Support reactions are developed directly in response to the loads applied to the structure (and any self-weight of the structure). Remember, if our structure is subject to externally applied forces, reactions must be developed at supports to ensure the structure remains in a state of static equilibrium. Once the supports for a structure have been appropriately modelled, we must determine the magnitude and direction of the various reactions developed at the supports. 3.4 Reaction Forces & Statical Determinacy One of the key takeaway messages from this section is that, although we’ve presented 3 neatly defined models of support, we must be careful in placing too much trust in the output of analytical modelling and recognise that actual foundations rarely adhere perfectly to this idealised behaviour. This is one of the reasons why it typically takes a minimum of 4 or 5 years of post-graduation experience before professional chartership can be achieved. During the first 3 to 5 years as a practicing engineer, the intuition for relating real-world behaviour to mathematical models is developed. An intuitive understanding for how well analytical models approximate real-world structural behaviour is something that comes with practice and experience.īroadly speaking, a formal training in engineering will largely focus on developing knowledge of analytical models and how to manipulate them. The ability to appropriately map real-world behaviour onto our analytical models is a key part of becoming a competent engineer and is essential for any engineering analysis. Three foundation support configurations (top three) and their potential models (bottom two). But supports and their associated restraints will make more sense when we consider them applied to structures below.įig 6. This might be hard to visualise or understand right now, because we’re considering the support models in isolation. a moment reaction can be generated at the support a horizontal reaction force can be generated at the support a vertical reaction force can be generated at the support If a structure is supported by (or restrained by) a built-in support, the structure will in theory be restrained against: 3.1 Built-in / cantilever / encastre supports There are three common support/restraint models we employ to approximate real-world foundation behaviour. We also need to know what kind of support or restraint a particular foundation or support type will provide to our structure. In order to design foundations that will not experience unacceptable settlements for example, we need to know the forces and moments imposed by the structure. That reaction force is equal in magnitude to the gravitational force imposed by the object onto the surface.Īs engineers it’s important that we understand the forces and moments our structures impose on their supports or foundations. Therefore, the reaction force in this case is the force imposed by the support surface back onto the object. Newton’s third law states that for every action there is an equal (magnitude) and opposite (direction) reaction. We saw above that an object resting on a horizontal surface, experienced a reaction force. 3.0 Supports and Reaction Forces and Moments We’ll see this in action when we evaluate support reaction forces and moments below. As there are three equations we can solve for up to three unknown forces or moments. We often use these three equations to identify unknown forces or moments in a system that is in a state of static equilibrium. If any of these conditions is not satisfied, the object will not be in a state of static equilibrium and will undergo a change in its velocity (i.e. We could equally condense this and say simply that the sum of all forces on an object must equal zero, Note here that we are considering the orthogonal components of forces experienced by an object separately. If we again only consider forces and moments in a 2D plane (as we did for our discussion of equivalent systems in the previous tutorial), we can say that an object is in a state of static equilibrium if it satisfies the following three conditions, For now, we will just concern ourselves with objects in a state of static equilibrium. Such objects are said to be in a state of dynamic equilibrium. From this we note that equilibrium can also apply to moving objects provided they are moving at a constant velocity. This is a direct result of Newton’s first law which states that a body remains at rest or moving with a constant velocity unless an unbalanced force acts upon it. The inertia force present due to the fact that the mass has an acceleration,Īn object is said to be in a state of static equilibrium if the sum of all forces and moments acting on the object are zero and the object is at rest.The drag force due to the presence of air,.The tension force in the suspension cable,.Free body diagram for a swinging pendulum (right).
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