May 25, 2008, 1:15 PM
Post #1 of 7
Numerous comments on this forum and elsewhere indicate that there is a widespread and fundamental misconception of the construction used for low-rise housing in Mexico, and in other countries with significant earthquake risk. Popular belief has it that the castillos and dalas (vertical and horizontal reinforced concrete elements) form a load-bearing frame, and the bricks are therefore merely non-structural infill. This (potentially dangerous) view appears to arise through people from NoB equating it with the timber-framed construction that is prevalent there.
Mexican House Construction - a Common Misunderstanding
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In fact this type of construction is not framed at all, but is a form of masonry wall construction. Here, instead of a frame, it is the entire walls that form the load-bearing system. The structural elements are the bricks/blocks/stone and mortar from which the walls are made. This system normally includes all external walls; internal walls are frequently also load-bearing.
In countries that are seismically stable, plain unreinforced masonry is used. Here, the only large forces taken into account are gravitational, and these can be resisted by plain brick/block/stonework. The exception to this is over door and window openings, where additional support is required; this is usually provided by a lintel (often of reinforced concrete), or sometimes by an arch.
However, this type of construction is vulnerable in a significant earthquake, where lateral loads are involved. Being brittle, masonry materials are liable to fracture when subjected to these forces. Whilst the compressive strength of these materials is good allowing them to withstand significant vertical loads, they are poor at handling the shear stresses that may arise from an earthquake. The consequent poor cohesion within walls and connectivity between them can then lead to structural failure.
To address these limitations, in areas of seismic risk two enhancements to plain masonry wall construction are commonly used. One, referred to as "reinforced masonry", involves placing horizontal and vertical steel reinforcing bars at frequent intervals within the wall, and using tie columns to connect the reinforced walls. The other is called "confined masonry", and is the familiar technique being described here.
Confined masonry wall construction consists of sections of unreinforced masonry wall confined between reinforced concrete members (the dalas and castillos). Relative to plain unreinforced masonry, this gives the following benefits:
a) Connectivity is greatly improved through the tensile strength and connections in the steel; thus wall sections that might otherwise come apart are held together.
b) The ductibility of the reinforced concrete allows some absorption of seismic forces.
c) Stability of the walls is greatly improved, allowing a height-to-thickness ratio well over double that which would be acceptable with plain unreinforced masonry. This makes it feasible to build houses with very thin walls (most houses in Mexico have walls only 1/2 brick thick).
d) Strength of the walls is also improved. This could be a significant benefit with the dubious quality of bricks in Mexico, which are often insufficiently fired and thus soft and lacking in strength (my architect used to go through the deliveries and throw out such bricks).
It is worth noting that structures comprising a reinforced concrete frame infilled with brick are also quite common. These are normally used for commercial buildings of higher rise and/or with larger openings. Being a frame, the reinforced concrete part is built first, and is designed to completely resist all loads. Although this frame is likely to be much more substantial than the dalas and castillos used in confined masonry, it may appear to have the same function. However, these are actually two quite distinct types of construction, based on completely different design principles.
To summarize two basic points for those steeped in framed construction:
1. The dalas and castillos in confined masonry wall construction are not designed to act as a load-bearing frame, but instead to improve the earthquake resistance of masonry walls.
2. The bricks and mortar, being laid first, are patently not infill; in fact they are the primary elements of the walls, which over their entire length support the floors and roof. Since the dalas are laid on top of and are bonded to the brick, it would seem to be only common sense that the bricks help support the load.
The following links substantiate these points and give further information:
Still-skeptical readers may want to search for pages on "confined masonry" for themselves. It should be easy to ascertain that this term does indeed describe the technique used predominantly for house construction in Mexico. Further reading should then make clear that the basis of this technique is as described above.
Current state of my own construction:http://www.flickr.com/photos/9085850@N08/sets/72157602510502146/