An information series from the national authority on concrete masonry technology ALLOWABLE STRESS DESIGN OF CONCRETE MASONRY LINTELS TEK 17-1C Structural (2009) INTRODUCTION Lintels and beams are horizontal structural members designed to carry loads above openings. Although lintels may be constructed of grouted and reinforced concrete masonry units, precast or cast-in-place concrete, or structural steel, this TEK addresses reinforced concrete masonry lintels only. Concrete masonry lintels have the advantages of easily maintaining the bond pattern, color, and surface texture of the surrounding masonry and being placed without need for special lifting equipment. Concrete masonry lintels are sometimes constructed as a portion of a continuous bond beam. This construction provides several benefits: it is considered to be more advantageous in high seismic areas or areas where high winds may be expected to occur; control of wall movement due to shrinkage or temperature differentials is more easily accomplished; and lintel defl ection may be substantially reduced. More detailed information and wider ranging design aides for concrete masonry lintels are presented in Design Tables for Concrete Masonry & Precast Concrete Lintels (ref. 6) and Structural Masonry Design System (ref. 7). DESIGN LOADS Vertical loads carried by lintels typically include: (1) distributed loads from the dead weight Uniform load Triangular load Concentrated loads Uniform load over portion of span Lintel Clear span Effective span Figure 1 Typical Lintel Load Components Related TEK: 14-7B, 17-2A Keywords: allowable stress design, design examples, lintels, openings in walls NCMA TEK 17-1C 1
of the lintel, the dead weight of the masonry above, and any fl oor and roof loads, dead and live loads supported by the masonry; and (2) concentrated loads from floor beams, roof joists, or other beams framing into the wall. Axial load carried by lintels is negligible. Most of these loads can be separated into the four types illustrated in Figure 1: uniform load acting over the effective span; triangular load with apex at mid-span acting over the effective span; concentrated load; and uniform load acting over a portion of the effective span. The designer calculates the effects of each individual load and then combines them using superposition to determine the overall effect, typically by assuming the lintel is a simply supported beam. Arching Action For some confi gurations, the masonry will distribute applied loads in such a manner that they do not act on the lintel. This is called arching action of masonry. Arching action can be assumed when the following conditions are met (see also Figure 2): masonry wall laid in running bond, suffi cient wall height above the lintel to permit formation of a symmetrical triangle with base angles of 45 o from the horizontal as shown in Figure 2, Table 1 Lintel Weights, D lintel, lb/ft (kn/m) A Nominal Nominal wall thickness, lintel in. (mm) height, 6 (152) 8 (203) 10 (254) 12 (305) 16 (406) in. (mm) Lightweight CMU 8 (203) 38 (0.56) 51 (0.75) 65 (0.95) 80 (1.2) 108 (1.6) 16 (406) 76 (1.1) 103 (1.5) 131 (1.9) 160 (2.3) 216 (3.2) 24 (610) 114 (1.7) 154 (2.3) 196 (2.9) 240 (3.5) 324 (4.8) Normal Weight CMU 8 (203) 43 (0.63) 58 (0.84) 73 (1.1) 88 (1.3) 118 (1.8) 16 (406) 86 (1.3) 116 (1.7) 145 (2.1) 176 (2.6) 236 (3.5) 24 (610) 128 (1.9) 174 (2.5) 218 (3.2) 264 (3.9) 354 (5.2) A Face shell mortar bedding. Unit weights: grout = 140 pcf (2,242 kg/m 3 ); lightweight masonry units = 100 pcf (1602 kg/m 3 ); normal weight units = 135 pcf (2,162 kg/m 3 ). Superimposed wall load 8 in. (203 mm) minimum 45 h = effective span 2 Lintel End bearing 4 in. (102 mm) minimum (typ) Wall height Clear opening Effective span (see Figure 1) Figure 2 Arching Action 2 NCMA TEK 17-1C
at least 8 in. (203 mm) of wall height above the apex of the 45 o triangle, minimum end bearing (4 in. (102 mm) typ.) is maintained, control joints are not located adjacent to the lintel, and suffi cient masonry on each side of the opening to resist lateral thrust from the arching action. Lintel Loading The loads supported by a lintel depend on whether or not arching action can occur. When arching is not present, the lintel self-weight, the full weight of the wall section above the lintel and superimposed loads are considered. Self weight is a uniform load based on lintel weight (see Table 1). When arching occurs, the wall weight supported by the lintel is taken as the wall weight within the triangular area below the apex (see Figure 2 and Table 2). This triangular load has a base equal to the effective span length of the lintel and a height of half the effective span. Any superimposed roof and floor live and dead loads outside this triangle are neglected, since they are assumed to be distributed to the masonry on either side of the lintel. Loads applied within the triangle need to be considered, however. Uniform loads that intersect the 45 o triangle are typically distributed as follows. The portion of the load intersecting the 45 o triangle is distributed over the lintel span, i.e., the load is assumed to spread out at 45 o angles so that the uniformly distributed load on the lintel is reduced by the ratio of the width of load within the triangle to the lintel span. Distributed loads that do not extend entirely across the 45 o triangle may be distributed to only a portion of the lintel. Concentrated loads are assumed to be distributed as illustrated in Figure 3. The load is then resolved onto the lintel as a uniform load, with a magnitude determined by dividing the concentrated load by this length. In most cases, this results in a uniform load acting over a portion of the lintel span. When a lintel or other beam supports unreinforced masonry, 2005 Building Code Requirements for Masonry Structures (ref. 1) limits lintel defl ection to the clear lintel span divided by 600 or to 0.3 in. (7.6 mm) to limit damage to the supported masonry. By referencing this document, the 2006 International Building Code (IBC) (ref.3) contains the same provision. In the 2008 Building Code Requirements for Masonry Structures (ref. 2) and 2009 IBC (ref. 4) by reference, this provision was changed to adequate stiffness to limit defl ection that adversely affect strength or serviceability which applies to all masonry. For beams or lintels supporting un- Table 2 Wall Weights A, B Wall weights (lb/ft 2 ) for wall thicknesses, in. (mm), of: Grouted Lightweight units Normal weight units cells: 6 (152) 8 (203) 10 (254) 12 (305) 16 (406) 6 (152) 8 (203) 10 (254) 12 (305) 16 (406) None 23 29 34 37 44 30 39 45 49 58 48 in. o.c. 28 37 44 51 64 36 47 55 63 78 40 in. o.c. 30 39 47 54 67 37 48 58 66 82 32 in. o.c. 31 41 50 58 73 39 51 61 70 88 24 in. o.c. 34 45 55 65 83 41 55 66 77 98 16 in. o.c. 40 53 66 78 103 47 63 77 90 118 Full grout 57 77 98 120 163 64 87 109 132 177 A Assumes face shell mortar bedding. Unit weights: grout = 140 pcf (2,242 kg/m 3 ); lightweight masonry units = 100 pcf (1602 kg/m 3 ); normal weight units = 135 pcf (2,162 kg/m 3 ). kn/m 2 = lb/ft 2 x 0.04788. B Additional wall weights can be found in TEK 14-13A (ref. 5). NCMA TEK 17-1C 3
Table 3 Allowable Shear and Moment Capacities for Concrete Masonry Lintels (width x height) A Bottom cover, in. (mm): No. 1.5 (38) 2 (51) 2.5 (64) 3 (76) Steel of V all M all V all M all V all M all V all M all size: bars lb lb-in. lb lb-in. lb lb-in. lb lb-in. 6 x 8 lintels No. 4 1 1,279 16,691 1,170 14,369 1,062 12,182 953 10,136 No. 5 1 1,266 18,709 1,157 16,028 1,048 13,511 939 11,168 No. 6 1 1,252 20,174 1,143 17,203 1,034 14,426 925 11,849 6 x 16 lintels No. 4 1 3,022 60,355 2,913 58,087 2,804 55,821 2,695 53,558 No. 5 1 3,009 80,278 2,900 75,531 2,791 70,892 2,682 66,364 No. 6 1 2,995 89,629 2,886 84,228 2,777 78,956 2,668 73,814 6 x 24 lintels No. 4 1 4,765 96,892 4,656 94,598 4,547 92,304 4,438 90,012 No. 5 1 4,751 147,158 4,643 143,640 4,534 140,125 4,425 136,613 No. 6 1 4,738 191,919 4,629 184,711 4,520 177,606 4,411 170,604 8 x 8 lintels No. 4 1 1,730 20,460 1,580 17,650 1,430 14,990 1,290 12,500 No. 5 1 1,710 23,170 1,560 19,890 1,420 16,810 1,270 13,930 No. 6 1 1,690 25,210 1,550 21,550 1,400 18,120 1,250 14,930 No. 4 2 B 1,730 25,460 1,580 21,860 1,430 18,480 1,290 15,320 No. 5 2 B 1,710 28,140 1,560 24,030 1,420 20,190 1,270 16,620 8 x 16 lintels No. 4 1 4,090 61,110 3,940 58,820 3,800 56,540 3,650 54,250 No. 5 1 4,070 92,550 3,930 89,050 3,780 85,560 3,630 80,860 No. 6 1 4,060 109,740 3,910 103,210 3,760 96,830 3,610 90,600 No. 4 2 B 4,090 107,750 3,940 101,420 3,800 95,240 3,650 89,200 No. 5 2 B 4,070 123,960 3,930 116,510 3,780 109,240 3,630 102,150 8 x 24 lintels No. 4 1 6,460 97,900 6,310 95,590 6,160 93,280 6,010 90,980 No. 5 1 6,440 148,990 6,290 145,440 6,140 141,900 5,990 138,360 No. 6 1 6,420 207,820 6,270 202,840 6,120 197,860 5,980 192,880 No. 4 2 B 6,460 190,840 6,310 186,300 6,160 181,760 6,010 177,220 No. 5 2 B 6,440 264,990 6,290 255,050 6,140 245,260 5,990 235,600 10 x 8 lintels No. 4 1 2,190 23,810 2,000 20,570 1,810 17,500 1,630 14,620 No. 5 1 2,160 27,170 1,980 23,360 1,790 19,780 1,600 16,430 No. 6 1 2,140 29,760 1,950 25,480 1,770 21,470 1,580 17,720 No. 4 2 2,190 29,990 2,000 25,790 1,810 21,840 1,630 18,140 No. 5 2 2,160 33,430 1,980 28,600 1,790 24,080 1,600 19,870 10 x 16 lintels No. 4 1 5,170 61,630 4,980 59,330 4,790 57,040 4,610 54,740 No. 5 1 5,140 93,500 4,960 89,970 4,770 86,450 4,580 82,940 No. 6 1 5,120 127,610 4,930 120,080 4,750 112,720 4,560 105,540 No. 4 2 5,170 119,870 4,980 115,360 4,790 110,700 4,610 103,740 No. 5 2 5,140 144,910 4,960 136,290 4,770 127,870 4,580 119,650 4 NCMA TEK 17-1C
Table 3 Allowable Shear and Moment Capacities for Concrete Masonry Lintels (width x height) A (continued) Bottom cover, in. (mm): No. 1.5 (38) 2 (51) 2.5 (64) 3 (76) Steel of V all M all V all M all V all M all V all M all size: bars lb lb-in. lb lb-in. lb lb-in. lb lb-in. 10 x 24 lintels No. 4 1 8,150 98,600 7,960 96,280 7,780 93,970 7,590 91,650 No. 5 1 8,130 150,260 7,940 146,700 7,750 143,140 7,570 139,580 No. 6 1 8,100 209,870 7,920 204,850 7,730 199,840 7,540 194,830 No. 4 2 8,150 192,650 7,960 188,080 7,780 183,510 7,590 178,940 No. 5 2 8,130 292,290 7,940 285,280 7,750 278,280 7,570 271,290 12 x 8 lintels No. 4 1 2,640 25,400 2,420 23,140 2,190 19,790 1,960 16,560 No. 5 1 2,610 30,820 2,390 26,530 2,160 22,490 1,940 18,710 No. 6 1 2,580 33,930 2,360 29,090 2,130 24,540 1,910 20,300 No. 4 2 2,640 34,130 2,420 29,390 2,190 24,920 1,960 20,740 No. 5 2 2,610 38,300 2,390 32,820 2,160 27,670 1,940 22,880 12 x 16 lintels No. 4 1 6,240 62,030 6,020 59,720 5,790 57,420 5,570 55,110 No. 5 1 6,210 94,210 5,990 90,670 5,760 87,130 5,540 83,600 No. 6 1 6,190 131,170 5,960 126,190 5,740 121,220 5,510 116,250 No. 4 2 6,240 120,880 6,020 116,340 5,790 111,800 5,570 107,270 No. 5 2 6,210 164,010 5,990 154,330 5,760 144,860 5,540 135,620 12 x 24 lintels No. 4 1 9,840 99,130 9,620 96,800 9,390 94,470 9,170 92,150 No. 5 1 9,820 151,220 9,590 147,640 9,370 144,070 9,140 140,490 No. 6 1 9,790 211,410 9,560 206,370 9,340 201,330 9,110 196,300 No. 4 2 9,840 194,010 9,620 189,420 9,390 184,830 9,170 180,240 No. 5 2 9,820 294,720 9,590 287,680 9,370 280,650 9,140 273,620 16 x 8 lintels No. 4 1 3,555 25,730 3,252 23,454 2,950 21,183 2,647 18,919 No. 5 1 3,517 37,370 3,214 32,231 2,912 27,379 2,609 22,827 No. 6 1 3,479 41,444 3,177 35,606 2,874 30,109 2,571 24,967 No. 4 2 3,555 41,588 3,252 35,877 2,950 30,488 2,647 25,435 No. 5 2 3,517 47,137 3,214 40,477 2,912 34,214 2,609 28,364 16 x 16 lintels No. 4 1 8,396 62,598 8,093 60,278 7,791 57,959 7,488 55,642 No. 5 1 8,358 95,247 8,056 91,681 7,753 88,118 7,450 84,558 No. 6 1 8,320 132,815 8,018 127,796 7,715 122,781 7,413 117,771 No. 4 2 8,396 122,335 8,093 117,760 7,791 113,190 7,488 108,623 No. 5 2 8,358 185,320 8,056 178,313 7,753 171,314 7,450 164,256 A Grade 60 reinforcement. Metric equivalents: f' m = 1,500 psi (10.3 MPa); N = lb x 4.44822; N. m = lb-in x 0.112985; No. 4 bar = M 13; No. 5 = M 16; No. 6 = M 19. B For 8 in. (204 mm) lintels with two bars, low lift grouting is recommended for adjacent jambs to ensure proper grout flow and consolidation. NCMA TEK 17-1C 5
4 x wall thickness + width of bearing area of beam 45 Figure 3 Distribution of Concentrated Load for Running Bond Construction reinforced masonry the limitation remains as the clear lintel span divided by 600 but the 0.3 in. (7.6 mm) limit was removed. DESIGN EXAMPLE Beam 1 2 Clear span Effective span (see Figure 1) 1 2 45 Notes: 1) (2008 MSJC, ref. 2) Only the portion of the concentrated load shown intersecting the triangle over the lintel is applied to the lintel. 2) The 2005 MSJC (ref. 1) provides for the concentrated load to be distributed at a 45 o angle rather than at a 1:2 slope. In that case, no concentrated load would be applied to the lintel in this illustration. Design a lintel for a 12 in. (305 mm) normal weight concrete masonry wall laid in running bond with vertical reinforcement at 48 in. (1.2 m) o.c. The wall configuration is shown in Figure 4. Case 1 Arching Action Check for Arching Action. Determine the height of masonry required for arching action. Assuming the lintel has at least 4 in. (102 mm) bearing on each end, the effective span is: L = 5.33 + 0.33 = 5.67 ft (1.7 m). The height of masonry above the lintel necessary for arching to occur in the wall (from Figure 2) is h + 8 in. (203 mm) = L/2 + 8 in. = 3.5 ft (1.1 m). Because there is 18.0-7.33 = 10.67 ft (3.3 m) of masonry above the lintel, arching is assumed and the superimposed uniform load is neglected. Lintel Design Loads. Because arching occurs, only the lintel and wall dead weights are considered. Lintel weight, from Table 1, for 12 in. (305 mm) normal weight concrete masonry units assuming an 8 in. (203 mm) height is D lintel = 88 lb/ft (1.3 kn/m). For wall weight, only the triangular portion with a height of 3.5 ft (1.1 m) is considered. From Table 2, wall dead load is: D wall = 63 lb/ft 2 (3.5 ft ) = 221 lb/ft (3.2 kn/m) at the apex. Maximum moment and shear are determined using simply supported beam relationships. The lintel dead weight is considered a uniform load, so the moment and shear are, M lintel = D lintel L 2 /8 = (88)(5.7) 2 /8 = 357 lb-ft (0.48 kn-m) V lintel = D lintel L/2 = (88)(5.7)/2 = 251 lb (1.1 kn) For triangular wall load, moment and shear are, M wall = D wall L 2 /12 = (221)(5.7) 2 /12 = 598 lb-ft (0.81 kn-m) 6 NCMA TEK 17-1C
1,000 lb/ft (14.6 kn/m) superimposed live and dead uniform load 12 in. (305 mm) CMU f' = 1,500 psi (10.3 MPa) 5 ft 4 in. (1.6 m) Window 4 ft (1.2 m) 3 ft 4 in. (1.0 m) Figure 4 Wall Configuration for Design Example V wall = D wall L/4 = (221)(5.7)/4 = 315 lb (1.4 kn) Since the maximum moments for the two loading conditions occur in the same locations on the lintel (as well as the maximum shears), the moments and shears are superimposed and summed: M max = 357 + 598 = 955 lb-ft = 11,460 lb-in (1.3 kn-m) V max = 251 + 315 = 566 lb (2.5 kn) Lintel Design. From Table 3, a 12 x 8 lintel with one No. 4 (M 13) bar and 3 in. (76 mm) or less bottom cover has adequate strength (M all = 16,560 lb-in. and V all = 1,960 lb). In this example, shear was conservatively computed at the end of the lintel. However, Building Code Requirements for Masonry Structures (ref. 1 and 2) allows maximum shear to be calculated using a distance d/2 from the face of the support. Case 2 No Arching Action Using the same example, recalculate assuming a 2 ft (0.6 m) height from the bottom of the lintel to the top of the wall. For ease of construction, the entire 2 ft (0.6 m) would be grouted solid, producing a 24 in. (610 mm) deep lintel. Since the height of masonry above the lintel m is less than 3.5 ft (1.1 m), arching cannot be assumed, and the superimposed load must be accounted for. D lintel = 264 lb/ft (3.9 kn/m), from Table 1. Because the lintel is 24 in. (610 mm) deep, there is no additional dead load 18 ft due to masonry above the lintel. (5.5 m) W total = 264 lb/ft + 1,000 lb/ft = 1,264 lb/ft (18.4 kn/m) M max = wl 2 /8 = (1,264)(5.7) 2 /8 x 12 in./ft = 61,601 lb-in (7.0 kn-m) V max = wl/2 = (1,264)(5.7)/2 = 3,602 lb (16.0 kn) From Table 3, a 12 x 24 lintel with one No. 4 (M 13) reinforcing bar and 3 in. (76 mm) or less bottom cover is adequate (M all = 92,150 lb-in. and V all = 9,170 lb). NOTATIONS D lintel = lintel dead load, lb/ft (kn/m) D wall = wall dead load, lb/ft (kn/m) d = distance from extreme compression fi ber to centroid of tension reinforcement, in. (mm) f' m = specifi ed compressive strength of masonry, psi (MPa) h = half of the effective lintel span, L/2, ft (m) L = effective lintel span, ft (m) M all = allowable moment, in.-lb (N. m) M lintel = maximum moment due to lintel dead load, in.-lb (N. m) M max = maximum moment, in.-lb (N. m) M wall = maximum moment due to wall dead load moment, in.-lb (N. m) V all = allowable shear, lb (N) V lintel = maximum shear due to lintel dead load, lb (N) V max = maximum shear, lb (N) V wall = maximum shear due to wall dead load, lb (N) W total = total uniform live and dead load, lb/ft (kn/m) w = uniformly distributed load, lb/in. (N/ mm) NCMA TEK 17-1C 7
REFERENCES 1. Building Code Requirements for Masonry Structures, ACI 530-05/ASCE 5-05/TMS 402-05. Reported by the Masonry Standards Joint Committee, 2005. 2. Building Code Requirements for Masonry Structures, TMS 402-08/ACI 530-08/ASCE 5-08. Reported by the Masonry Standards Joint Committee, 2008. 3. International Building Code 2006. International Code Council, 2006. 4. International Building Code 2009. International Code Council, 2009. 5. Concrete Masonry Wall Weights, TEK 14-13B. National Concrete Masonry Association, 2008. 6. Design Tables for Concrete Masonry & Precast Concrete Lintels, TR091B. National Concrete Masonry Association, 2004. 7. Structural Masonry Design System, CMS10. National Concrete Masonry Association, Brick Industry Association, Western States Clay Products Association, and International Code Council, 2008. 8. Engineering Notes For Design With Concrete Block Masonry, Masonry Chronicles, Winter 2008-2009. Concrete Masonry Association of California and Nevada, 2009. NCMA and the companies disseminating this technical information disclaim any and all responsibility and liability for the accuracy and the application of the information contained in this publication. NATIONAL CONCRETE MASONRY ASSOCIATION 13750 Sunrise Valley Drive, Herndon, Virginia 20171 www.ncma.org To order a complete TEK Manual or TEK Index, contact NCMA Publications (703) 713-1900 Provided by: 8 NCMA TEK 17-1C