Explanation:

Plate girders are widely used in the construction of bridges and buildings. The main purpose of a plate girder is to support a load and transfer it to the columns or piers. A plate girder consists of a top and bottom flange connected by a web plate. The web plate is subjected to various types of forces such as shear, bending, and compression. To prevent buckling of the web plate, horizontal stiffeners are provided.

Compression Buckling of Web Plate:
Compression buckling of the web plate is a common failure mode in plate girders. When a compressive load is applied to the web plate, it may buckle out of plane. This buckling can cause the web plate to fail and the girder to collapse. To prevent this failure mode, horizontal stiffeners are provided. The horizontal stiffeners increase the stiffness of the web plate, which reduces the likelihood of buckling.

Shear Buckling of Web Plate:
Shear buckling of the web plate is another failure mode that can occur in plate girders. This failure mode is caused by shear forces acting on the web plate. The web plate can buckle in a similar manner as in compression buckling. However, the cause of shear buckling is different. Horizontal stiffeners also help to prevent shear buckling by increasing the stiffness of the web plate.

Yielding:
Yielding is another failure mode that can occur in plate girders. This failure mode is caused by excessive stress in the web plate. The web plate can yield, which can cause the girder to fail. Horizontal stiffeners can prevent yielding by increasing the strength of the web plate.

Conclusion:
In conclusion, horizontal stiffeners are provided in plate girders to prevent buckling of the web plate. The horizontal stiffeners increase the stiffness and strength of the web plate, which prevents buckling due to compression, shear, and yielding.