Design Steps for Structures Requiring Little Structural Calculation
Many masonry structures require little structural calculation. Their primary design steps are layout, design involving primarily structural layout, detailing, and material specification. In this section these steps are outlined. This section can be viewed as a summary of material previously presented in Chaps. 1 and 2.
1. Layout of overall structural configuration:
a. Modularity: Adjust the plan dimensions to the nominal dimensions of the units to be used.
b. Selection of the overall structural system: Locate walls in plan. c. Architectural details: Locate windows and doors.
2. Specify type of wall system according to desired level of waterpenetration resistance. In areas of severe driving rain, specify a drainage wall, or a fully grouted barrier wall with a thickness of at least 8 in. If a drainage wall is specified, it should have at least a 2-in. cavity, and be provided with drainage details (flashing and weepholes). Further details of water-penetration resistance are addressed at the end of this chapter.
3. Specify masonry units: a. Specification of clay units: Decide whether building brick (ASTM C62), facing brick (ASTM C216), or some other kind of unit (e.g., ASTM C652, hollow brick) is required. Specify the grade of unit (SW, MW, or NW) based on the weathering index at the building’s geographic location. b. Specification of concrete units: Decide whether conventional hollow units (ASTM C90) or some other type of unit is required.
4. Specify mortar: Specify an ASTM C270 mortar, Type S or Type N (normally by proportion), and cementitious system (cement-lime, masonry-cement, or mortar-cement). The proportion specification is normally preferable to the property specification, because it avoids the additional cost of testing, and the additional difficulty of having to decide what to do if the test results do not comply with the required values. The property specification permits some savings in material costs, in return for increases in costs due to testing. Although it is theoretically not required, it is useful to insert the words “by proportion” in the specification as additional protection against inadvertent and possibly improper mortar testing. If tensile bond strength is important, use either cementlime mortar or mortar-cement mortar. While water-penetration resistance can be enhanced by using cement-lime mortar, this choice is probably not as important as choosing a properly specified drainage wall.
5. Specify grout: Normally, specify a coarse grout, conforming to the proportion specifications of ASTM C476 (one part Portland cement or other cements, three parts sand, and two parts pea-gravel). The amount of water should be sufficient to obtain a slump of about 11 in.
6. Specify accessories: For simple structures with single-wythe walls, the only accessories will be deformed reinforcement, conforming to ASTM A615 (new steel). This is simply prescribed (e.g., #4 bars @ 48 in. on centers) based on seismic design category or other considerations. Joint reinforcement could also be specified. a. For hollow units of concrete or clay, vertical reinforcement is placed in the continuous vertical cells, and horizontal reinforcement is placed in bond beam units (units with depressed webs). b. For solid units of concrete or clay, deformed reinforcement (vertical or horizontal) can be placed only in grouted spaces between wythes. Bed-joint reinforcement can be placed in the bed joints of a single-wythe wall.
c. Specify connectors, flashing, and sealants.
7. Specify construction details:
a. Foundation dowels.
b. Splices between foundation dowels and vertical reinforcement (required lap length depends on bar diameter).
c. Foundation details: See Sec. 2.9.3.
d. Roof connection details: See Sec. 2.9.3.
8. Construction process:
a. Decide whether or not to wet clay units: Check if the IRA exceeds 30, or use a field test (see if 20 drops of water, placed in a quarter-sized circle, are absorbed in 90 s or less). If the IRA exceeds 30, or if the drops are absorbed in the field test, wet the units briefly before laying.
b. Place units in running or stack bond. Tool joints using concave tooling.
c. Place reinforcement.
d. Pour grout (clean cells, mist the cells with water, grout by highlift
or low-lift procedures).
e. If possible, cure the masonry by keeping it damp.