34. Channels

Channels are placed against walls in place of I beams to receive the wall arches.

35. Properties Of Wrought-Iron Channels

To find the safe load equally distributed, divide the coefficient by the span in feet. For a safe centre load take one half the quotient.

Note. - Inasmuch as there is a great diversity in published tables of safe load for beams, etc., every one must judge for himself what proportion of the elastic strength of the beam will best suit his purpose.

36. Steel Channels

To find the safe load equally distributed, divide the coefficient by the span in feet. For a safe centre load take one half the quotient.

37. Zee Bars

Zee Bars are used in a similar manner, but prove more suitable than channels in many cases, as the one leg of the zee bar is built in with the wall, preventing any passage of fire from one story to another. See list of zee bars in common use, under chapter on Wrought-iron Columns.

38. Floors Should Be Rigid

The strength of a beam may be sufficient to carry the imposed load, but the risk of vibration is also to be considered. Since repeated vibrations may injure the connections with columns and masonry, the floor should be rigid, as rigidity prevents vibration.

In certain circumstances it is advisable to consider whether it may not be more economical to use deep beams placed well apart than a greater number of light ones set at shorter intervals.

39. Elastic Limit

Under no circumstances should a beam be strained beyond the limits of elasticity; in other words, deflected so much that after the removal of the load it will not return to its former position before being loaded.

40. Maximum Deflection

A beam that may be quite strong enough to carry a given load may deflect under this load more than is desirable, and stiffness being a different quality from strength, about one thirtieth (1/30) of an inch per foot of clear span is the usual maximum of deflection that is permissible: under ordinary loads this is attained when the clear span is about twenty-six times the depth of the beam.

The deflection of iron is somewhat greater than that of steel, experiments having proved that a steel beam will deflect slightly less than an iron beam of the same section under the same load.

41. Framed Beams

Care should be exercised, in placing and spacing the beams, to see that there is not an excess of strength or too little used, and that beams are placed for support of partitions, framing of stair wells, floor lights, and to observe proper spacing and levels for arches. In the annexed plan the beams are shown as drawn on floor plans of buildings.

Fig. 2 is an elevation of beam K, with the header or crossbeam L framed, or coped, into it at A (see also connection A, Fig 3). The distance F should be given on plan between centres of beams, unless there are special reasons for making a fixed distance between flanges of K beam at M.

The distance E on elevation of beam K is generally three inches, with 13/16-inch holes for 3/4-inch-dtameter tie rods, 15/16-inch for 7/8-inch rods, and 1 1/16inch for one-inch rods.

42. Tie Rods

Tie Rods may be placed from four to six times the depth of beam apart, and holes are usually punched in the centre of web; but they should be placed as low as possible, so as not to be seen, and not injure under side of arch. Fig. 4 at B shows a smaller framed into a larger beam, and flush on bottom, with the angle-iron knees on the web close in between the flanges. Fig. 5 is the opposite view of Fig. 3, and shows two knees attached to the framed beam, the centre of holes regulated by the size of angles to be used. The rivets or bolts, Figs. 4 and 5, should be proportioned to resist the shearing moment; that is, the strain at the joint multiplied by the leverage or average distance of the bolt or rivet B from the face of joint.

43. Beam Connections

It is not an uncommon thing to make the connection by resting the cross-beam on the flange of its supporter. As the fitting is not accurately done, it is not customary to depend on the lower flange taking the load, for the reason that if the beam is to resist bending strains symmetrically, these strains should be transmitted through axis of beam, which would not occur if the cross-beam rested on one flange of its supporter. It would not be felt injuriously, however, if the connection did not occur at middle of beam, or if a corresponding beam occurred directly opposite.

In Fig. 4 a section of portion of floor is shown. C distance is allowed for the passing of pipes, and is generally two inches; D distance is filled with a light fire-proof material, such as ashes, etc.