Science Focus Topic 6 Notes: Designing with Forces
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Engineers use their knowledge of forces to create designs that will most likely prevent the structures
Three key methods to help structures withstand forces are:
- distribute the load (in this way no one part of the structure carries
most of the load)
- direct the forces along angled components (so that forces hold
pieces together instead of pulling them apart)
- shape the parts to withstand the specific type of force acting on them
- Problem 1 - Frame structures experience load forces which can push
or pull them out of shape (Illustration p. 321)
- Problem 2 - A Horizontal beam structure that is supported only
at both ends will bend in the middle (Illustration p. 322)
- Problem 3 - Solid beams are often too heavy and use too much material
(Illustration p. 323)
Flying Buttresses - are columns on the outsides of a structure that
connect to the building near the top and are used to support the outer walls in much the same way that two sides
of an arch support each other. (Use an illustration)
All materials have their limitations. Materials
can be strengthened or weakened as they are made. (Concrete - if the correct recipe is followed, the concrete can
be very strong (compressive strength), but if the proportions are incorrect, the resulting concrete can crumble
and fail, however it does not have very good shear or torsion strength. Shear forces can be fatal in metal if the
shear strength is not analyzed when the metal is manufactured. The cooling process can eliminate almost all defects
if it is done properly.
(Swing Figure 4.57)
Using Frictional Forces
- The force of friction resists movement between two surfaces that rub
together. A brick wall is held together and kept evenly spaced with mortar, which helps
to create large friction forces between each brick.
- Friction is also important in frame structures. The friction between the nail and the wood keeps
the nail in place and the joints solid. Different types of nails provide differing amounts of friction. Squeaks
in floors are caused by fasteners that have loosened.
- Friction between the ground and the bottom of a structure is an important design consideration.
Friction holds the structure in place when external forces (wind) are acting on it. Too little, or too much friction
can cause problems (moving chairs across the floor).