Compressive Membrane Action

What is Compressive Membrane Action?
Compressive Membrane Action is an inherent strength enhancement which develops when a slab tends to move against lateral restriction.
Although this is the definition given for compressive membrane action, I know this is not clear enough to understand.

Well, lets take an example!









Picture 1 : (Pictures Courtesy: http://www.answer.com/)

From the above example we can see that neutral axis of a simply supported slab strip is located near the mid depth. What will happen when the slab strip started to crack? The slab will fail.

However, if a slab is laterally restrained, then it behaves differently. The initial behaviour prior to crack formation illustrated in the Figure1.

Since the tensile strength capacity of the slab is lower than compressive strength capacity, cracks will occur at tension zones. In the fixed end slab, the tension zones are in three locations. Top face of the slab strip near the supports and mid span bottom face.

Figure 1: Before cracks

As a result of cracks, the neutral axis migrates to the very compressive zone. Thus the resultant structure can be explained as it is on Figure 2.

Figure 2: After cracks

The neutral axis shift leads to a new structural form. The difference between the structure prior and after the neutral axis shift can be explained in Figure 3. Due to the cracks hinges forms. Thus structure tends to expand from the loading point towards the in plane. Thus if there is a lateral restraint then it induce compressive membrane action.

Figure 3:

When the structure loaded, any vertical deflection should be accompanied by a lateral expansion. If the lateral expansion is restrained with a fixed end, then there will be lateral force induced and thus compressive membrane action induce (See Figure 4).

Figure 4: CMA