A beam is a horizontal element of structure that carries weight mainly through tension. Beams bear or sit on columns or walls to transfer the weight from above to below. Loading a beam causes the top of the beam to be in compression (under the source of the weight) and the bottom in tension. Beams carry roofs and floors. Small beams over doors and windows are called lintels. Beams that project beyond the column (free of support) are called cantilevers. Cantilevers can be 1/3 of the length of the beam. A rule of thumb for beams is that they need to be ½ their span deep. In ancient times, a horizontal structure was called the entablature and had three parts: architrave (main beam); frieze (cross beams and ornamental sculpture) and cornice (overhanging crown). Beams were originally made of wood, stone or marble, but today are made of steel, concrete and glass!
Activity 1 – Types of Beams By Support
Four basic types of beams are classified by their supports. The most common type is the simply supported beam which sits on top of two columns- one which is fixed and the other is rolling. The second type is the built-in beam that is fixed in place between two walls or fixed supports. The third type is a cantilever beam that has one end fixed and the other end free. Cantilevers are subject to the force of bending (or movement up and down like a diving board. An example of a cantilever is when you hold your arm out at shoulder height. The longer you hold it out, the more stress you begin to feel in your shoulder, and when you get very tired you start to lower your arm! Your arm parallels the natural gravity of cantilevered beams. An important rule of thumb for cantilevered beams is that they can extend 1/3 the length of the beam from the support. The fourth basic type of beam runs along the width with intermittent supports holding it up throughout its span.
Draw and label the four basic types of beams.
Activity 2 – Build A Beam, Load A Beam
Beams carry and transfer weight. How do beams react when loaded with weight? Using simple materials from around your house (toilet paper tubes, pieces of paper, plastic cups), construct experiments to learn how beams react to loads, to see tension in action, and to learn how loads are transferred from above to below.
Stand two empty toilet paper tubes upright on the floor approximately 8” apart. Place an 8 ½” x 11” piece of paper on top of the two tubes or cups allowing for approximately 1” overhang. First, take an apple and place it in the middle of the beam; see how the paper beam immediately collapses, bending in the middle. The top of the paper is in compression and the bottom is in tension. Next, take the same piece of paper and fold it like an accordion with 1” folds. Now place the paper beam on top of the two tubes allowing for approximately 1” overhang off of each toilet paper tube. Replace the lemon carefully in the middle of the beam and see that the paper beam can now carry the apple! The paper beam is stronger when it is folded and can carry more weight. Take a picture of your paper beam holding something and upload it into the gallery!
Activity 3 – Beam of Different Materials
Beams come in many shapes and sizes and many lengths. Some beams are solid wood, some are laminated wood, some are hollow (some steel beams) and some beams are made of concrete with reinforcing steel inside of them. Structural Engineers and architects choose the type of beams to use in the construction of a building based on the material’s availability, weight, span, strength, fire resistance, dimension, and aesthetic. A rule of thumb for a beam is that it must be as many inches deep as the number of feet it spans. This rule of course varies with the strength and type of different materials. For example, take a look at this listing of diverse types and the size and average spans of Wood Joists and Rafters.
Activity 4 – Beams of Different Shapes
Beams come in different materials and in different materials they are often different in shape. For example, concrete beams, constructed in factories come in many shapes. They come as “T” beams, inverted “T” beams , I shaped beams, and even hollow square beams.
Be a beam engineer!
Activity 5 – Finding Beams
Take a 15 minute walk around your neighborhood with a digital camera. Take pictures of as many different beam types as you can find. See if you can find an entablature, a lintel, and a cantilever. Print your beam pictures, identify the material if possible, name as many parts as you can and label the pictures neatly. Label bearing areas, places of tensionandcompression, and overhangs or cantilevers.
- Which structural force involves stretching?
- What three parts does the entablature have?
- A rule of thumb for the depth of the beam to its span is:
- An overhang is also called a:
- A small beam over a door or window is called a lintel.