An experimental test program consisting of eleven reinforced concrete box girders with a length of 1300 mm, a cross-section of 350 x 350 mm and 200 x 200 mm an inner cross-section was carried out. Four control specimens were tested: two as box girders, one solid beam and one box girder with an opening. Two specimens with openings were retrofitted with steel plates and CFRP around the opening and two specimens without openings were retrofitted longitudinally with steel plates and CFRP sheets. Two specimens were strengthened with U-shaped steel plates and CFRP sheets. One specimen was strengthened with longitudinal and U-shaped CFRP sheets. All the specimens were tested under pure torsion up to failure. To validate the finite element model, numerical models were used and their results were compared to experimental results. After verification, different variables were studied such as the influence of the opening location, number of openings, area of the opening, change in longitudinal and transverse steel amount, and strengthening around the opening. The comparative study revealed that the number of openings and their dimensions are the most important factors influencing the torsional moment. Also, the strengthening around the opening compensates the torsional strength that was lost due to the opening.

An experimental test program consisting of eleven reinforced concrete box girders with a length of 1300 mm, a cross-section of 350 x 350 mm and 200 x 200 mm an inner cross-section was carried out. Four control specimens were tested: two as box girders, one solid beam and one box girder with an opening. Two specimens with openings were retrofitted with steel plates and CFRP around the opening and two specimens without openings were retrofitted longitudinally with steel plates and CFRP sheets. Two specimens were strengthened with U-shaped steel plates and CFRP sheets. One specimen was strengthened with longitudinal and U-shaped CFRP sheets. All the specimens were tested under pure torsion up to failure. To validate the finite element model, numerical models were used and their results were compared to experimental results. After verification, different variables were studied such as the influence of the opening location, number of openings, area of the opening, change in longitudinal and transverse steel amount, and strengthening around the opening. The comparative study revealed that the number of openings and their dimensions are the most important factors influencing the torsional moment. Also, the strengthening around the opening compensates the torsional strength that was lost due to the opening.