Unreiniforced brick walls are constructed with 3 (or more) parallel layers, called wythes - essentially 3 walls built adjacent to one another. Each wythe is separated from its neighbor by a 2" wide air gap; thus, 3 bricks at 3" plus 2 spaces at 2" equals a 13" thick wall (more for more wythes - more than 3 wythes used for large vertical loads, such as 5 stories).
A header course is laid between each wythe at regular intervals, usually
7 courses (a course is a horizontal run of bricks, all at the same height.) The header course consists of bricks laid perpendicular to the length of the walls and reaches from the center wythe to one of the exterior wythes. These header courses serve to tie the three wythes together so the wall can resist some lateral forces acting on the face of the wall, such as wind loads. This is how brick walls have been built (more or less) since bricks were invented.The problem is, the only thing holding the bricks, and thus the wall, together is: 1) the shear strenght of the mortar for forces in line with the wall (along the length of the wall), which is OK since the shear strength can be quite high, although degrades with time and weather., and 2) the tensile strength of the mortar for forces perpendicular to the face of the wall (think of leaning on a wall and pressing on the face of the wall -that is a force perpendicular to the face of the wall)
- these are termed out-of-place forces. Tension (and compression) is produced from the bending of the wall as it spans between the ground and the floor/roof above. Mortar has very low tensile strength.
An earthquake produces accelerations in all directions. Force is equal to mass times acceleration, and the brick wall has a lot of mass (weight); so the brick wall is subjected to shear, compression, and tensile stresses. These stresses can get quite high very quickly due to the mass of the brick. When the tensile capacity of the mortar is exceeded, the wall fails, usually quite dramatically. Shear failures also occur, but these are usually less dramatic, resulting in large diagonal cracks in the wall.
The majority of the failures that you see in unreinforced brick walls is due to out-of-plane tensile failures and failure of the connection between the wall and the floor/roof framing. Both these failure modes result in collapse of the wall.
This was demonstrated (but ignored) in the 1906 San Francisco earthquake, and again (this time duly noted) in the 1933 Long Beach earthquake. In Long Beach, many brick schools collapsed, and would have been horrible had it not occurred early in the morning. As a result of the '33 event, the Field Act was passed in California requiring all schools to be designed to resist eathquake forces. Modern seismic codes are descendants of the Field Act. Unreinforced masonry is no longer allowed in CA, and there are mandatory retrofit ordinances throughout the state. In Los Angeles, all unreinforced brick buldings have been either retrofit or torn down (Interesting note: if you watch Seinfeld, look at the exterior shot of Seinfeld's apartment building. You will see square plates at regular intervals on the outside of the wall - these are washer plates for the retrofit wall anchorage system - indicating that the building is in Los Angeles.).
Reinforced brick walls are constructed with 2 wythes of 3-/12" brick with a 2" wide space between, making a 9" thick wall. Reinforcing steel is placed, horizontally and vertically, in the void and then it is filled with grout. Now the reinforcing steel resists the tension forces, and the grout and mortar act in shear. Wall anchorage to the floor/roof is required by code.
Hope this helps.
Rob B, S.E.
I'm tempted to re-edit the grammar by removing commas and adding paragraphs but just bear in mind this is an engineer (I presume) writing to sci.geo.earthquakes.
(In a book these paragraph spacings work and are in fact normal in all types of books. On screen, the reader is assaulted by any close typing, the same way the unpleasantnesses of fonts inceases with size on screen but dimish off.)
In bricklaying I've never seen the term wythe used for a wall leaf. I thought withies was willow as used for making wattles.
The sand and cement used in a brick wall is always weaker than the bricks themselves for the purpose of allowing movement. The widths of the joints being crucial; not so much in holding the bricks together as in keeping them precise distances apart.
Forces "perpendicular to the face of the wall" are catered for in the size of the wall as in an house -where the floors and other walls act as buttresses, or the wall is given buttresses at specific distances along the wall.
And finally, the 2" gap he speaks of is a comparatively recent invention called cavity walling. It is usually accompanied by metal cross bracing not header bricks. Bricks -being twice as long as their width, would be unsuitable for use in spanning cavity walls.
Is he right in what he thinks or what?