Lintels and Openings
Brickwork needs to be supported over openings. There are five methods of doing this: brick arches, steel bars or angles, prefabricated reinforced concrete, prefabricated brickwork and directly from the reinforced concrete structure. The steel angles (known as shelf angles), can provide all the support, or be attached to the primary structure such as a reinforced concrete frame, to which the load is transferred.
Brick arches of various curvatures are a traditional method of carrying brickwork over an opening. Constructing an arch requires a timber formwork ("centering") to be constructed first, and removed after the arch is completed and the mortar sufficiently set. If the radius of curvature is large enough, it can be achieved by using ordinary rectangular bricks with slightly tapered joints; but in a tight arch, the bricks have to be tapered, which is much more time-consuming. An arch also requires a substantial amount of adjoining wall to stabilise it against the outward thrust of the arch. (See Brickwork Design - Arches)
Short spans, for example up to 1000 mm, can be spanned by a flat bar, but longer spans need angles or stronger sections. As the lintel will extend not less than 100 mm beyond the opening (AS 3700, 4.12), the thickness of the leg in the bed joint should be no greater than the thickness of the joint, typically 10 mm. Steel should be protected against corrosion usually by hot-dip galvanising, the density of which will depend on the corrosive level of the environment, however stainless steel can also be used in highly corrosive environments. (See Metal ties and Inclusions under Weather Resistance.)
The bottom of the steel angle will, in most cases, be exposed to view at bed joint level. This can be left in its galvanised state or painted for aesthetic reasons.
If a brick only soffit is desired then the bed joints of a number of courses above the opening can be reinforced with galvanised steel rods or proprietary stainless steel helical strips (such as "Helifix"). The installation will need to be designed by an engineer and AS 3700 5.7.3 (b) calls for 15 mm of cover to the steel. Alternatively, a proprietary galvanised steel bar lintel system (such as "Hesbia") embedded in the bed joint above the opening can be used. One lintel per leaf will be needed. In these installations, it is important that the engineer's or manufacturers' instructions are properly followed.
Shelf angles are typically bolted to the supporting frame and the brickwork built from it. Proprietary systems of inserts in the reinforced concrete with slotted angles allow for adjustment to ensure that the angle is where it should be.
Flashing and weep holes will need to be provided as with lintels supported on brickwork.
Toga building, York St., Sydney, Architects, Peddle Thorp and Walker.
Precast concrete and prefabricated brickwork lintels
The heights of these lintels are in multiples of brick courses and they are the full width of a leaf. They are not designed for use with face brickwork and need to be rendered to be visually acceptable. The load carrying capacity of the precast concrete lintels increases significantly when they act compositely with the brickwork above and spans of 3000 mm can be readily achieved. Fire ratings range from 60 to 120 minutes.
Fire rating of lintels
Brickwork readily meets most fire rating requirements. However, in some situations an exposed steel lintel may need the same fire rating as the brickwork, but it usually easier to support the brickwork in other ways than try to fire rate the steel.
Specification C1.1 "Fire Resisting Construction" of the BCA requires that:
A lintel must have the FRL required for the part of the brickwork in which it is situated, unless it does not contribute to the support of a fire door, fire window or fire shutter, and -
- (a) it spans an opening in
- (i) a wall of a building containing only one storey; or
- (ii) a non-loadbearing wall of a Class 2 or 3 building; or
- (b) it spans an opening in masonry which is not more than 250 thick and -
- (i) not more than 3m wide if the masonry is non-loadbearing; or
- (ii) not more than 1.8m wide if the masonry is loadbearing and part of a solid wall or one of the leaves of a cavity wall.
Openings in non-loadbearing walls
The limitations imposed on openings in a loadbearing wall are removed when there is a loadbearing frame and the wall is simply an infill. The heads over openings can be supported in one of two main ways.
- The head of a small opening can be supported by a lintel (of galvanised steel, concrete, or reinforced brickwork, as it would be in a loadbearing wall.
- The head of any sized opening can be supported from the floor above.
The latter allows openings of any length. It requires the head to be aligned with the underside of the floor slab or beam above.
The illustration of Hilversum Town Hall (Willem Dudok, architect, 1924) is typical of a style that uses large expanses of brickwork supported above very wide openings. It is obvious that the brickwork above the openings is supported by the structural frame of the building, rather than by a lintel.
The head of an opening can be aligned with the bottom of a floor beam by locating a supporting nib or shelf angle at the bottom edge of the beam. It may be difficult to detail the shelf angle if it is exposed at the head, and it would not be feasible to fire-protect it in this position. Therefore it is more likely that a concrete detail would be used. However, if the designer wants to show continuous brickwork adjacent to the opening, a shelf angle could be used up to the edge of the opening.
In the adjacent diagram, part of the outer leaf has been removed to show the relationship of the angle and concrete nib. Flashing has also been omitted for clarity. Note that a soft joint will be required under the shelf angle.
Alternatively, if the building is only two or three storeys high and the outer leaf can be self-supporting for its full height, there should be a vertical control joint at the edge of the opening to allow the full-height wall to expand vertically while the part supported on the slab can move downward slightly due to the shrinkage and deflection of the concrete structure. Depending on the growth characteristics of the bricks and the properties of the frame, the relative movements may be in single-digit millimetres, but this is enough to produce very noticeable cracking at the corners of the opening if it is not allowed for. Notice that, if the outside leaf is self-supporting, there is no soft joint in that leaf at the floor level.
As discussed, non-loadbearing brickwork needs to be tied to its supporting frame, but the anchors (or ties) need to allow for movement, particularly that caused by the long term expansion of bricks. Some proprietary ties and their typical use are shown.