This section is from the book "The American House Carpenter", by R. G. Hatfield. Also available from Amazon: The American House Carpenter.

In the following table are recorded the results of experiments made to test the capability of the various materials named to resist the effects of transverse strain. The figures are taken from the author's work, Transverse Strains, before referred to.

Material. | B. | F. | e. | a. |

Resistance to Rupture. | Resistance to Flexure. | Extension of Fibres. | Margin for Safety. | |

Georgia Pine......... | 850 | 5900 | .00109 | 1. 84 |

Locust............ | 1200 | 5050 | •0015 | 2.20 |

White Oak................... | 650 | 3100 | •00086 | 3.39 |

Spruce......... | 550 | 3500 | •00098 | 2.23 |

White Pine......... | 500 | 2900 | •OO14 | 1.71 |

Hemlock............. | 450 | 2800 | •00095 | 2.35 |

White Wood.......... | 600 | 3450 | •00096 | 2.52 |

Chestnut......... | 480 | 2550 | •00103 | 2.54 |

Ash......................... | 900 | 4000 | •00111 | 2 82 |

Maple............ | 1100 | 5150 | •0014 | 2.12 |

Hickory.............. | 1050 | 3850 | •0013 | 2.91 |

Cherry............ | 650 | 2850 | •001563 | 2.03 |

Black Walnut........ | 750 | 3900 | •00104 | 2.57 |

Mahogany(St. Domingo).. | 650 | 3600 | .00116 | 2.l6 |

,, (Bay Wood).................. | 850 | 4750 | •00109 | 2.28 |

Cast Iron, American...... | 2500 | 50000 | ||

,, English..... | 2100 | 40000 | ||

Wrought Iron, American... | 2600 | 62000 | ||

,, English................ | 1900 | 60000 | ||

Steel, in Bars........... | 6000 | 70000 | ||

Blue Stone Flagging... | 200 | |||

Sand Stone....... | 59 | |||

Brick, common..... | 33 | |||

,, pressed......... | 37 | |||

Marble, East Chester........ | 147 |

The figures in the second column, headed B, denote the weight in pounds required to break a unit of the material named when suspended from the middle, the piece being supported at each end. The unit of material is a bar one inch square and one foot long between the bearings. The third column, headed F, contains the values of the several materials named as to their resistance to flexure, as explained in Arts. 302-305, Transverse Strains. These values of F, as constants, are used in the rules. The fourth column, headed e, contains the values of the several materials named, denoting the elasticity of the fibres, as explained in Art. 312, Transverse Strains. These values of e, as constants, are used in the rules.

The fifth column, headed a, contains for the several materials named the ratio of the resistance to flexure as compared with that to rupture, and which, as constants used in the rules, indicate the margin of safety to be given for each kind of material. The figures given in each case show the smallest possible value that may be safely given to a, the factor of safety. In practice it is generally taken higher than the amount given in the table. For example, in the table the value of B, the constant for rupture by transverse strain for spruce, is 550.

Now, if the dimensions of a spruce beam to carry a given weight be computed by the rules, using the constant B, at 550, the beam will be of such a size that the given weight will just break it.

But if, in the computation, instead of taking the full value of B, only a part of it be taken, then the beam will not break immediately; and if the part taken be so small that its effect upon the fibres shall not be sufficient to strain them beyond their limit of elasticity, the beam will be capable of sustaining the weight for an indefinite period; in this case the beam will be loaded by what is termed the safe weight. Or, since the value of a for spruce is 2.23 in the table, if, instead of taking B at 550, its full value, only the quotient arising from a division of B by a be taken - or 550 divided by 2.23, which equals 246.6 - then the beam will be of sufficient size to carry the load safely. Therefore, while the constant B is to be used for a breaking weight, for a safe load the quotient of - is to be used. But, again, if a be taken at a its value as given in the table, the computed beam will be loaded up to its limit of safety. So loaded that, if the load be increased only in a small degree, the limit of safety will be passed, and the beam liable, in time, to fail by rupture.

Therefore, as the values of a, in the table, are the smallest possible, it is prudent in practice always to take a larger than the table value. For example, the table value of a for spruce is 2.23, but in practice let it be taken at 3 or 4.

Continue to: