Go to Top

Offset (exterior)

When we are looking for a form of a membrane we work with a certain geometry: vertices, cusps, straight edges, arcs, etc. This geometry is determined by a series of points with their coordinates X,Y, Z.

Once we have found a form we are interested in, we pass to analysis or patterning. To do that it is often necessary to add the rest of structural elements that are not membrane: cables, masts, arcs, etc. These elements have some actual dimensions which generate serious interference between them and make it impossible to realize such a structure when attempting to coincide with a series of points.

Let’s look at a typical example, we have generated a structure with a high point:

Solamente una membrana

Only a membrane

We are now going to add four tripods, one at each vertex. We do it and obtain the following final structure:

Estructura completa

Completed structure

If we zoom to the top corner we see that vertex does not work with real dimensions, since there are a series of elements that intersect the membrane.

Problemas en el vértice

Problems at vertex

The intersection of the rear cables is not serious. We will be able to resolve the issue of the intersection with the “ears” that serve to connect the cables.

Se mantienen los ejes

The axes are preserved

But on the part of the membrane, we find ourselves in a situation without a solution: membrane penetrates the tube. Moreover, we need to put all the detail of the membrane anchor to the tube.

We can trim the canvas, so that we come closer to the theoretical model, but even then we will build the detail in the theoretical space.

Trying to cut the membrane to simulate the theoretical model
indir-4 indir-5

The best and most common method is to separate the vertex of the membrane from the rest of the structure, using tensors, shackles or various other gadgets. But, of course, if we want to do this in the digital model, we have to create space (offset) between the apex of the structure and the apex of the membrane. The WinTess3 program, when we are in the Form finding module, if we click with the right mouse button on a vertex, or a apex, we get a pop-up menu that allows us to select the Offset option. Enter the value of the offset in cm and the program generates a bar at this length at the apex. This bar has the length of the offset and the tangent direction to the membrane at this point (the direction of the resultant of the prestressing)

Offset en el vértice

Offset in the vertex

We can now generate the rest of the structure, in this case, the tripod of the high point. If the offset created is long enough, the membrane does not intersect anything to the mast.

La membrana ya no intersecta al mástil

The membrane no longer intersects to the mast

Ramon Sastre,
June 2012