Masonry, generally made up of bricks or blocks placed in rows and interspersed with cementitious mortar, is essentially a composite ceramic material. Ceramic materials by their very nature are highly resistant to compression, rigid, hard and have low electrical and thermal conductivity. These properties make them useful building materials; however, ceramics are also very brittle, meaning they have low tensile strength compared to other materials, such as steel.
Masonry is not only weak in tension, but as a result of traditional construction methods, weak areas also exist within masonry structures. This is because the bricks or blocks are laid in rows and set with mortar. Mortar forms joints between individual clay or concrete masonry units, both horizontally and vertically, where inherent structural weaknesses exist. It is at these points that 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 horizontal “bed joints”, with greater strength perpendicular to bed joints provided by the shear effects of “interlocking” (overlapping) masonry units at alternate layers.
For slabs or walls constructed of isotropic materials (ie, materials whose properties do not deviate based on orientation) and supported on all four sides, it is typical for the material to “traverse” the shortest distance. This means that most of the forces will be absorbed by the slab or wall in an orientation relative to the shortest distance between supports. Drywall panels are no different in that they are isotropic in the sense of their stiffness and, like a reinforced concrete floor slab, a vertical drywall panel also requires support (as a result of the lateral load imparted on it). him, which is generally by virtue of wind pressures). Therefore, a wall panel built as part of a typical dwelling will generally extend vertically, between the ground and a supported floor or ceiling.
The disadvantage of vertically extending wall panels is that, when subjected to lateral wind pressures, the resulting flexing of the panel subjects the bed joints to tensile forces and, as explained above, these are the most severe points. weak in a masonry wall panel. Therefore, to reinforce wall panels that would otherwise run vertically, it is necessary to install reinforcing “shear” walls. This ensures that at least a part of the panel extends horizontally and that the stress in the wall panel is supported by shear effects that occur as a result of keying the masonry units in the vertical direction. These reinforcing supports can be provided by properly designed masonry returns or otherwise steel frame framing.
In the UK, the approved Building Standards Document A for Structures describes the limiting dimensions for a masonry wall or pier. BS5628 part 1, (the code of practice for the structural use of unreinforced masonry) specifies that no lateral load resistant wall panel shall have dimensions (defined by support positions) of more than 50 times its effective thickness, which , for a cavity wall formed by two sheets of 100 mm masonry is 6.65 m. The successor to BS5628, Eurocode 6, stipulates limiting dimensions for wall panels in relation to span distances and thicknesses, but states that these dimensions are for the purpose of ensuring adequate serviceability (so that finishes not deteriorate) instead of the final allowable load limits above. failure.
So why is it important to make sure masonry walls are adequately supported against lateral loads? Well, there are two answers to that question: one is serviceability and the other is the ultimate structural capacity before failure.
Clearly we don’t want our wall to sag as a result of wind loading, so there is a clear incentive here to ensure the wall panel is strong enough not to collapse, but what about the service capacity? What worries us? Surely if a wall doesn’t fail then there’s nothing to worry about? Well, it depends on your attitude towards construction.
You probably haven’t noticed it before, but if you look closely at wall paneling in many older buildings, you will often see a “bow” or curvature of the wall panel vertically. This is an effect of a wall panel not being properly designed for maintenance. Wall panel bows over time due to inadequate lateral support caused by defects such as poor tie-down and inadequate load transfer at grade, in combination with progressive creep effects due to moisture absorption, frost attack and thermal expansion and contraction. A wall panel like this will not only show up in structural studies reflecting a property’s value, but over time it can also result in the wall panel collapsing.
How can these problems be remedied or better yet prevented? To know this we need to know why they occur. There are a number of reasons why this kind of thing happens. Often this is due to inadequate fastening of the wall to a floor or ceiling, due to insufficient provision of ties in the cavity, or simply because the floor is not capable of acting as a horizontal support transferring lateral forces from the panel. from the wall back to the cut walls. the property. The above issues can be resolved in some cases by retroactive binding. This last issue is where things become more complex.
In order for the floor to be able to transfer lateral forces, it is required to be rigid enough to act as a diaphragm, transferring forces from the sidewall panel to other masonry returns. In other words, the floor panel must be rigid, and there must also be sufficient return walls in the building. This is where the dark art of engineering judgment on lateral stability can come into play. In the event that there are not enough returns on the property, it is possible that there could be a major structural failure; therefore, we must be very careful with these things.
If you are considering removing a substantial wall panel from a property to create a large open space, or if you are building a masonry structure with very few masonry walls, be prepared to change the layout so that there is is sufficient masonry, or be prepared for the installation of a side load resistant steel frame. Choosing these options comes down to how much you’re willing to pay in design fees, because a masonry structure typically requires much less design input from a structural engineer than a steel structure.