longer workability time is required, WRAP 12 or ADESILEX PG2 may be used.

6) Remove all excess WRAP 11 or ADESILEX PG1 from the CARBOPLATE with a flat trowel.

7) Position the CARBOPLATE and apply an even pressure over the entire surface with a hard rubber roller.

8) Eliminate all excess resin with a flat trowel, taking care not to move the carbon fibre plate. If more than one layer of CARBOPLATE is required, once the WRAP 11 or ADESILEX PG1 have hardened, carefully peel the second protective backing from the plate if it has not already been removed before placing it in position.

(ref. “Design Guide” procedure G.1.2 and technical specifications G.1.2.1, G.1.2.2 and G.1.2.3)*.

 

APPLICATION TECHNIQUE FOR “WRAP” FABRICS USING THE “WET TECHNIQUE”

Procedure

1) Prepare the substrate (as per procedure on page 26).

2) Apply an even coat of WRAP PRIMER 1 by brush or with a roller. If the substrate is particularly absorbent, apply a second coat of primer once the previous one has been completely absorbed.

3) Skim and even out the surface with a 1 to 2 mm thick layer of WRAP 11 applied with a notched trowel while the WRAP PRIMER 1 is still wet. If epoxy adhesive with a longer workability time is required, WRAP 12 may be used.

4) Smooth over the surface of the adhesive with a flat trowel to eliminate even the smallest irregularities from the surface.

5) Impregnate the pieces of fabric before laying them on the surface. This step may be carried out either manually or with suitable tools and equipment. For manual impregnation, cut the WRAP fabric to the size required with scissors and soak it for a few minutes in a plastic container (preferably rectangular) filled to around 1/3 with WRAP 21. Remove the fabric from the container, leave it to drip for a few seconds and then press it lightly without twisting to completely remove all the excess resin. Wear protective rubber gloves when carrying out this operation. As an alternative to manual impregnation, simple equipment with a bowl and a series of rollers may be used; this makes it easier and safer for the operator to saturate the fabric and remove the excess resin. This system is particularly recommended when a large number of installations on large surfaces need to be carried out and guarantees that the resin is distributed evenly in every part of the fabric.

6) Position the WRAP fabric immediately after impregnation, making sure that it is spread on evenly without creases. Wear protective rubber gloves for this operation.

7) Go over the surface of the fabric with a WRAP ROLLER to make sure the adhesive and resin completely penetrates through the fibres. This operation is also necessary to eliminate air bubbles trapped in the fabric.

8) Wash the WRAP ROLLER with thinners immediately after use.

(ref. “Design Guide” procedure G.1.3 and technical specifications G.1.3.1, G.1.3.2, G.1.3.3, G.1.3.4 and G.1.3.5)*.

 

APPLICATION TECHNIQUE FOR “WRAP” FABRICS USING THE “DRY TECHNIQUE”

Procedure

1) Prepare the substrate (as per procedure on page 26).

2) Apply an even coat of WRAP PRIMER 1 by brush or with a roller. If the substrate is particularly absorbent, apply a second coat of primer once the previous one has been completely absorbed.

3) Skim and even out the surface with a 1 to 2 mm thick layer of WRAP 11 applied with a notched trowel while the WRAP PRIMER 1 is still wet If epoxy adhesive with a longer workability time is required, WRAP 12 may be used.

4) Smooth over the surface of the adhesive with a flat trowel to eliminate even the smallest irregularities from the surface.

5) Apply an even coat of WRAP 31 with a brush or short-haired roller on the WRAP 11 or WRAP 12 while it is still wet.

6) Position the WRAP fabric immediately after applying the resin, making sure that it is spread on evenly without creases. Wear protective rubber gloves for this operation.

7) Go over the surface of the fabric with a WRAP ROLLER to make sure the adhesive and resin completely penetrate through the fibres. This operation is also necessary to eliminate air bubbles trapped in the fabric.

8) Apply another coat of WRAP 31 over the WRAP fabric. Go over the surface of the impregnated fabric with a WRAP ROLLER to eliminate air bubbles trapped in the fabric.

9) If any other product is to be applied on the surface of the fabric, we recommend dusting the WRAP 31 with sand while it is still wet.

(ref. “Design Guide” procedure G.1.3 and technical specifications G.1.3.1 to G.1.3.5)*.

 

 

 

Flexural strengthening of floor joists

 

This type of strengthening may be achieved by applying one or more sheets of CARBOPLATE or one or more layers of uniaxial fabric, such as WRAP C UNI-AX, WRAP C UNI-AX HM, WRAP G UNI-AX, WRAP B UNI-AX or WRAP S FABRIC to the strained areas of the joist.

 

FRP FORMULA – DESIGN OF FLEXURAL STRENGTHENING FOR REINFORCED CONCRETE FLOOR JOISTS

The objective of the intervention is to increase the load-bearing capacity of floor joists due to a change in use that generates overloads.

Characteristics of the materials used in the calculation

Joist section: H = 190 mm + 50 mm;

Width of floor slab: W = 500 mm;

Width of joist: b = 120 mm;

Area of steel in tension zone: As = No. 3 ø10 mm ≈ 314 mm²

Area of steel in compression zone: not available ≈ 10 mm²

Steel: FeB38k (fyk ≥ 375 N/mm²)

Concrete class: C 20/25

 

Design bending moment acting on the section:

• Msd, ULS = 20 kN*m

 

The analysis of the loads generates a bending moment in the section higher than the initial bending moment.

 

 

DESIGN/CALCULATION PROCEDURE

The analysis is carried out using the semi-probabilistic “Limit State Design” approach, taking into consideration the combined loads and then multiplying them by a safety factor.

 

ACTIONS/STRESSES

 

 

 

The technique proposed, using products from the WRAP SYSTEM line, is applied “in parallel” to the existing structure. It acts in conjunction with the structure rather than replaces it, so that by working together the strength and load-bearing capacity of the strained areas of the floor joists are increased without modifying the distribution or stiffness of the masses.

TYPE OF STRENGTHENING ➠ No. 1 plate of CARBOPLATE E 170/100

Flexural capacity of the strengthened member:

Mrd, ULS post-strengthening = 35.14 kN*m/joist ➠ Msd < Mrd

 

APPLICATION TECHNIQUE FOR “CARBOPLATE” PLATES

Procedure

For flexural strengthening of joists with CARBOPLATE, refer to the previous pages regarding flexural strengthening of beams.

(ref. “Design Guide” procedure G.1.2 and technical specifications G.1.2.1, G.1.2.2 and G.1.2.3)*.

 

APPLICATION TECHNIQUE FOR “WRAP” FABRICS USING THE “DRY TECHNIQUE” AND THE “WET TECHNIQUE”

Procedure

For flexural strengthening of joists with WRAP, refer to the previous pages regarding flexural strengthening of beams.

(ref. “Design Guide” procedure G.1.3 and technical specifications G.1.3.1 to G.1.3.5)*.

 

 

Flexural strengthening of wooden beams

 

This type of strengthening may be achieved by applying one or more sheets of CARBOPLATE or one or more layers of uniaxial fabric, such as WRAP C UNI-AX, WRAP G UNI-AX or WRAP B UNI-AX, to the strained areas of the beam.

FRP WOOD – DESIGN OF FLEXURAL STRENGTHENING FOR WOODEN BEAMS

The objective of the installation is to increase the load-bearing capacity of wooden beams due to a change in use that generates overloads.

Characteristics of the materials used in the calculation

• Depth of the beam: h = 320 mm;
• Width of the beam: b = 230 mm;

 

Design bending moment acting on the section:

• Msd, ULS = 95 kN*m

 

DESIGN/CALCULATION PROCEDURE: “WOOD FRP” CALCULATION

 

TYPE OF STRENGTHENING ➠ No. 1 plate of CARBOPLATE E 170/50

Flexural capacity of the strengthened member in the section after strengthening:

Mrd, ULS post-strengthening = 115.93 kN*m ➠ Msd < Mrd

 

APPLICATION TECHNIQUE FOR “CARBOPLATE” PLATES

The plates may be applied by bonding them to the inner face of the wooden beam or by inserting them in specially cut seats (ref. ReLuis Guidelines 3.3.2.1.7).

Procedure

1) Support the floor slab with appropriate equipment to eliminate all stresses and any deformation caused.

2) Thoroughly clean the substrate to remove all traces of loose material from the surface.

3) Brush-apply a coat of WRAP PRIMER 100 (fluid epoxy impregnator in water dispersion for consolidating and priming wooden structures).

4) Apply a layer of WOOD PASTE 140 (thixotropic epoxy adhesive for structural consolidation of wooden members) on the inner face of the wooden beams to cover all the surfaces on which the plates are to be positioned.

5) Apply another layer of WOOD PASTE 140 in correspondence with the area where the plates are to be bonded, remove the protective plastic coating (peel-ply) from the plates of CARBOPLATE (pultruded carbon fibre plates preimpregnated with epoxy resin) and position them on the inner face of each beam.

6) Apply a thin layer of WOOD PASTE 140 on the inner face of the plates that have just been positioned and dust the surface with dry quartz sand while the resin (WOOD PASTE 140) is still wet to create a rough surface.

(ref. “Design Guide” procedure G.4.4 and technical specifications G.4.4.1, G.4.4.2 and G.4.4.3)*.