A domestic house would not be subjected to the same imposed loads as the office block therefore the proposed use of the building is very important when estimating the imposed loads. This estimation will then be taken into account in the when deciding what materials to use for the construction.
Wind loads play a much more important role in modern construction than they did in the past. In Victorian construction heavy masonry was a prominent feature which was not affected by wind loads. In modern construction where a steel framework is used wind loads affect the strength and stability of the building. Due to the fact that modern structures are constructed from lightweight materials the dead weight of the building may not be sufficient to hold it firmly in position. As a result the structure has to be:-
- Braced - to resist the horizontal load.
- Anchored to the ground - to prevent the structure from being blown away.
Two different examples of external bracing are, a high rise office building and a bridge, the high rise office has a vertical wind-bracing system is used to protect against wind loads, this system is usually hidden within the walls of a building while the bridge has a horizontal wind-bracing system.
When wind flows around a building it can produce quiet high suction pressures especially at the edges of the building. It is important if the building is to be protected that the cladding is firmly fixed to the structure and that the roof is firmly secured. The suction pressures increase with a decrease in pitch of a roof therefore it is important that flat roofs are firmly held down.
b)
Concurrent forces are forces that do not have the same , but they do act through the same point.
Non-Concurrent forces are forces that do not have a common point of intersection.
Coplanar forces are forces which act in a single plane, thus the forces are parallel to the plane and their points of application are in the plane.
c)
Point loads apply the whole load of the item to the same point whereas uniformly distributed loads distribute the total weight of the item equally spread over the item’s entire contact area, the contact area is large compared to size and weight of the load.
The maximum stress caused by a static point load will be twice the maximum stress
caused by a uniformly distributed load of the same total amount.
Compressive stress is when there is force pushing in the structure from each end so it is squashing it.
Tensile stress is the opposite to compressive stress as this is when the force is pulling from each end of the structure so it is stretching it.
Shear stress is when the structure is being pushed in opposite directions at each end of the structure.
Bending stress is when the structure has a force acting on it in one place which therefor makes the structure bend.
b)
A shear force has two signs associated with it, one defined by its direction, and one by the sign of the surface. If they are both positive or both negative, the shear force is positive. If one sign is negative and the other positive, the shear force is negative.
c)
The difference between positive and negative bending is pretty simple, if the beam is bending in an anti-clockwise direction it is positive and if it is bending in a clockwise direction, it is negative.
If a steel beam has an elastic limit of 200N/mm² and an ultimate bending stress at 300N/mm². The structural consequences of a bending stress of 250N/mm² would be that the beam will become weaker and the elastic limit will increase and therefore the beam will not be as safe as it was and may need to be replaced if it was part of a structure.
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