Formwork for Concrete Structures by R.L.Peurifoy and G.D- By EasyEngineering.net

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Properties of Form Material 85
Lumber
and
wedge
Breakback Wall
thickness
Lumber
and
wedge
FIGURE 4-2 Form snap tie for concrete walls. (Source: Dayton Superior
Corporation)
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Form Ties
Form ties are placed between wall forms to resist the lateral pressure
against wall forms resulting from concrete. Form ties serve two purposes:
they hold the forms apart prior to placing the concrete, and
they resist the lateral pressure of the concrete after it is placed.
There are many varieties of ties available, which may consist of
narrow steel bands, plain rods, rods with hooks or buttons on the
ends, or threaded rods, with suitable clamps or nuts to hold them in
position. Although most ties are designed to break off inside the concrete,
there are some that are tapered to allow them to be pulled out
of the wall after the forms are removed. After the form ties are
removed, the holes can be filled with a suitable grout material.
In ordering form ties, it is necessary to specify the thickness of the
wall, sheathing, studs, and wales. Ties are available for wall thicknesses
from 4 to 24 in. Figure 4-2 shows a standard snap tie that is
commonly used in formwork for walls. The tension load of a standard
snap tie is generally 3,000 lb. Coil ties are also available, which
have tension load capacities that exceed 15,000 lb. Form ties are further
discussed in Chapter 9.
Manufacturers specify the safe loads that can be resisted by their
ties, which generally range from 3,000 to 5,000 lb. Special high strength
ties are available for loads that exceed 50,000 lb. ACI Committee 347
recommends a safety factor of 2.0 for form ties. When high-strength
ties are used, the bearing capacity of the tie clamp against the supporting
member, such as the wale, must have adequate strength to
keep the member from crushing.
Concrete Anchors
Anchors are mechanical devices that are installed in the concrete to
support formwork or to provide a means of lifting a concrete member,
such as tilt-up wall panels. Anchors placed in the form before the
placement of con-crete are sometimes referred to as “inserts.” Generally
anchors of this type are used for lifting the concrete member.
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