67. Cast-Iron Lintels

When the fracture of an iron lintel is produced by vertical pressure, the fibres or molecules of the lower section are separated by extension; consequently a lintel of the section shown in Fig. 1, when the weight is imposed, will have the bottom flange C in a state of tension and the top rib D in a state of compression, and the parts of the section generally will be extended or compressed according to their distance from the neutral axis A - B.

If cast-iron lintels were required to serve to the full extent of the power of the metal to resist rupture, the base or bed should be five times the top flange, cast iron in compression being 5 to 1 in tension.* On the other hand, they are seldom subjected in practice to the ultimate load, and when the flanges are as 1 to 4 the proportion meets the requirements of practice.

Allowance of an excess of strength should be made where a lintel is loaded more on one side than on the other, as it always has a tendency to twist and thus produce fracture.

In the plain cast-iron lintel, Fig. 2, the weakest part is the top of the vertical part C. It cannot resist compression in a degree corresponding to its tenacity, and by many experiments made as to its strength it is found that fracture is produced by the crushing of the vertical part C; by stiffening the web, as shown in Fig. 5, the strength is somewhat increased (see test, lintel B). This also occurs when the lintel is built in with the wall, being thus braced by the surrounding masonry.

* In first-class foundries a high average is 20,000 pounds per square inch in tension. This gives 4 1/2 to I.

68. Skew-Back Lintels

Skew-Back Lintels are placed over openings to receive the thrust of the arch, and to make a square-finished ppening for windows or doors. The bed of lintel acts as a tie bar between the skew-backs.

69. Lintels For Iron Fronts

Lintels For Iron Fronts are placed between upright and continuous columns of fronts, or a continuous line of lintels is placed on top of the columns to receive the cornice and masonry. The ribs with holes in the ends are for bolting the lintels to each other and to columns. (See article Iron Fronts).

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70. Sidewalk Lintels

Sidewalk Lintels as shown by Fig. 7, are made to receive the granite sidewalk A and vault lights B. The lintel should be one inch lower than the granite, to form a rebate for the vault-light frame. Where several lintels are in one line, they are connected and bolted together similarly to lintels for fronts, Fig. 6.

71. Window-Head Lintel

This acts as a finished window head and lintel at the same time. It is set flush with wall outside, the section at A being formed for the head of window frame.

72. Double-Web Lintels

Where lintel beds are required to be over 16 inches wide more than one web is used. For handling and moulding it is best to make the beds not more than 24 inches wide; if wider than this, several lintels with double webs should be placed side by side.

73. Window Sills

For openings where the window-head lintels are used the sills are made of cast iron 3/8 of an inch thick, with a wash on top and lip on back to receive the sill of window frame.

74. Rule For Breaking Weight At Middle

Mr. Hodg-kinson gives for breaking weight W, when the bottom flange is four times the top flange, W=(ca'd)/l, in which W= breaking weight at middle; / = span in inches ; d = extreme depth in inches; a = area of bottom flange; c = 26, a constant derived from experiment where the weight was in tons of 2240 pounds and the dimensions all in inches; and therefore W = (26a'd)/l, where a is a factor of safety equal to 5, W= (26ad)/(al), or a = (Wal)/(26d).

Example

What should be the area of the bottom flange of a lintel to sustain safely a load at middle of 10 tons, span 12 feet, height of lintel at centre 12 inches ? By formula, a = (10x5x144)/(26x12)= 23.1 inches.

75. Web

The web at end need only be thick enough to resist the shearing strain upon the metal. Owing to the tendency to fracture when the casting is cooling, the thickness of web should not be less than the flange.

76. Tests Of Cast-Iron Lintels

The results of some tests on cast iron, among which were two cast-iron lintels, by Geo. A. Just, C.E., for the Jackson Architectural Iron Works, illustrated in vol. XXII (Feb. 1890), Transactions of the Am. Soc. C. E., gave results corresponding nearly with those of the above formula for W, when the lower flange was four times the top flange.

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The lintels here illustrated were proportioned as follows:

Lintel A

Weight.................306 pounds.

Length.............6 feet 3 1/8 inches.

Width of bottom flange.....12 "

Depth at centre............ 12 "

" " ends..............4 "

Average thickness of flange.13/18 "

" web....7/8 "

Clear span............5 feet 6 "

Ultimate strength... .98,450 pounds.

Lintel B

Weight................275 1/2 pounds.

Length..............7 feet 4 inches.

Width of bottom flange .. .12 "

Depth at centre............8 "

" " ends..............4 "

Average thickness of flange. 5/8 "

" web...3/4 "

Clear span...................6 feet.

Ultimate strength.....40,030 pounds.

Both lintels were subjected to a centre load. When 81,700 pounds had been applied to lintel A, its deflection was approximately 5/8 inch. Upon releasing the load an approximate set of 1/8 inch was found, and the lintel broke in the centre, its load being applied practically at one point.

In the case of lintel B, failure took place by tearing of the lower flange, the web having been stiffened by triangular brackets, and the lintel broke at the flaw about 6 inches from the centre, its load being distributed over the web for a length of 8 inches.

By formula, W = (26a'd)/l.

Then for lintel A,

W = (26x9.75x12)/66 = 46 tons(2240 lbs.) or 103,040 lbs.; and for lintel B,

W = (26x7.5x8)/72= 22 tons, or 49,280 lbs.

From the above formula and tests, lintels in ordinary use, as Figs. 3 and 5, may be employed, - the circumstances of the case regulating their proper use; - a factor of safety of 5 being adopted.