1. Check the local building code for the allowable load capacity.
2. Call your nearest block supplier and ask for a copy of one of his test reports to verify the tested load capacity and use it with an appropriate factor of safety.
Blocks are manufactured having a wide range of strengths. Here in UK, the weakest in the code is 2.8N/sq mm (400 psi) and they go up to '35 or greater' N/sq mm (5000 psi). The allowable bearing stress depends on the block strength and the mortar mix you choose, and then you have to consider things like slenderness of the wall or column, eccentricity of the load, factors of safety depending on block manufacturing standards, workmanship standards and so on. So it isn't that simple - get hold of a copy of your local code!
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High bearing pressures occur at high localised stresses, for example where lintels bear onto masonry. These stresses disperse quickly through the masonry at 45 degrees. This is recognised in the codes some of which allow an increase in the value of local design compressive stresses.
BS5628 considers 3 types of bearings: type 1 & 2 where the element applying the load is rigid, or provided with a stiff spreader, the difference between 1&2 being the "area of bearing of the load and its proximity to the end of the wall". Type 3 bearing comprises a spreader beam located at the end of a wall and spanning in its plane.
Based on this this the stress value under each bearing type must not exceed (1.25comp strength of masonry, fk/safety factor for material strength,Ym) for type 1 bearing; 1.5fk/Ym for type 2 condition; 2fk/Ym for type 3.
The additional factors of safety are applied to the loads acting on masonry (c.f steel, concrete), and also to the materials of construction, as masonry is a site assembled material, and the finished strength of the material is related to the degree of workmanship involved in its assembly (i.e. skill of the bricklayer), the factor of safety for materials takes into account the degree of supervision given to the work.
