Dead and live loads
From Wikipedia, the free encyclopedia
Dead and Live loads are terms used in mechanical and structural engineering, especially where analysis of real world objects are required. A 'load' refers to any type of force exerted on an object, which may be in the form of a 'weight' (gravitational force), a pressure, or anything which affects the object in question.
Contents |
[edit] Dead loads
Typically dead loads are considered those which are static, i.e. do not change over the course of normal operations of the object. For example, in the analysis of a staircase where the handrails are attached to the main structure but are not the subject of the analysis and so not included in the model the dead load would be considered to be -
- Self weight of the staircase (9.81 x mass)
- Force exerted by the weight of the handrails, applied to the point of attachment to the staircase (again calculated as 9.81 x mass)
[edit] Live loads
Live loads, sometimes referred to as dynamic loads include all the forces that are variable within the object's normal operation cycle. Using the staircase example the live load would be considered to be -
- Pressure of feet on the stair treads (variable depending on usage and size)
- Wind load (if the staircase happens to be outside)
[edit] Real world usage
The reason for splitting loads into these categories is not always apparent, and in terms of the actual load on the object there is no difference between dead or live loading. For the most part, the split occurs for use in safety calculations or ease of analysis on complex models.
When considering the feasibility of a structure, safety always takes precedent and because of this governing bodies around the world have regulations to which structures have to adhere. Using the example of the staircase, if it was intended for use in the UK it would have to follow British and European Standards
- BS 4592 - Industrial type flooring and stair treads
- BS 5395 - Code of practice for the design of straight stairs
- Other standards specific to the application (e.g. BS 14122-3:2001 - Permanent means of access to machinery. Stairways, stepladders and guard-rails)
Within these standards a safety factor is usually determined where the structure should be able to withstand a certain force above the maximum expected load. Once again using the staircase example, assuming it is an indoor medium-usage industrial staircase the current safety factor would be 1.4 times the maximum stress imposed by the dead load and 1.6 times the maximum stress imposed by the live load. The reason for the disparity between values, and thus the reason the loads are initially categorised as dead or live is because while it is not unreasonable to expect a large number of people ascending the staircase at once (or the wind speed increasing, snowfall or any other live load increase), it is less likely that the structure will experience much change in its permanent load. The same can be said of many structures and so it is convenient to assess loading based on its application.
[edit] Calculating combined loads
It is worth noting that on first inspection it seems you should find the maximum stress for each of dead and live, factor them and add them together. This will give you a massively overestimated stress result. The combination needs to be applied with great care and almost exclusively programmatically because you may only combine two stress results at the same point. Since the maximum stress is very rarely at the same place in a structure for dead and live it may well be the case that the overall increase is a fraction of the addition of the two maximum stresses and in a completely different position two either of the two original maximums. To clarify, take the staircase analysis. The maximum stress under dead load appears at the foot of a support beam and it is 80 Nmm-2, at this point the stress from the live load is 5 Nmm-2. The maximum stress under live load is 60 Nmm-2 and appears at the corner of the second stairtread where the dead load stress is 30 Nmm-2. At a third point the stresses from both dead and live are 50 Nmm-2. Given these figures you can see that the combined load cases for each point would be:
- 1. 80 x 1.4 + 5 x 1.6 = 120
- 2. 30 x 1.4 + 60 x 1.6 = 138
- 3. 50 x 1.4 + 50 x 1.6 = 150
As you can see the maximum combined stress appears away from both the original maxima but is still well under the 275 Nmm-2 yield point of the structural steel this staircase is made of so in this case we could say that the structure is safe.
[edit] See also
- The Hotel New World disaster was caused by a miscalculation of the dead load of the building.
