ALLOWABLE STRESS DESIGN TABLES FOR REINFORCED CONCRETE MASONRY WALLS
The combination of concrete masonry and steel reinforcement provides a strong structural system capable of resisting large compressive and flexural loads. Reinforced masonry structures have significantly higher flexural strength and ductility than similarly configured unreinforced structures and provide greater reliability in terms of expected load carrying capacity at failure.
Two methods of designing reinforced concrete masonry structures are commonly used:
- allowable stress design, based on service level loads and proportioning members using conservative allowable stresses.
- strength design, based on a realistic evaluation of member strength subjected to factored loads which have a low probability of being exceeded during the life of the structure.
Capacities of reinforced concrete masonry determined by the allowable stress design method are included herein. Capacities of reinforced concrete masonry determined by the strength design method are included in Strength Design of Concrete Masonry Walls for Axial Load and Flexure, TEK 14-11B (ref.3).
ALLOWABLE STRESS DESIGN
The wall capacities of Table 3 are determined in accordance with the requirements for allowable stress design of reinforced concrete masonry contained in Chapter 2 of Building Code Requirements for Masonry Structures (ref. 2). A more detailed discussion of the allowable stress design method, as well as provisions governing materials and construction for reinforced concrete masonry, are contained in Allowable Stress Design of Reinforced Concrete Masonry, TEK 14- 7A (ref. 1).
Tables 1 and 2 list the maximum bending moments and shears, respectively, imposed on walls simply supported at top and bottom subjected to uniform lateral loads with no applied axial loads.
WALL CAPACITY TABLES
Table 3 contains the maximum bending moments and shear loads that can be sustained by various reinforced walls, without exceeding the allowable stresses defined in Building Code Requirements for Masonry Structures (ref. 2). These wall strengths can be compared to the loads in Tables 1 and 2 to ensure the wall under consideration is not loaded beyond its design capacity.
The values in Table 3 are based on the following criteria:
- Allowable stresses:
- f’m = 1500 psi (10.3 MPa)
- Em = 900f’m or 1,350,000 psi (9,310 MPa)
- Es = 29,000,000 psi (200,000 MPa)
- Type M or S mortar
- running bond or bond beams at 48 in or less o.c.
- reinforcement spacing does not exceed the wall height
- walls are grouted only at cores containing reinforcement
- where indicated, allowable stresses are increased by ⅓, as prescribed in Building Code Requirements for Masonry Structures (ref. 2), section 18.104.22.168, for load combinations including wind or seismic loads
- due to space limitations, metric equivalents are not provided in Table 3 except for reinforcement bar sizes.
Metric equivalents can be obtained by applying the following conversion factors:
in x 25.4 = mm
in²/ft x 2117 = mm²/m
lb-in/ft x 0.0003707 = kN-m/m
lb/ft x 0.01459 = kN/m
A warehouse wall will span 34 ft (10.4 m) between the floor slab and roof diaphragm. The walls will be 12 in. (305 mm) thick. What is the required reinforcing steel to support a wind load of 20 psf (0.96 kPa)?
From interpolation of Tables 1 and 2, respectively, the wall must be able to resist:
M = 34,800 lb-in/ft (12.9 kN-m/m)
V = 340 lb/ft (4.96 kN/m)
Assuming d = 8.625 in. (219 mm), from Table 3 (for load combinations including wind or seismic) no. 7 bars at 48 in. (1219 mm) on center provides sufficient strength:
Mr = 38,512 lb-in/ft (14.3 kN-m/m) > M OK
Vr = 5345 lb/ft (77.9 kN/m) > V OK
Note: Since wind loads can act in either direction, two bars must be provided in each cell when using off center reinforcement – one next to each faceshell. Alternatively no. 6 bars at 24 in (19M at 610mm) or no. 8 at 40 in (25M at 1016 mm) could have been used in the center of the wall.
As = net area of steel per foot of wall length, in.²/ft (mm²/m)
b = effective width of compression zone, in. (mm)
d = distance from extreme compression fiber to centroid of tension reinforcement, in. (mm)
Em = modulus of elasticity of masonry, psi (MPa)
Es = modulus of elasticity of steel, psi (MPa)
Fb = allowable compressive stress due to flexure, psi (MPa)
Fs = allowable tensile stress in reinforcement, psi (MPa)
Fv = allowable shear stress in masonry, psi (MPa)
f’m = specified compressive strength of masonry, psi (MPa)
M = applied moment, in.-lb/ft (kN⋅m/m)
Mr = resisting moment of wall, in.-lb/ft (kN⋅m/m)
V = applied shear, lb/ft (kN/m)
Vr = resisting shear of wall, lb/ft (kN/m)
- Allowable Stress Design of Reinforced Concrete Masonry, TEK 14-7A. National Concrete Masonry Association, 2004.
- Building Code Requirements for Masonry Structures, ACI 530-05/ASCE 5-05/TMS 402-05. Reported by the Masonry Standards Joint Committee, 2005.
- Strength Design of Concrete Masonry Walls for Axial Load & Flexure, TEK 14-11A. National Concrete Masonry Association, 2003.
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