column weighing three tons. The boom of course buckled and was out of commission in an instant. A new and larger derrick was finally used.

From what was previously stated it appears that derrick accidents are at times very serious and most regretable. It is incumbent upon the inspector as well as upon the erection superintendent of each job, to carefully examine all parts and accessories of each derrick as often as possible, in order to avoid accidents and injuries to men and structures.

CHAPTER IX.

Iron in Retaining Walls and Footings.

RETAINING WALLS.

In the erection of tall buildings, as soon as the excavations will allow, and as early as possible, sheath piling is driven along the sidewalk and the material is removed to make room for a retaining wall. There are three kinds of retailing walls in common use:

The most usual is the brick retaining wall. The Code requires that such walls shall be laid in cement mortar, like all the walls below curb, and the width of the retaining wall at the base must not be less than 1/4 of the height of the wall. No iron is used in this kind of retaining walls.

The most graceful retaining walls which in the same time are stronger and take up less room in the cellar, are the reinforced concrete walls. One inch reinforcing bars spaced about 18 inches on centres vertically, and cross bars about 2 ft. on centres and about 1 inch thick are commonly used. Of course in walls reaching 30 to 40 feet, brick work may be out of question and the concrete wall will be designed in the usual way. The only objection to concrete retaining walls is the need of forms and the time lost in the setting of the concrete.

A third form of retaining walls often reaching over 30 ft. is sometimes used. This consists of heavy channels, placed vertically against the embankment about four feet apart, and braced against the main structure by means of steel beams, and with circular brick arches in between.

PIERS FOR COLUMNS.

For tall buildings in general the column piers are carried down to rock, using caissons if necessary. Where the rock is not far from the proposed cellar bottom, the walls between columns are also started on rock. Where caissons are used or where the rock is too low, steel beams are placed from column pier to column pier, and then the brick wall is started on top of these beams. Where the ground is soft and the rock is not within economic distance from the surface spread footings or piles may be used.

For lighter structures the piers may be left out, and the columns may rest directly on a spread footing carried by the

soil at the bottom of the excavation. Where no piers are used the Building Code allows a bearing capacity of:

I ton per square foot on soft clay.

2 tons per square foot on clay and sand in layers, wet and springy.

3 tons per square foot on loam, clay, or fine sand, firm and dry.

4 tons per square foot on coarse sand, stiff gravel or hard clay.

These values are allowed where no tests are made. In all doubtful cases or where the owner wants a larger bearing allowance the Building Department will make tests at the expense of the owner. These tests are generally carried out as follows:

Upon a timber platform constructed for the purpose, the load per square foot which is proposed to impose upon the soil is first applied and allowed to remain undisturbed for at least forty-eight hours. During this time measurements are being taken once each twenty-four hours or oftener in order to determine the settlement, if any. After forty-eight hours 50 per cent of the first load is added, and the total load is left undisturbed for at least six days, careful measurements and reading being taken once in twenty-four hours, or oftener, in order to determine the settlement. The test is not considered satisfactory or the result acceptable unless the proposed safe load shows no appreciable settlement for at least two days and the total test load shows no settlement for at least four days.

The accepted safe load shall not exceed two-thirds of the final test load.

Piers. Before a pier is built, the pier hole must be inspected and approved. Where piers have to go down to solid rock, a man gets into the pier hole and sounds the bottom with a crow bar. Good rock is known by its general appearance and by a fairly clear ringing sound which it gives when struck with a bar. All soft spots must be cleaned out before the pier hole is approved. In some cases, although very seldom, these tests fail to indicate to the inspector whether solid rock, or simply a large boulder has been struck, unless great care is exercised.

Sometimes piers have been erected on top of old sewers or old well holes. These are dangerous cases and mostly met with in smaller buildings where the excavations are not carried far below curb.

After the pier bottoms have been approved, they are filled in with a mixture not poorer than I cement, 2 sand, and 4

broken stone or gravel. This is required by the Building Code. Each pier must be brought to the proper elevation on top,, and must be allowed to set hard before placing any load on top of it.

If after setting the piers come too high, on account of incorrect levelling, the top of the piers are cut down to within 3/4 in. below the bottom of the column footing, whether it be a cast iron base or a grillage. This 3/4 inch space allows for proper grouting.

In one case about fifty piers came too low by from 2 to 4 inches, all due to the leveler's mistake in starting from a wrong bench mark. Wooden forms had to be built around each pier, after the pier surface was made very rough; water was abundantly supplied to flush the pier and then a rich concrete was dumped on top to the required elevation. In order to make absolutely certain that these instructions were carried out, the builder cut down not less than one foot from the top of each pier. This insured a real rough surface of contact between the new and the old work.

For piers carried down to rock in caissons, the Code allows fifteen tons per square foot. For piers carried down to rock in open trenches or in sheet piling, only eight tons per square foot is allowed. This difference is due to the fact that in caisson work the caisson helps making a pier of a uniform cross section. The caisson will keep the mass together until set, and even then the caisson as a rule is left in place, and this adds some more strength to the pier. On the other hand in open trenches the pier may be irregular in cross-section, and the grout between stones may be lost by absorption into the soil, making the pier useless near the edges.

For piers carried down in caissons to gravel or hard clay, the Building Code allows ten tons per square foot.

Loads as high as 30 tons per sq. ft. may be allowed on good rock, where the piers are reinforced near the top by two or more rows of horizontal 1⁄2 in. steel round bars, placed about six inches on centres and about six inches apart vertically.

GRILLAGE. Rolled beams, channels or girders are generally used to distribute the column loads upon the top of the piers. The Building Code requires that all grillage beams shall be provided with proper bolts and separators, to keep them in place at a proper distance apart. It is also specified that all grillage must be inclosed and filled in solid with concrete. This is usually done by setting the grillage on wooden wedges, at the proper elevation and about 1⁄2 in to 3/4 in. above the pier. A form is then built around the grillage and the

concrete poured in. Where the beams are too close together, grout, or fine gravel concrete will have to be used to fill in the spaces in between the beams.

Separators are placed to keep the beams properly spaced. In the same time separators stiffen the web of the beams and for this reason they are generally placed directly under the column. The separators for grillage are mostly one inch gas pipe cut to length, and provided with 3/4 in. bolts. Other means for stiffening the webs of grilliage beams and for preventing them from crippling under the load, is to use heavier standard beams with thicker webs, or two channels back to back with a plate in between and riveted together, or even stiffener angles against the webs of grillage beams like in an ordinary plate girder.

Where two layers of grilliage are used under a column, the upper grillage in good work is bolted to the lower grillage. Some engineers insist however, that a space of about 1⁄2 in. should be left between the two grillage layers for grouting. Of course a grout of one part cement and one sand in such a thin layer will stand about six thousands lbs. per sq. inch before being crushed into powder and the objection that this grout will be crushed under the load may be disregarded. The reason for grouting in between rather than having the grillages in contact is that rolled sections are seldom of exactly the same depth. In fact their depth will vary in some cases more than 8 in. Consider now a column footing made of a lower and an upper grillage with no grout in between the two, and with the upper grillage consisting of three I beams. If, for instance, the middle beam of this upper grillage is not of full depth by 8 in., such a beam will be useless, because it will not carry any load until the other two beams, overloaded as they may be, will cripple in the web for % in. Grouting in between the two layers would tend to avoid these.