Loadbearing walls in small buildings
In the case of a small building, brick walls will often have the ability to also support other parts of the building (that is, to be loadbearing) without any particular modification. Masonry has traditionally been used as the principal loadbearing system for buildings, ranging from small single-storey housing to fairly tall commercial and industrial buildings. In most cases, some thought will have to be given to the form and detailing of the walls to make sure they are suitable to carry these loads.
In order to carry vertical loads, the wall has to be continuous from top to bottom. Ideally, openings should be rather narrow and in-line vertically, rather than wide or haphazardly located on the elevation.
Since walls rely on intersecting with each other to provide some of their stability, continuous vertical openings would turn the wall into a series of isolated piers. This layout would only be efficient if the floors each served to tie the separate piers together at each level.
Openings in loadbearing walls
The Building Code of Australia requires lintels over openings in loadbearing walls to have the same Fire Resistance Level (FRL) as the wall itself, unless it falls within the scope of certain exemptions. The exemptions allow non-fire-rated lintels in single storey buildings, and in other buildings where the span does not exceed 1.8m and the wall or leaf is not more than 150mm thick. In practice, this is another reason for limiting the width of openings in loadbearing walls.
Layout of walls to support floor loads
In order to use the walls to support floor loads, we first have to consider a suitable span for the floor structure. Conventional timber joist floors seldom span more than 4 or 5 metres. Domestic concrete slabs only improve on these spans a little, while commercial flat concrete slabs commonly cover 6 to 8 metres between supports, and floors using steel or concrete beams can extend these limits a little. Slab systems can be continuously supported on walls, but beam systems usually need thickened piers under the beams. Therefore there are several different systems that might be considered:
If the building is narrow enough, the external walls alone can support the entire floor structure. The floor slabs (or slab and beam systems) span right across. (See Support of Slabs on Brick Walls)
If there are sufficient internal walls (as, for example, in a hotel with a series of small rooms) then the internal and external walls together might support the entire floor load. In this case, it often happens that the internal walls have only small doorways, while the external walls have large windows. Therefore, most of the load will be taken directly by the internal walls.
External walls and internal columns
If the building requires large internal spaces without walls, the floor loads might be carried by the external walls and a series of internal columns. This was a common system for many warehouses and woolstores at the end of the nineteenth century, when either heavy timber or cast iron construction was used for the floors and internal columns.
In this case the floors could be a concrete "flat slab" supported on the internal columns, with or without thickened drop panels around the columns. Alternatively, they could be either timber or concrete floors with timber, steel or concrete main beams between the columns.
Stability of loadbearing walls
Early in the design of a building, decisions will be made about the overall structural system and the character of the facades. These decisions will take into account the characteristics of masonry walling systems, and in turn they will influence the development of masonry detailing.
In order to resist the horizontal loads, walls rely on either their own thickness, or the support provided when two walls meet at right angles. In modern buildings there is no need to use the very thick walls of the Victorian era, and adequate stability can usually be achieved either by having a lot of intersecting walls (as in the hotel-type plan above), or by articulating the wall itself to provide both strong modelling and stability.
The principal vertical loads acting on any wall will be its own weight, and if it is loadbearing, also the loads from parts of the building's floors and roofs. It must be able to support these loads.
An external wall will be subjected to horizontal wind loads. It must be able to resist the effect of the wind, which will be either to overturn the wall as a unit, or to bend a panel of walling inward or outward between its supports. In this respect, a loadbearing wall is stabilised to some extent by the effect of the vertical load on top of it. Because of being attached to a floor or roof structure at the top of the wall, it also is stabilised more than a freestanding wall would be.
In seismic areas, walls will also be subject to earthquake loads, which will generally have the effect of overturning either individual walls, or the building as a whole. When severe seismic action is expected, masonry can be reinforced to increase its ductility.
Two or three-storey loadbearing buildings
It is now common practice to use loadbearing walls for apartment buildings up to three storeys. If all the floor plans are the same, there are plenty of internal walls to carry the loads, and the maximum span of the floors is only as large as the biggest room. The loads are carried on the internal walls and the inside leaf of the external cavity walls. This allows the outside leaf to continue for the full height without interruption, avoiding any problems of how to treat exposed floor slabs, and avoiding the need for horizontal expansion joints or flashing at floor levels.
The outside leaf has little stability of its own, and relies on the cavity ties to tie it back to the stiff box-like arrangement of internal walls. Therefore the ties must be designed to be adequate for the purpose, and to be corrosion-resistant so that they keep doing their job for the life of the building. (See Weather Resistance of Metal Ties & Inclusions.)
The structural requirements are discussed in more detail in Lawrence (2000). The main problems are likely to be parts of the walls, usually the exterior walls, where there are large openings and only small lengths of wall between them.
The load carrying capacity of the walls is also affected by its slenderness, and if the storey height is much greater than the usual 2.5m to 3m the reduction can be severe. This can also occur if split-level layouts, atriums, or voids joining two levels are included in the planning. In these cases, it is possible to thicken the inside leaf, or include piers or offsets in the walls, or reinforce the brickwork. By expressing deep piers on the inside or outside of the building, or using deep reveals to openings, the character of the building can be changed.
It is also possible, and fairly simple, to construct a diaphragm wall, in which the two leaves are some distance apart but connected by cross walls. This defeats the waterproofing advantage of having a continuous cavity, so it would generally be applicable to freestanding walls either fully within the building enclosure, or fully external to it.
Multi-storey loadbearing buildings
Buildings up to 10 or 12 storeys have been constructed from loadbearing brickwork, both in Australia and overseas. In these cases the structural requirements become more severe, both because of the additional load of the building, and also because of the increased effect of wind loads. Usually the strength of the bricks and of the mortar have to be increased, and it is common for the lower storeys to require full-brick (230mm) thick walls, at least in parts.
Many brickworks can and do produce high-strength bricks, but if they are required to test and certify them at a particular strength, the cost will increase, and the range of colours and finishes might be reduced. Testing and certification of the mortar strength and the techniques of laying (such as ensuring full bed joints) might also add something to the cost. On the other hand, in a multistorey building with an appropriate plan layout, the use of loadbearing brick walls can save the cost of a separate structural frame, and of the details where the walls abut columns and beams.
Examples of a number of apartment buildings, from five to 11 storeys in height built during the 1960s, are presented in Krantz (c1967).
One of the tallest buildings of loadbearing brickwork was the Monadnock building in Chicago (Burnham and Root, architects, 1890), which has walls 1800 thick at the base. These stylised floor plans give an indication of the relative thickness of the walls at the ground level (upper plan), and partway up.
Monadnock building, Chicago
Support of slabs on brick walls
Brickwork tends to expand, whereas concrete shrinks. Also, the concrete slab will deflect and rotate slightly at the edges when it carries its own load. For these reasons, a bond-breaking layer is used under the slab when it is supported on a brick wall. Commonly, two layers of aluminium-cored dampcourse material, or of zincalume steel in low-corrosion areas, are used for this purpose. This layer prevents the slab bonding to the wall, which would otherwise cause a crack just below the slab.
If there is a full-width flashing in the outside leaf, that will isolate the brickwork from the slab it sits on. The inside leaf does not need a flashing or dampcourse, but if the bricks are expected to be highly expansive, a dampcourse would allow it to move independently of the concrete.
It is preferable not to support the slabs on the outside leaf of the external walls. This avoids showing the slab on the elevation, and therefore avoids the flashing details required when the cavity is crossed. It also avoids any problems caused by differential brick growth between the two leaves (if they use different bricks), and different thermal movement because the outside leaf is subject to the weather.
If the slab does extend across both leaves, either a drip bar or a drip groove is required in the underside of the slab in the cavity region, to prevent moisture travelling across on the soffit of the slab and damaging the internal wall.