Q.
a) With the use of diagram, label the elements of brick wall openings. Elaborate the functions of each element. (10 marks)
b) Using diagrams indicate the elements of casement window. Explain the types of these windows. (15 marks)
(25 marks, 2014 Q4)
A.
a) Earlier question on "elements of a brick wall" is referred.
Wall Expansion Joints -
long-term moisture expansion of the brick and thermal movement of the brick wythe* result in an increase of size of approximately 1” for each 100’ of wall length. This movement is outward from the center line of the wall section. Vertical expansion joints, including the sealant with which they are closed, must accommodate this movement. This is the same concept like that of the Bridges, and Cement Floors.
* A wythe is a continuous vertical section of masonry one unit in thickness.
The presence of openings in the wall changes the manner in which the movement takes place. Each wall section with a different horizontal length expands outward from its center line. Thus, wall components are moving different distances and perhaps in opposite directions. That movement causes stress concentrations and cracks unless vertical expansion joints are positioned properly.
Support of masonry above openings
There are several means of carrying masonry above openings. The four means of supporting masonry above openings are: an arch, a reinforced brick beam, a loose steel lintel and a shelf angle. The first three are supported by the masonry at the jambs; a shelf angle is attached to a structural frame. For each of these, expansion joint location will influence how much load the structural element carrying the masonry above the opening must support.
ARCH
A structural brick arch needs a sufficient length of horizontal masonry to resist its thrust. An expansion joint near either jamb prevents this from occurring. Thus, for an arched opening, expansion joints must be located away from the opening beneath the arch. A horizontal distance of two-thirds of the arch span from the vertical edge of the opening to the expansion joint should suffice for a segmental arch. Similarly, any expansion joint above the arch will prevent arching action from occurring. So, none can be placed there. See Figure 2.
If a steel angle supports a non-structural arch, treat it as a loose lintel for expansion joint locations. If an expansion joint is desired above the arch, locate it only above the crown of the arch.
REINFORCED BRICK BEAM
A reinforced brick beam needs sufficient bearing area at the jambs to transfer its vertical reaction to the masonry below. Further, attempting to accommodate horizontal movement of the beam on its bearing is difficult. There is a mortar joint at this junction and the coefficient of friction is relatively high.
If there is an expansion joint at either jamb, arching cannot take place in the masonry above the opening. As a result, the beam must be designed to carry the weight of the rectangular section of brickwork above the opening. So, let’s not put a vertical expansion joint at the jamb of an opening spanned by a reinforced brick beam.
Placing it at the end of the beam bearing is possible, but at least 3’ from the vertical edge of the opening is recommended.
LOOSE LINTEL
Loose lintels, single structural steel angles placed in the brick wythe, offer more options for expansion joint locations. Since the coefficient of friction of the angle on the brickwork or of the brickwork on the lintel is lower than that of a mortared joint and bond is broken by the presence of the angle, it is possible to place the expansion joint at the jamb of the opening. However, doing so requires that the lintel be designed to carry the entire weight of the rectangular section of brickwork
above the opening.
In most cases this is the logical loading condition. Even if the expansion joint is moved to the end of bearing of the lintel, the brickwork above will not be able to develop arching action and the lintel still has to be designed for the full area above the lintel. Placing the expansion joint past the end of the lintel by at least 3’ from the vertical edge of the opening is recommended in order to get arching action above the opening.
Then the lintel can be designed for a 45º triangular area of masonry above the opening. However the vertical expansion joint should then continue through the brickwork adjacent to the opening. Which location is preferred? That is a matter of
aesthetic preference. Engineers do not go there.
SHELF ANGLE
A shelf angle is supported by the building frame or structure. It may be continuous around the perimeter of the building or over just one or a series of openings. The masonry above the shelf angle is, therefore, supported by structure at the height of the shelf angle.
The brickwork horizontally adjacent to the opening is supported by the foundation or structure at a lower level. For continuous shelf angles, the vertical expansion joints can be placed at any desired location. For shelf angles over a single or series of openings, the vertical expansion joint must be placed at the ends of the shelf angle.
That is, since the support conditions are different, these two sections of brickwork must be separated by a vertical expansion joint. This is a steadfast rule. Expansion joints at or near jambs are more difficult to detail properly when reinforced brick beams or loose lintels are used. With expansion joints at the jambs, be sure to detail the bearing ends so horizontal movement can occur.
Ref:
Wikipedia on "Wythe" at
http://en.wikipedia.org/wiki/Wythe
J Gregg Borchelt. 2007. Brick Expansion Joints and Wall Openings - design element minimizes movement cracking. The Story Pole, Vol 38 No 4.