# Why are basement walls subject to tensile forces?

I am teaching a mixture of English and Civil Engineering to a student, while not an Engineer.

One of our textbooks states:

Underground walls have to resist both tensile and compressive forces

Compressive: I can understand. The weight of the building above the wall, and the force of the earth pressing in against the walls from the sides.

But where are the forces trying to pull the walls apart?

• Are “underground walls” the same as “basement walls” ? Jan 29 at 0:11

All walls experience tensile stress. This is not a force trying to pull the wall apart as such. The stress comes from lateral forces that can arise from two sources.

Firstly, a simple lateral force on the wall, for example in a basement, the outside soil. Secondly, the weight of the building above causes the walls to bow outwards. This second source of force is why large walls have buttresses, to reduce bowing. See for example cathedrals with flying buttresses.

A wall with a bow experiences compression on the concave face and tension on the convex face. Most building materials, like stone or concrete, are weak in tension - again a reason for butressing to reduce stress to acceptable levels.

• Actually, the load on walls don’t cause them to bow “outward”, the load exceeds the maximum height of the wall before it bows in or out. The flying buttresses just keep the wall perfectly straight…(keeping it from bowing in or out. ) Jan 29 at 2:40

When you bend a beam or a wall then the following happens:

the outer curved surface is in tension,

the inner curved surface is in compression and

the centre of the beam / wall is zero stress. The stress increases the further from the centre line you measure.

• I'm not sure this is universally true. A beam or wall that's retained at each end could be in tension throughout its section. Jan 28 at 17:28
• The center of a beam (or wall) when it bends is not in zero stress. The center is the highest for horizontal shear. Jan 29 at 2:42
• Bending stress is highest the further from the center. (Bending stress is affected by the extreme fiber in tension or compression.) Jan 29 at 2:46
• It’s called “slenderness ratio” when the wall (or column) has too much load for it’s height. You can look it up. Jan 29 at 23:29

There are two kinds of basement walls: 1) slab or beam type, and 2) retaining wall type.

1. The slab or beam type is the most popular. It is designed to span from the footing up to the first floor framing above. The floor framing supports the wall at the top and the footing supports the wall at the bottom. This creates a slab-like (or beam-like) structural design. So, when the soil presses against the wall it puts the wall in bending, (with compression on one side of the wall and tension on the other) much like a typical beam. Most engineers like to think of the wall as a beam so they can calculate how much reinforcing is required per foot.

2. Sometimes the wall is designed as a retaining wall and is supported only at the footing. So, when soil presses against the wall it bends like a retaining wall with compression on one side of the wall and tension on the other.