Masonry Magazine August 1966 Page. 33
TABLE I-MINIMUM THICKNESS AND MAXIMUM SPAN FOR CONCRETE MASONRY SCREEN WALLS
| Construction | Minimum Nominal Thickness, (1) | Maximum Distance Between Lateral Supports (Height or Length, Not Both) |
| - | - | - |
| | | Nominal Thickness of Wall, inches |
| | | 4 | 6 | 8 | Other Nominal Thicknesses (1) |
| Nonload-Bearing Reinforced: | | | | | |
| Exterior | 4" | 10-0 | 15-0 | 20-0 | 30 t |
| Interior | 4" | 16-0" | 24'-0" | 32'-0" | 48 t |
| Nonreinforced: | | | | | |
| Exterior | 4" | 6'-8" | 10-0 | 13'-4" | 20 t |
| Interior | 4" | 12-0 | 18-0" | 24'-0 | 36 t |
| Loadbearing: | | | | | |
| Reinforced | 6" | Not recommended | 12"-6" | 16′-8" | 25 t |
| Nonreinforced | 6" | | 9-0 | 12′-0 | 18 t |
| *Total steel area, including joint reinforcement, not less than 0.002 times the gross cross-sectional area of the wall, not more than two-thirds of which may be used in either vertical or horizontal direction, | | | | | |
design
Design of screen block walls depends upon a number of factors: function, location (exterior or interior), aesthetic requirements, and provisions of local building code. They are used extensively for the following types of construction: (1) interior partitions, (2) free-standing walls supported on their own foundations, (3) veneers, and (4) enclosed panels in masonry walls. Screen wall partitions are designed as nonload-bearing panels with primary consideration given to adequate anchorage at panel ends and/or top edge, depending upon type of lateral support furnished. Free-standing walls include such assemblages as fences and other exterior nonbearing screens that receive lateral stability from a structural frame braced to an adjacent structure or designed as a cantilever from the foundation. The structural frame may consist of reinforced concrete masonry columns, pilasters, and beams, or may incorporate structural steel members in similar manner. When designed as veneer, the concrete masonry screen wall is attached to a structural backing with metal ties in the same manner as other types of masonry veneer. Likewise, the nonload-bearing screen used to fill an opening in a load-bearing masonry wall is designed in the same manner as a window.
Nonload-bearing screen walls should have a minimum thickness of 4 inches, and maximum clear spans as shown in Table I. For load-bearing screen walls the minimum thickness should be increased to 6 inches. The maximum distance between lateral supports can be measured vertically or horizontally, but need not be limited in both directions.
Adequate anchorage should be provided between screen walls and lateral supports, and the supports should be designed to transfer horizontal stresses to the structure and into the ground. Anchorage of free-standing screens to their supporting framework is accomplished by various means, Fig. 2. Veneer is anchored to the supporting structure by means of corrosion resistant wire ties or corrugated sheet metal spaced 24 inches o.c. vertically and horizontally, or spaced to comply with the local building code, Lateral support may be obtained from cross walls, piers, columns, posts, or buttresses for horizontal spans, and from floors, shelf angles, roofs, or spandrel beams for screen walls spanning the vertical direction.
Wind load should be considered on exterior screen walls. Design load may be based upon net wall area, or upon gross area, depending upon local code requirements. A shape factor of 1.3 is suggested as satisfactory when net area of screen wall surface is employed.
Due to fragile nature of screen walls, the use of steel reinforcement is recommended wherever it can be embedded in mortar joints, in bond beam courses, or grouted into continuous vertical or horizontal cells. Horizontal joint reinforcement, two No. 9 gauge wires or equivalent, placed 12 or 16 inches on center is recommended when screen wall units are laid in stack bond.
Screen walls classified as fully reinforced, Table I, should contain a minimum amount of reinforcement equal to 0.002 times the gross cross sectional area of the wall with not more than 3 of this minimum placed in either direction.
When screen walls support vertical loads, the allowable compressive stress should be limited to 10 percent of the strength of the unit, but not greater than 50 psi on gross area. In some instances, the compressive stress at the base of non-load-bearing screen walls will govern maximum unsupported height. Where screen block are not laid in a continuous mortar bed, (intermittent bond), the allowable stresses should be reduced in proportion to the reduction in the mortar bedded area, Fig. 3.