Masonry, generally consisting of bricks or blocks laid in courses and interspersed with cementitious mortar, is in essence a composite ceramic material. Ceramic materials by their very nature are high in compressive strength, stiff, hard and low in electrical and thermal conductivity. These properties make them useful building materials, however ceramics are also very brittle, which means that they have a low tensile strength in comparison to other materials, such as steel.
Not only is masonry weak in tension, but as a result of the traditional construction methods there are also areas of weakness within masonry structures. This is because the bricks or blocks are laid in courses and bedded in mortar. The mortar forms joints between the individual clay or concrete units of masonry, both horizontally and vertically, at which there are inherent structural weaknesses. It is at these points where wall panels, columns and piers are most likely to fail as a result of lateral loading. The weakest joints in a masonry wall panel are at the horizontal “bed joints”, with enhanced strength perpendicular to the bed joints provided by the shear effects of “keying” (overlapping) of the masonry units at alternate layers.
For slabs or walls constructed from isotropic materials (that is to say materials whose properties do not deviate dependent upon orientation) and supported on four sides it is typical that the material will “span” the shortest distance. This means that the majority of the forces will be accommodated by the slab or wall in an orientation relative to the shortest distance between supports. Masonry wall panels are no different because they are isotropic in the sense of their stiffness, and, like a reinforced concrete floor slab, a vertical masonry wall panel also requires support (as a result of lateral load imparted upon it – which is generally by virtue of wind pressures). A wall panel constructed as part of a typical dwelling will therefore generally span vertically – between the ground and a supported floor or roof.
The disadvantage of the wall panels spanning vertically is that when subjected to lateral wind pressures the resultant bending of the panel subjects the bed joints to tensile forces – and as previously explained these are the weakest points in a masonry wall panel. Therefore, in order to reinforce the wall panels which otherwise would span vertically, it is necessary to install buttressing “shear” walls. This ensures that at least a proportion of the panel spans horizontally, and that the stress on the wall panel is carried by the shear effects occurring as a result of keying of the masonry units in the vertical direction. These buttressing supports can be provided by suitably designed masonry returns, or otherwise steel frame structures.
In the UK, the Building Regulations Approved Document A for structures outlines the limiting dimensions for a buttressing masonry wall or pier. BS5628 part 1, (the code of practice for the structural use of unreinforced masonry) specifies that no lateral load-resisting wall panel should have dimensions (defined by support positions) of greater than 50 times its effective thickness, which, for a cavity wall formed of two 100mm leaves of masonry is 6.65 m. The successor to BS5628, Eurocode 6, stipulates wall panel limiting dimensions relating to span distances and thicknesses, though it states that these dimensions are for the purpose of ensuring adequate serviceability (so that finishes do not deteriorate) rather than ultimate limits of allowable load before failure.
So why does ensuring that masonry walls are adequately supported against lateral loads matter? Well, there are two answers to that question – one is of serviceability and one is of ultimate structural capacity before failure.
Clearly we do not want our wall to fall down as a result of wind loading, so there is a clear incentive here to ensure that the wall panel is sufficiently strong that it will not collapse, but what about serviceability? What are we worried about? Surely if a wall doesn’t fail then there is nothing to worry about?.. Well, it depends on your attitude towards construction.
You probably haven’t noticed before, but if you look carefully at wall panels on a lot of older buildings you will quite often see a “bowing” or curving of the wall panel vertically. This is an effect of a wall panel which is inadequately designed for serviceability. The wall panel bows over time due to inadequate lateral support caused by defects such as poor tying and inadequate load transfer at floor level, in combination with the progressive effects of creep due to moisture absorption, frost attack and thermal expansion and contraction. A wall panel such as this will not only show up in structural surveys which reflect on the value of a property, but may also over time result in the collapse of the wall panel.
How can these problems be remedied or, even better, prevented?.. In order to know this we need to know why they occur. There are a number of reasons why these sorts of things occur. Often this is due to inadequate restraint strapping of the wall to a floor or ceiling, due to insufficient cavity tie provision or simply that the floor is not capable of acting as a horizontal support by transferring lateral forces from the wall panel back to shear walls in the property. The former issues can be resolved in some cases by tying retrospectively. The latter issue is where things become more complex.
In order that the floor can transfer lateral forces it is required to be sufficiently stiff that it acts as a diaphragm – transferring the forces from the side wall panel back to other masonry returns. In other words the floor panel needs to be stiff and there also need to be sufficient return walls in the building. This is where the dark art of engineering judgement over lateral stability can come into play. In the event that there are insufficient returns in the property it is possible that there could be a large structural failure – therefore we need to be very careful about these things.
If you are considering removing a substantial wall panel from a property to create a big open space, or you are constructing a masonry structure with very few masonry walls, be prepared to either change your layout so that there is sufficient masonry, or otherwise be prepared for the installation of a lateral load- resisting steel frame. The choice of these options comes down to how much you are willing to pay in design fees, because a masonry structure generally requires much less design input by a structural engineer than a steel structure.
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Source by Sam E Dean
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