Foundations of tensile buildings

Foundations of traditional buildings (structures of stone, brick, wood, steel, etc.) are very different. Depending on the type of soil, its stratification, the type of the structure (frames, continuous…) and the type and magnitude of loads, a typology of diverse foundations is generated: isolated, strip, off-centre footings; foundation beams and slabs; piles, etc.

However, despite this diversity, often a common factor is found. The predominant load on the foundation is compression. A force pushing down the foundation and must be countered by the resistance of the soil beneath the foundation (or next, friction, in some types, such as certain piles).

It is true that we also can find other cases with other types of loads; especially in structures with columns built in the foundations (at least this is the usual way to calculate the structures). These columns have at its bottom shear forces and bending moments that have to be balanced. Shear forces are usually not very important and, therefore, in most cases the problem is solved with the tensile or compression resistance offered by horizontal braces that are arranged between different footings.

As for the bending moment, it may be important at the border columns of frames and especially in the off-centre footings (for example, under border walls). In these cases solutions are diverse, but often strap beams between this footing and the next one inwards is the most usual solution.

Even so, in all these cases, vertical load of compression is the most important load and the one which determines the type of foundation to use.

In tensile buildings we can also find foundations like those just discussed, for example, in the central vertical mast of a conoid-shaped roof, but these cases are less frequent.

In general, foundations of typical tensile buildings can be classified depending on the load to be resisted, in any of these types:

1. Vertical pulling load (vertical cable)
2. Leaning pulling load (any cable or guy-rope fixed to the ground)
3. Leaning pushing load (tilted masts, base of an arch, etc.).
4. Vertical pushing load (vertical mast)5. Bending moment, with vertical and horizontal loads of lesser importance (especially vertical columns supporting a membrane, without stays or guy-ropes)

As we can see, only type 4 is what we would call traditional foundation. Type 3 could be considered traditional if the horizontal component was very small or if it was possible to establish a grid of straps between these and other footings. If not, we’ll see that the design of the foundation is quite different.