Q: CI 530.1-ASCE 6-TMS 602 - Specification for Masonry Structures permits sizeable variations in the dimension of head joints. The joints are permitted to be as little as 1/8 in. and as large as ¾ in. This range seems to be excessive.
Why are the variations so great in the code?
A: This code was primarily designed for structural masonry. The requirements were not intended to address aesthetic concerns.
In most walls, head joints play very little, if any, role in the structural strength of the masonry. In running bond, it is the units themselves that help the masonry span horizontally. In stack bond masonry that lacks the interlock of the masonry units, the bed joints are reinforced. Therefore, the head joints have no significant structural demands, even in properly designed and constructed stack bond masonry systems.
Occasional variations in mortar joint thickness generally do not detract from the appearance of the wall. The frequency and location of these variations are key factors when evaluating the impact on appearance. Many architectural specifications include tighter tolerances in the head joint width for aesthetic reasons.
However, it is important to consider the dimensional tolerance of the units being used to construct the walls. The permissible length differences in many brick units may have a significant impact on joint variations. Sometimes, in order to line up head joints, the width must be increased or decreased.
Type FBS modular brick units can vary in length from 7 3/8 in. to 7 7/8 in. Therefore, since the permitted range in lengths for Type FBS units is ½ in., it would seem reasonable to allow a somewhat greater range in the joint tolerances.
The tolerance for FBA units can be much less restrictive than FBS. FBA units are selected to produce different architectural effects resulting from non-uniformity in size and texture, and tolerances of these units vary. In many cases, it may not be possible to maintain even the tolerances listed in ACI 530.1-ASCE 6-TMS 602 when Type FBA units are used.
The type of building is also an important consideration. For instance, a wall in a warehouse building probably does not need to meet the same joint tolerance as those in a highly visible facade. In fact, joint variations from 1/8 in. to ¾ in. are relatively common in such buildings. Fig. 1, 2, and 3 illustrate the walls of a warehouse/office building where the head joint varies from 1/8 in. to ¾ in. within a 2-ft distance.
Even with this close proximity of these wide and narrow joints, the texture of the brick units helps to conceal this variation. Holding masonry walls in warehouses to tight tolerances can increase costs without any significant benefit to the building.
Tight head joint tolerances do not always produce a more attractive masonry wall. One of the important features of masonry is its hand-crafted appearance. Precise masonry units with tight head joint tolerances produce a very uniform industrial appearance that may not be desirable in many applications. A certain amount of variation adds to the character of the masonry wall.
Norbert V. Krogstad is a consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.