Stair Forms

Concrete stairway forms require accurate layout to ensure accurate finish dimensions for the stairway. Stairways should always be reinforced with rebars (reinforcing bars) that tie into the floor and landing. They are formed monolithically or formed after the concrete for the floor slab has set. Stairways formed after the slab has set must be anchored to a wall or beam by tying the stairway rebars to rebars projecting from the walls or beams, or by providing a keyway in the beam or wall. You can use various stair forms, including prefabricated forms. For moderate-width stairs joining typical floors, a design based on strength considerations is generally not necessary. Figure 7-16 shows one way to construct forms for stair widths up to and including 3 feet. Make the sloping wood platform that serves as the form for the

Figure 7-14.-Joining wall form panels at a corner.

Figure 7-15.-Form for panel wall and columns.

underside of the steps from 3/4-inch plywood. The platform should extend about 12 inches beyond each side of the stairs to support the stringer bracing blocks. Shore up the back of the platform with 4-by-4 supports, as shown in figure 7-16. The post supports should rest on wedges for easy adjustment and removal. Cut 2-by-12 planks for the side stringers to fit the treads and risers. Bevel the bottom of the 2-by-12 risers for easy form removal and finishing.

Beams and Girders

The type of construction used for beam and girder forms depends upon whether the forms are to be removed in one piece or whether the sides are to be

Figure 7-16.-Stairway form.

Figure 7-17.-Typical beam and girder form.

stripped and the bottom left in place until the concrete has hardened enough to permit removal of the shoring. The latter type of form is preferred, and details for this type are shown in figure 7-17. Although beam and girder forms are subjected to very little bursting pressure, they must be shored up at frequent intervals to prevent sagging under the weight of fresh concrete.

The bottom of the form should be the same width as the beam and should be in one piece for the full width. The sides of the form should be 1-inch-thick tongue-and-groove sheathing and should lap over the bottom as shown in figure 7-17. The sheathing is nailed to 2-by-4-inch studs placed on 3-foot centers. A 1-by-4-inch piece is nailed along the studs. These pieces support the joist for the floor panel, as shown in figure 7-18, detail E. The beam sides of the form are not nailed to the bottom. They are held in position by continuous strips, as shown in detail E. The

Figure 7-18.-Assembly of beam and floor forms.

crosspieces nailed on top serve as spreaders. After erection, the slab panel joists hold the beam sides in position. Girder forms (figure 7-17) are the same as beam forms except that the sides are notched to receive the beam forms. Temporary cleats should be nailed across the beam opening when the girder form is being handled.

The entire method of assembling beam and girder forms is illustrated in figure 7-18. The connection of the beam and girder is illustrated in detail D. The beam bottom butts up tightly against the side of the girder form and rests on a 2-by- 4-inch cleat nailed to the girder side. Detail C shows the joint between the beam and slab panel, and details A and B show the joint between the girder and column. The clearances given in these details are needed for stripping and also to allow for movement that occurs due to the weight of the fresh concrete. The 4-by-4 posts (detail E) used for shoring the beams and girders should be spaced to provide support for the concrete and forms. They should be wedged at the bottom to obtain proper elevation.

Figure 7-19 shows you how the same type of forming can be done by using quick beams, scaffolding, and I-beams-if they are available. This type of system can be set up and taken down in minimum time.

Oiling and Wetting Forms

You should never use oils or other form coatings that may soften or stain the concrete surface, prevent the wet surfaces from water curing, or hinder the proper functioning of sealing compounds used for curing. If you cannot obtain standard form oil or other form coating, you can wet the forms to prevent sticking in an emergency.

OIL FOR WOOD FORMS.- Before placing concrete in wood forms, treat the forms with a suitable form oil or other coating material to prevent the concrete from sticking to them. The oil should penetrate the wood and prevent water absorption. Almost any light-bodied petroleum oil meets these specifications. On plywood, shellac works better than oil in preventing moisture from raising the grain and detracting from the finished concrete surface. Several commercial lacquers and similar products are also available for this purpose. If you plan to reuse wood forms repeatedly, a coat of paint or sealing compound will help preserve the wood. Sometimes lumber contains enough tannin or other organic substance to soften the concrete surface. To prevent this, treat the form surfaces with whitewash or limewater before applying the form oil or other coating.

OIL FOR STEEL FORMS.- Oil wall and steel column forms before erecting them. You can oil all other steel forms when convenient, but they should be oiled before the reinforcing steel is placed. Use specially compounded petroleum oils, not oils intended for wood forms. Synthetic castor oil and some marine engine oils are examples of compound oils that give good results on steel forms.

APPLYING OIL.- The successful use of form oil depends on how you apply it and the condition of the forms. They should be clean and have smooth surfaces. Because of this, you should not clean forms with wire brushes, which can mar their surfaces and cause concrete to stick. Apply the oil or coating with a brush, spray, or swab. Cover the form surfaces evenly, but do not allow the oil or coating to contact construction joint surfaces or any reinforcing steel in the formwork. Remove all excess oil.

OTHER COATING MATERIALS.- Fuel oil, asphalt paint, varnish, and boiled linseed oil are also suitable coatings for forms. Plain fuel oil is too thin to use during warm weather, but mixing one part petroleum grease to three parts of fuel oil provides adequate thickness.

Form Failure

Even when all form work is adequately designed, many form failures occur because of human error, improper supervision, or using damaged materials. The following list highlights some, but not all, of the most common construction deficiencies that supervisory personnel should consider when working with concrete:

Inadequately tightened or secured form ties;

Inadequate diagonal bracing of shores;

Use of old, damaged, or weathered form materials;

Use of undersized form material;

Shoring not plumb;

Failure to allow for lateral pressures on form work; and

Failure to inspect form work during and after concrete placement to detect abnormal deflections or other signs of imminent failure.

There are many reasons why forms fail. It is the responsibility of the Builder to ensure that the forms are correctly constructed according to design, and that proper techniques are followed.

REINFORCED CONCRETE LEARNING OBJECTIVE: Upon completing this section, you should be able to determine the types of ties for and placement of reinforcing steel.

Concrete is strong under compression, but relatively weak under tension. The reverse is true for steel. Therefore, when the two are combined, one makes up for the deficiency of the other. When steel is embedded in concrete in a manner that assists it in carrying imposed loads, the combination is known as reinforced concrete. The steel may consist of welded wire fabric or expanded metal mesh, but, more often, it consists of reinforcing bars, or more commonly "rebar."

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