SECTION III. CONSTRUCTION
4-45. The portion of a structure that extends above the ground level is called the superstructure. The portion below ground level is called the substructure. The parts of the substructure that distribute building loads to the ground are called foundations. Footings are installed at the base of the foundations to spread the loads over a larger ground area to prevent the structure from sinking into the ground. Forms for large footings such as bearing wall footings, column footings, and pier footings are called foundation forms. Footings or foundations are relatively low in height since their primary function is to distribute building loads. Because the concrete in a footing is shallow, pressure on the form is relatively low. Therefore, it is generally not necessary to base a form design on strength and rigidity.
SIMPLE FOUNDATION FORMS
4-46. Whenever possible, excavate the earth and use it as a mold for concrete footings. Thoroughly moisten the earth before placing the concrete. If this is not possible, you must construct a form. Because most footings are rectangular or square, it is easier to erect the four sides of the form in panels. The following steps and figures helps explain this process:
Step 1. Build the first pair of opposing panels (see [a] in Figure 4-7 to the footing width. Nail vertical cleats to the exterior sides of the sheathing. Use 2-inch dressed lumber for the cleats and space them 2 1/2 inches from each end of the exterior sides of the (a) panels and on 2-foot centers between the ends. Use forming nails driving them only part way for easy removal of the forms.
Figure 4-7. Typical foundation
Step 2. Nail two cleats to the ends of the interior sides of the second pair of panels (see [b] in Figure 4-7). The space between them should equal the footing length, plus twice the sheathing thickness. Nail cleats on the exterior sides of the (b) panels, spaced on 2-foot centers. Erect the panels into either a rectangle or a square, and hold them in place with form nails. Make sure all reinforcing bars are in place.
Step 3. Drill small holes on each side of the center cleat on each panel less than 1/2 inch in diameter to prevent paste leakage. Pass number 8 or number 9 black annealed iron wire through these holes and wrap it around the center cleats of opposing panels to hold them together as shown in Figure 4-7.
Step 4. Mark the top of the footing on the interior side of the panels with grade nails. For forms 4 feet square or larger, drive stakes against the sheathing as shown in Figure 4-7. Both the stakes and the 1- by 6-inch tie braces nailed across the top of the form keep it from spreading apart. If a footing is less than 1 foot deep and 2 feet square, you can construct the form from 1-inch sheathing without cleats. Simply make the side panels higher than the footing depth, and mark the top of the footing on the interior sides of the panels with grade nails. Cut and nail the lumber for the sides of the form as shown in Figure 4-8.
Figure 4-8. Small footing form
FOUNDATION AND PIER FORMS COMBINED
4-47. It is best to place the footing and a small pier at the same time. A pier is a vertical member, either rectangular or round, that supports the concentrated loads of an arch or bridge superstructure. The form that is build for this type of construction is shown in Figure 4-9 below. The footing form should look like the one in Figure 4-7. You must provide support for the pier form while not interfering with concrete placement in the footing form. This can be accomplished by first nailing 2 by 4s or 4 by 4s across the footing form, as shown in Figure 4-9, that serve as both supports and tie braces. Nail the pier form to these support pieces.
Figure 4-9. Footing and pier form
BEARING WALL FOOTING
4-48. Figure 4-10 below shows footing formwork for a bearing wall and Figure 4-11 shows bracing methods for a bearing wall footing. A bearing wall, also called a load-bearing wall, is an exterior wall that serves not only as an enclosure, but also transmits structural loads to the foundation. The form sides are 2 inch lumber whose width equals the footing depth. Stakes hold the sides in place while spreaders maintain the correct distance between them. The short braces at each stake hold the form in line.
Figure 4-10. Typical footing form
Figure 4-11. Methods of bracing bearing wall footing form
COLUMN AND FOOTING FORMS
4-49. Square column forms are made from wood whereas round column forms are made from steel or cardboard impregnated with waterproofing compound. Figure 4-2 shows an assembled column and footing form. After constructing the footing form follow the steps below:
Step 1. Build the column form sides in units first and then nail the yokes to them.
Step 2. Determine the yoke spacing from Table 4-1 according to the procedure described in paragraph 4-43.
Step 3. Erect the column form as a unit after assembling the steel reinforcement and tying it to the dowels in the footing.
Step 4. Place reinforcing steel in columns as described in Chapter 6. The column form should have a clean-out hole in the bottom to remove construction debris. Be sure to nail the pieces of lumber that you remove for the clean-out hole to the form. This way you can replace them exactly before placing concrete in the column.
4-50. A panel wall is an exterior or curtain wall made up of a number of units that enclose a structure below the roof line. It is built only strong enough to carry its own weight and withstand wind pressure on its exterior face.
4-51. Panel-wall units should be constructed in lengths of about 10 feet for easy handling. The sheathing is normally a 1-inch (13/16-inch dressed) tongue-and-groove lumber or 3/4-inch plywood. Make the panels by first nailing the sheathing to the studs. Then connect the panels as shown in Figure 4-12 below. Figure 4-13 below shows the form details at the wall corner. When placing concrete panel-walls and columns at the same time, construct the wall form as shown in Figure 4-14. Make the wall form shorter than the clear distance between the column forms to allow for a wood strip that acts as a wedge. When stripping the forms, remove the wedge first to facilitate form removal.
Figure 4-12. Method of connecting panel wall unit form together
Figure 4-13. Detail at corner of panel wall form
Figure 4-14. Form for panel walls and columns
SPREADERS AND TIES
4-52. Spreaders keep the sides of the wall form at the proper distance from each other until you place the concrete. Tie wires or tie-rods hold the sides firmly in place so that the weight of the freshly placed concrete does not push them apart. Figures 4-15 and 4-16 below show the installation of tie wires and tie-rods. Use the following steps to install them:
Figure 4-15. Detail at corner of panel wall form
Figure 4-16. tie-rod and spreader for wall form
Step 1. Use tie wires (see Figure 4-15 above) only for low walls or when tie-rods are not available. Number 8 or number 9 black annealed iron wire is recommended, but you can use barbwire in an emergency.
Step 2. Space the individual ties the same as the stud spacing but never more than 3 feet apart.
Step 3. Place a spreader near the location of each tie wire or tie-rod, as shown in view 2 of Figure 4-15. Form each tie by looping a wire around the wale on one form side, bringing, it through the form and looping it around the wale on the opposite form side.
Step 4. Make the wire taut by inserting a nail between the two strands and twisting. When using the wedge method shown in view 1 of Figure 4-15, twist the ends of the wire strands around a nail driven part way into the point of a wedge. Remove the nail and drive the wedge until it draws the wire taunt.
Step 5. Use pull wires to remove the spreaders as you fill the form so that they are not embedded in the concrete. Figure 4-17 below shows an easy way to remove spreaders. Loop a wire around the bottom spreader and pass it through off-center holes drilled in each succeeding spreader.
Figure 4-17. Removing wood spreader
Step 6. Pull on the wire to remove one spreader after another as the concrete level rises in the form. Use tie-rods if rust stains are unacceptable or if the wall units must be held in an exact position.
Step 7. Space spreaders as shown in Figure 4-17 the same as studs. After stripping the form, break off the rod at the notch located slightly inside the concrete surface. For a better appearance, grout in the tie-rod holes with a mortar mix.
4-53. Various types of stair forms, including prefabricated ones, can be used. A design based on strength is not necessary for moderate-width stairs joining typical floors. Figure 4-18 below 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 underside of the steps from 1-inch tongue-and groove-sheathing. The platform should extend about 12 inches beyond each side of the stairs to support the stringer bracing blocks. Support the back of the platform with 4 by 4 posts. Space the 2 by 6 cleats nailed to the post supports on 4-foot centers. The post supports should rest on wedges for easy adjustment and removal. Cut the 2 by 12 planks for the side stringers to fit the tread and risers. Bevel the 2 by 12 risers as shown.
Figure 4-18. Stairway form
STEEL PAVEMENT FORMS
4-54. When possible, use standard steel pavement forms that can be set fast and accurately. They contain the concrete and provide a traction surface for form-riding equipment. These steel plate sections are 10 feet long and are normally available in slab thicknesses of 6, 7, 8, 9, 10, and 12 inches. The usual base plate width is 8 inches. Locking plates rigidly join the end sections. Three 1-inch diameter steel stakes per 10-foot section of form are required.
After driving the stakes, set the form true to line, them grade and clamp it to the stakes using special splines and wedges. Set the forms on the subgrade ahead of the paver and leave them in place for at least 24 hours after placing the concrete. Oil the forms immediately after setting them and before depositing the concrete. After oiling the form completely, wipe the top side or traction edge free from oil. Remove the forms as soon as practical, then clean, oil, and stack them in neat piles ready for reuse.
OILING AND WETTING FORMS
4-55. Make sure that any oil or other form coating that is used on forms does not 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 standard form oil or other form coating is not obtainable, wet the forms to prevent sticking. Use water only in a emergency.
OIL FOR WOOD FORMS
4-56. Before placing concrete in wood forms, treat the forms with a suitable oil or other coating material to prevent the concrete from sticking to them. The oil should penetrate the wood and prevent water absorption, making it environmentally safe. 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. A coat of paint or sealing compound helps to preserve the wood if you plan to reuse wood forms repeatedly. Sometimes lumber contains enough tannin or other organic substance to soften the surface concrete. To prevent this condition, treat the form surfaces with whitewash or limewater before applying the form oil or other coating.
OILFOR STEEL FORMS
4-57. Oil steel column and wall forms before erecting them. You can oil all other steel forms when convenient but before reinforcing steel is in place. Use specially compounded petroleum oils, not oils intended for wood forms. Synthetic castor oil and some marine engine oils are typical examples of compound oils that give good results on steel forms.
4-58. The successful use of form oil depends on the method of application and the condition of the forms. Forms should be clean and have smooth surfaces. Therefore, do 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
4-59. 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 enough thickness.
4-60. Safety precaution should be practiced at all times. When work is progressing on elevated forms, take precautions to protect both workers on scaffolds and ground personnel.
4-61. Protruding nails are the main source of formwork accidents. Observe the following safety precautions during construction:
4-62. Observe of the following safety precaution when stripping forms:
4-63. Even when all formwork is adequately designed, many form failures occur because of some human error or improper supervision.
4-64. The following items are the most common construction deficiencies in wall-form failures and should be considered by supervisory personnel when working with concrete:
OVERHEAD SLAB FAILURES
4-65. The Following Are common Construction Deficiencies In Overhead slab Failure:
OTHER GENERAL CAUSES OF FAILURE
4-66. The following are miscellaneous causes of form failure:
4-67. This list is not all inclusive; there are many reasons why forms fail. It is the responsibility of the form designer to ensure that the concrete will not fail when the designs are properly interpreted and constructed. It is the responsibility of the builder to make sure that the design is correctly constructed and that proper techniques are followed.
|David L. Heiserman, Editor||
Copyright © SweetHaven
Revised: June 06, 2015