Beams AHL Topic 9: Structures

Beam Beams are structural members that are subject to loads acting normally to their longitudinal axis. The loads create shear stresses and bending moments and cause the beam to bend or flex. Beams are classified according to the way they are supported; for example, cantilever beams are rigidly supported at one end with the other end free.

Beams Beams are designed to transfer forces and distribute loads through the beams. Cantilever beam supporting a walkwaySolid wood beam to support a ceiling

A statically determinate beam, bending under an evenly distributed load. A bridge uses beams to support loads at supports. The supports experience tension and compression forces which causes it to flex. The engineering term for this is called bending moment. When a beam is under load or under its own self weight, the top portion or the flange of the beam has compression forces and the bottom has tension forces. These forces are equal and opposite to maintain static equilibrium in the cross section of the beam. To get a feel for this, cross your fingers together and the while keeping them together, stretch them out so the palms are facing away from you like you are yawning with your hands out above or in front of you. Notice the tension in your fingers. Your palm should feel like it is in a stretch or tension and the back side of your hands should look more wrinkle than usual. It is more wrinkle than usual because the skin has compressive forces in it and that is why there is compression and tensions forces in a beam.

Historical development The materials used to create beams have changed as new materials and manufacturing techniques have been developed. Solid wood beams were used, but were very bulky. Concrete beams were developed with metal rods inside to stiffen. Metal sectional beams can be riveted together and reduce the amount of material needed in the beam.

Sectional beams (members) A steel I shaped beam Circular shaped beams L shaped beam The shapes of sectional beams makes effective and economic use of materials.

Sectional members Sectional members of a structure can be manufactured from sheet materials, such as laminated veneer lumber. LVL can be used in place of other more expensive wooden beams where the finished product is hidden by other forms of cladding.

LVL LVL is an engineered wood product that uses multiple layers of thin wood assembled with adhesives. It offers several advantages over typical milled lumber: it is stronger, straighter, and more uniform. It is much less likely than conventional lumber to warp, twist, bow, or shrink due to its composite nature. Made in a factory under controlled specifications, LVL products allow users to reduce the onsite labour. They are typically used for headers, beams, rim board, and edge-forming material.

Factor of safety of beams The factor of safety in beams is essential as beams support weight and have to withstand a range of forces. A bridge with beams needs to be able to withstand a higher load as you would normally need. The roof of a building would need to withstand a higher load than the weight of the roof. WHY?