Fig 12 shows a beam used to support a shop sign that projects outward from a building wall. The beam is fixed rigidly at one end to the wall and the sign ha...
Question 1 Report
Fig 12 shows a beam used to support a shop sign that projects outward from a building wall. The beam is fixed rigidly at one end to the wall and the sign hangs from the free end. The weight of the sign causes the beam to bend.
(a) Name the type of beam shown in Fig 12. [1]
(b) Give one way the beam could be strengthened to better support the sign. [1]
Answer Details
(a) The beam shown in Fig 12 is a cantilever beam. [1]
A cantilever is a beam that is rigidly fixed (built-in) at one end and unsupported at the other. The fixed end must resist both the vertical load and the bending moment created by the weight of the sign acting at a distance from the wall. The upper fibres of the beam near the wall are placed in tension (being stretched) while the lower fibres are in compression (being squeezed), creating a bending stress distribution.
(b) One way to strengthen the beam: [1]
Add a diagonal support strut (brace) from the wall to the underside of the beam. This triangulates the structure, converting the cantilever into a supported bracket. The diagonal member carries the load in compression, transferring the weight of the sign more directly into the wall and greatly reducing the bending moment in the beam. Other acceptable methods include: using a deeper cross-section beam (increases the second moment of area), using a hollow box-section (efficient strength-to-weight ratio), using a stronger material such as steel, or adding a gusset plate at the wall junction.
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