Masonry Magazine February 1993 Page. 16
TABLE 2
SEISMIC PERFORMANCE CATEGORIES (SPC)
| Effective Peak Velocity Related Acceleration (A,) | Seismic Hazard Exposure Group |
|---|---|---|
| | I | II | III |
| A≤0.05 | A | A | A |
| 0.05≤A≤0.10 | B | B | C |
| 0.10≤A≤0.15 | C | C | D |
| 0.15≤A≤0.20 | C | D | D |
| 0.20≤A | D | D | E |
If courses are horizontally offset less than one-quarter of the unit length (stack bond), the minimum horizontal reinforcement shall be 0.0007 times the gross cross sectional area of the wall. This reinforcement shall be satisfied with uniformly distributed joint reinforcement or with horizontal reinforcement spaced not over 4 ft. and fully embedded in grout or mortar.
Some model codes have added additional requirements to those listed above. These requirements include:
1. At least one wythe of a cavity wall shall be designed and reinforced in accordance with ACI 530/ASCE 5. The other wythe shall be tied to its backup and reinforced with a minimum of one No. 9 wire gage (W1.7) at a maximum spacing of 16 inches on center. The wall wythes shall be tied in accordance with ACI 530/ASCE 5 Section 5.8.2.2.
2. Masonry screen walls, with perforations that preclude the use of reinforced cores, laterally supported but not otherwise connected on all edges by a structural frame of concrete, masonry or steel, shall conform to the following requirements.
a. All screen walls shall be reinforced in accordance with this section. Joint reinforcement shall be considered effective in resisting stresses. The units of a panel shall be so arranged that either the horizontal or vertical joint containing reinforcing is continuous without offset. This continuous joint shall be reinforced with joint reinforcement having a minimum steel area of 0.03 square inch (2-9 gage wires). Joint reinforcement shall be embedded in mortar or grout.
b. In calculating the resisting capacity of the system, compression and tension in the spaced wires are permitted to be utilized. Joint reinforcement shall not be spliced and shall be the widest that the mortar joint will accommodate allowing ½ inch of mortar cover.
The requirements for extra steel at the tops and bottoms for walls, at openings and at the ends of walls are requirements that were based on traditional requirements of the Uniform Building Code.
Figure 2 shows a typical wall reinforced in accordance with these provisions. These requirements are arbitrary and are based of construction methods prevalent on the west coast. Some of the provisions create quite a problem with traditional midwest and east coast construction methods. As a result significant changes to the ACI 530/ASCE 5 Design Standard are being worked on in committee to remove the problem without sacrificing safety.
Some states are also working on revisions to these requirements to make them more workable. In the meantime, however, the provisions must be followed until changed or until approval for alternate design is approved by a building official. The provisions for the reinforcement of stack bonded walls are also based on old earthquake codes. Table 4 shows the amount of reinforcement that is needed to comply with this requirement.
In addition to using joint reinforcement alone to satisfy these requirements, joint reinforcement in conjunction with bond beam steel, can be used. In this situation the total area of steel for the full height of the wall is calculated. The area of steel supplied by using a 2-9 ga. wires @ 16" o.c. for the full height is then calculated and subtracted from the total to determine the amount of steel required for the bond beam at the top of the wall.
For example, given a 10 ft. high wall (15 courses) made with 8 in. block; the required area of steel is .061 x 10 or 0.61 in. Standard joint reinforcement at 16 in. o.c. will provide 0.026 x 10 or 0.26 in.. The balance of steel (0.61-0.26) 0.35 must be