The effectiveness of CFRP strengthening of steel plate girders with web opening subjected to shear

The present study is experimental by nature; it involves applying shear buckling test on seven steel-plated girders that are subjected to shear loading. Four of these girders represent the strengthened girders whereas the other three are reference girders. One of the latter type of girders has a square web opening; the second has a diamond web opening while the last one has no opening at all. The webs of the strengthened girders were adhesively attached to CFRP sheets of different patterns. This step was carried out to evaluate the most effective strengthening scheme by CFRP composite. The study aims at examining the effect of several parameters on the behavior of web girders. It further examines the percentage of increase in the ultimate shear capacity of the perforated girders that have a fixed location and size where the latter is equal to 40 percent of the web depth. Results have shown that, first, the ultimate shear load of the CFRP-strengthened girders with square web opening is higher than that of the reference girder with a square web opening; ranging from 8.7% to 15.7%. The obtained ranges depend on the orientation of the CFRP strips. Second, the ultimate shear load of the CFRP-strengthened girders with a diamond web opening is higher than the reference girder with a range of 9.8% to 21.5%. Again, the obtained ranges depend on the orientation of the CFRP strips. Analytically speaking, Von Mises stresses have been used to predict the ultimate shear load of girders with square and diamond web openings and without web opening.


Introduction
Many research studies have been so far conducted on the use of carbon fiber reinforced plastic composite in both strengthening and repairing the webs of girders.These previously conducted studies mainly focused on the application of CFRP to girders with axial or bending stresses.As far as the investigations on strengthening and repairing web girders using CFRP are concerned, they have been reported few.
The standard strengthening techniques used with steel-plated girders include bolting or welding.However, these techniques are not void of disadvantages; cases in point are the effect of corrosion, the fatigue-sensitivity of the repaired system to the stress concentration resulted from either the bolting or welding techniques.Another shortcoming is the long period of service interruption.Recently, alternative girder strengthening techniques have been used, such as that of the external bonded (CFRP) composites.Such techniques help prevent web buckling in steel-plated girders by adding stiffness to the web of girder.In other words, the stiffness of CFRP provides the required high stiffness, tensile strength and corrosion /fatigue resistance to the girders [1,2,3,4].
In modern buildings and bridges, large ducts for aircondition system, sanitary pipes, sewage pipes, electric cables and other services need to be provided.However, to provide such services, an unacceptably large construction depth between storeys is needed.To solve this problem, a web opening needs to be made on each of the web girders.This helps in extending the pipes of services and reducing the floor height.However, such web openings change the distribution of stress at the web of girders and ultimately reduce the shear and bending strength capacity of the girder [2,3,4].
To restore the lost strength, the locations of these openings and the lost strength need first to be studied and measured, respectively.Second, the lost strength in these locations and the locations surrounding the openings needs to be strengthened with different patterns of CFRP.In addition to that, one should avoid making such openings in locations where the shear stresses are high [5].

Material properties
The properties of the materials used in the tested plate girders include the following:

Steel
Table (1), below, shows the details and properties of the steel plates used to form the seven steel-plated girders obtained from the Tensile strength test, as shown in Figure (1).In this study, the modules of elasticity and poison '

CFRP Laminate
The type of CFRP used to strengthen the girders is sika-warp-230 c produced by Sika Corporation, as shown in Figure (2) and Table (2), where the latter is about the properties of CFRP provided according to manufacturer data sheet attached to the CFRP laminate.

Adhesive epoxy
The suitable adhesive material used with the carbon fiber strips is Sikadur®-330, as shown in Figure (3).This type of adhesive consists of two parts: part A, resin, which is white and part B, hardener, which is grey.The main properties of the adhesive are provided according to manufacturer data sheet attached to the adhesive package, as shown in Table (3).

Type and details of the plate girders
Two types of plate girders have been used in this study.The first type is without web opening whereas the second is with a web opening.The second type is further sub-divided into two types: square and diamond web opening.The length of web opening is 240 mm and the shear stresses in the webs are relatively low in comparison to the bending stresses in the flanges.As a result, the 2mm web plate is generally chosen to be much thinner than the flanges which are (6mm) [7].For more details about the girders, consider Figure (4) and Table (4) where three of the girders are reference whereas the other four are strengthened girders with different patterns of CFRP laminate.

Types of strengthening
Type1 includes strengthening the web of girders using CFRP strips of 10mm width from two sides.The first side is parallel to the flanges while the second is perpendicular to flanges, as it is used in (T2-D-1), (T2-S-1) girders shown in figure (5).All plate girders were simply supported by a pin and roller over 1200 mm span.The girders were tested up to the failure mode under the action of applied load at the mid span of the girder.Such an application helps ensure that each of two panels equally have half of the applied load; see Figure (7).The maximum load capacity of the tested machine (Avery) is 120 tons and the deflections of all girders were measured by a dial gauge of 0.01 accuracy installed at the mid span under the girders.The ultimate shear strength can be obtained as follows: The critical shear stress (τ cr ) in this stage is given through adopting Gardiff Method: Figure ( 9) shows the load deflection curves for the tested girders (T2-0) without web opening and (T2-S-0) with square web opening where both represent the reference girders.In addition, the girders (T2-S-1) and (T2-S-2) were strengthened by CFRP strips.Results have shown that the square web opening decreases the ultimate shear load resistance to 38%, while the girders (T2-S-1) and (T2-S-2), which were strengthened by CFRP strips, as in Type 1 and Type 2, show higher stiffener and a high load than that of (T2-S-0).Fig. 9. Load -Deflection Curves for (T2-0), (T2-S-0), (T2-S-1), and (T2-S-2) In the vein, the CFRP strips-strengthened girders were all painted in light grey.Besides, the test was conducted on only one panel of girders whereas the second panel was diagonally strengthened by a woodmade stiffener.This is because the girder is symmetrical in shape and has two panels.As it was previously mentioned, each panel should equally carry half of the applied load.Accordingly, and economically speaking, there is no need for the two panels to be tested.This is because both the first and second panels of the girders share common results and behavior.Figure (10) portrays the failed mode of all girders after the tests.In this respect, the researchers compared between the analytical and experimental results.In addition, the ultimate shear load was compared with its reference girder, as illustrated Table (5).3-The ultimate shear load for the girders with a square web opening strengthened by CFRP strips was higher than the reference girder with a square web opening.The ranges of the ultimate shear obtained are 8.7% and 15.7%, depending on the orientation of the CFRP strips;

4-
The ultimate shear load for the girders with a diamond web opening strengthened by CFRP strips are higher than that of the reference girder with a diamond web opening.The ranges of the ultimate shear obtained are 9.8% and 21.5%, depending on the orientation of the CFRP strips;

5-
The experimental results of the plate girders have shown the applicability and effectiveness of the strengthening technique used in girders with a web opening.Furthermore, the technique is shown to increase the shear strength of these girders; and

6-
The most effective form used to strengthen the web opening of plate girders was type (1).This type was strengthened by CFRP strips that were parallel and perpendicular to their flanges.

Fig. 7 .
Fig. 7. Test Setup for a sample of Plate Girder

2 (FigureFig. 8 .
Figure (8)  shows the load deflection curves for the tested girders (T2-0) without web opening and (T2-D-0) with diamond web opening as reference girders.Moreover, the (T2-D-1) and (T2-D-2) girders were strengthened by CFRP strips.Results have revealed that the diamond web opening decreases the ultimate shear load resistance to 44%, whereas the girders (T2-D-1) and (T2-D-2) which were strengthened by CFRP strips, as in Type 1 and Type 2, show a higher stiffener and a high load than that of (T2-D-0).

Table 1 .
Properties of Steel Plates Used

Table 4 .
Details of Plate Girders in (mm)

Table 5 .
The Analytical and Experimental Ultimate Shear Load for the Girders in comparison to their Reference Girders