When an industrial building is being designed (industrial warehouse, airport or train hangar…) it is indispensable to consider the existence of some kind of bridge crane for general use to run some of the typical activities of these kind of facilities.
A normal overhead bridge crane is an element made of a beam supported and guided by two rails on both ends that allow its movement, generally rolling. A moving hoist is mounted on the beam and its role is to lift the loads and move them along the beam.
In short, There are two movements for the load (not counting the vertical lift move, which is of no interest for structural calculations): a longitudinal movement with respect to the rails guided by the bridge crane and another longitudinal movement with respect to the beam that constitutes the beam (transversal with respect to the rails). The first one is the one that will involve a displacement of the load that will affect the rail beams since it is a load that is moving along them and the second one will involve a distribution of the load over them which will be proportional to the eccentricity over the bridge beam.
Implementing a calculation model that considers this type of loads implies an important workload, regarding the definition and situation of them, if every possible scenario of the states it can be found in is to be considered and, consequently, all the hypotheses and load envelopes for the design of the structure. Fortunately STATIK-7 has a tool with to generate loads which is very interesting in these cases.
Said tool is the ‘Generator for General Moving Loads’. With it you can generate loads, from a determined group of loads, in arbitrary positions along a rail. Taking into account that the positions of a group of loads are mutually exclusive (they appear alternatively), the generated loads will be checked for ‘exclusive overlapping’ (this option will appear checked and it will not be possible to change it in the dialog box. See the figure below). The rail can be made of various independent parts, like on the example of the bridge crane, in which we have two rails.
After creating a new load generator of the type ‘General moving load’, the buttons used to configure it are shown on the ‘Loads’ tab.
The load elements entered with the two first buttons define the group of moving loads. Said group can consist of any combination of loads and it can be positioned in any part of the rail (see the figure below).
Using the third button:
The handle point used to define the position of the group of loads will be created.
The fourth button:
Allows the creation of the positioning polyline, the points of which define the position of the group of loads (figure above). The loads, situated with respect to the handle point, will position themselves at each of the points defined by the polyline.
Let’s see an example of a bridge crane over a simple metallic structure like the following one. The bridge crane would be supported on the two points lateral beams that are pointed out:
Once the structure is introduced, the definition of the load generator must be done as it is usual in STATIK-7:
Then, one must introduce the loads. We will suppose a self-weight of the bridge crane, without the hoist, of 80 kN. The hoist with its maximum liftable load will be 400 kN. The load group will be made of two concentrated loads of 0.5 kN each ( total of 1kN) and it will have a handle load in between both of them. This group of loads will represent the two wheels on each of the beam supports.
To group both loads they have to be selected and, right clicking, attribute box has to be opened. In the options tab the name of the group should be defined in the highlighted area on the figure below:
After that, the polylines are introduced over the selected rails clicking on the points on which the moving load is going to be positioned. The polylines show up in green as it can be see in the following figure, where all the positioning points can also be seen:
Because the example given is a bridge crane, the displacement is materialised in parallel, that means the support points of the beam will advance at the same time over both rails. To implement this effect, the process of combining positions is done. In the example, this would entail combining opposite positions P1-P7, P2-P8, P3-P9, P4-P10, P5-P11 and P6-P12. To do that we will use the option on the properties box of the position point:
To combine P7 with P1 one simply has to open the properties box of P7, check the option ‘combining of position’ and indicate which point to combine it with (1):
This will be done as many times as couples we need.
In the attributes of the positioning points there is a factor that has great importance for the correct definition of these loads. That factor is the load factor:
This factor will allow the definition of the load distribution on a support and another one depending on the eccentricity of the load on the bridge beam. In the example, the bridge beam measures 13 m, if we assume that the load is at a distance of support P1 of 20% of the total length of the beam, we will have a distribution such as:
For the bridge beam:
80*0.5 = 40
For the hoist + the load:
0.8*400 = 320 on the closest support
0.2*400 = 80 on the furthest support
To show that distribution the corresponding load factor will be introduced on each of the attribute boxes for P1 and P7 (same with the rest of the couples):
We want to highlight the convenience of defining the group of loads with a value of 1 kN (as in this example) so that the load factor is the value of the real loads. In this way we can avoid having to introduce complex factors to affect the loads that can lead to error.
Finally, one has the following sequence of the load advance for this example with the asymmetrical distribution indicated:
If we want to take into account the different load positions of the bridge crane, it will be enough with defining different load generators, task that is, as we have seen, incredibly easy.
Once the generators are defined, the load envelopes are generated correctly and automatically, reflecting the fact that the loads will be in positions P1 and P7, P2 and P8, etc.
Imagine all the different load hypotheses that you would have to create as well as the complex creation of the enveloped. With STATIK that is a really simple task.