Diseño de una grua

Practice 2: Design of a truss type crane
Department of Continuum Mechanics and Structural Analysis

09/05/2011
100283716 Bravo Conde, Olalla 100276408 Gómez Lendínez, Daniel100276583 Gesé Bordils, Francisco Javier

Index

Index page 2
Previous considerations page 3
Design process page 4
Indeterminate trusspage7
Calculations page 8


Previous Considerations

The objective of this practice is to design a truss type crane by using a computational tool: ED-Tridim.
The first thing we want to notice here is that we had some problems when we tried to understand the restrictions of the problem, because we knew that the load must be placed at 12m of height and at 6m from thebase. But the question was if we could draw for example a straight line from the base to the load.
We decided on designing the truss like a usual crane because otherwise it will not make sense to have the load placed 6m far from the base if you can build the tower crane very close to the load reducing the stresses.
Another thing we have used during the design process is that the program computesonly the axil force but not the stress in the bar, so we assigned the same section to the entire crane to get the values of the axial force and later assign the minimum section which fulfils the stress of 200 MPa of the steel.

Design Process
The first idea was to build one of the simplest cranes in order to observe the distribution of the forces in the crane. Since it was the first design wecalled it crane (1).We were surprised because there were a lot of bars that weren’t working at all, so we thought that we could take advantage of this distribution by reducing the cross-section area as much as we could.

After that we tried to reduce the total length of the bars in the vertical part of the crane, so we did another crane design: crane (2). Nevertheless we realised that there weremore bars working and the maximum stress, located in the upper left bar (extreme) had increased.
We also tried with crane (3) to have the maximum base size in order to have the minimum reaction forces in the supports, but we noticed that the normal stresses in the bars of 450 were not negligible.

crane (3)
crane (3)
crane (2)
crane (2)
crane (1)
crane (1)

With the results of theprevious designs we though that the best way of distributing the bars along the tower of the crane was to form a square by joining bars of 45o. So we designed crane (4).
Once we decided on the structure of the vertical part (tower) we focused on the horizontal and upper part. The first idea in this step was to add one bar so that the stresses were more evenly distributed. However, this did not work.That was crane (5).
In crane (6) we reduced the height of the horizontal part of the crane to see the behaviour of the crane. The orientation of the bars was such that we could learn that the bigger stresses would be at the upper left corner.

crane (4)
crane (4)
crane (5)
crane (5)
crane (6)
crane (6)

So the idea was clear: increase the length of the bars in this zone in order toreach the best distribution of the load. We tried a first design with crane (7) and we realised that we were on a good track.
crane (9)
crane (9)
crane (8)
crane (8)
crane (7)
crane (7)

Having those results we developed a new design for the upper part; we thought that with a triangle we could at least decrease notably the total length of the bars.
The next designs produced good and bad...