How much load bearing capacity a concrete block can hold is determined by a number of factors such as how compact the soil is, the concrete mix, the age, (the hydration and hardening process of concrete causes it to become stronger over time), the size, shape and structural content all have to be taken into consideration. For example, the weight that is brought to bear upon a concentrated point or when it is distributed over a wider area. A block that is used to support an individual column will bear less weight than a load distributed across a network of support from other load bearing units or columns. When weight is uniformly distributed it requires twice as much weight to break it as it would were the weight concentrated at the centre.
Gray Car on Concrete
Concrete is measured for strength in PSI meaning pounds per square inch of pressure that it can bear. A typical residential patio is 4″ thick with an average of 3000 pounds per square inch ratio. It is designed to carry more than the weight of two automobiles. The average compact car is about 3,000 to 3,500 lbs. A mid size car, van or light truck is about 5,000 to 7,000 lbs. A full size truck is 7,000 to 12,000 lbs.
Two automobiles covering the same amount of space far outweigh an oversized heavy duty timber frame pergola. The overall total weight of an oversized pergola is nearly 4,000 lbs. which would compute to be around 1,000 lbs. of undistributed weight for each beam. Simulating this would be like placing 4 extra large sized men standing on one corner. After the weight is uniformly distributed between three other beams the PSI is considerably less.