Category CONCRETE FORMWORK

Table 9.2 was used to select the formwork system for the exterior ‘‘wall.’’ The selection was a self-raising form for the following rea­sons:

1. The self-raising form is suitable for buildings higher than 25 stories.

2. It can accommodate architectural concrete requirements.

3. The self-raising form is a crane-independent system which is suitable for restricted site conditions.

This case study was extracted from an article entitled ‘‘Build­ing Production and Quality into Architectural Concrete.’’ The whole article was devoted to the problem of selecting the form­work system. The author is an expert in selecting formwork sys­tems and he attributed his selection to the same reasons stated above. In his article, he explained that renting or buying another crane was impossible because of the site conditions in ‘‘downtown Denver.’’ It was also economically unfeasible.

[1] Dimension: 2 in. < thickness < 4 in. and width > 2 in.

[2] Beams and stringers: thickness > 5 in. and width > thickness + 2 in.

[3] Southern Pine Inspection Bureau

[4] Check local suppliers for availability before specifying Plyform Class II grade, as it is rarely manufactured.

[5] When exterior glue is specified, i. e. Exposure 1, stress level 2 (S-2) should be used.

[6] Properties and stresses apply only to APA RATED STURD-I-FLOOR and APA RATED SHEATHING manufactured entirely with veneers.

[7] APA RATED STURD-I-FLOOR 2-4-1 may be produced unsanded.

[8] May be available as Structural I. For such designation use Group 1 stresses and Table 2 section properties.

[9] C face and back must be natural unrepaired; if repaired, use stress level 2 (S-2).

[10] Type: APA B-B plyform class I with species group of face

ply = 2.

• Dry conditions.

• Thickness: 11/8 in.

[11] Consider 1-ft strip. It carries a load of 180 x 1 =

180 lb/ft.

[12] Dimension: 2 in. < thickness < 4 in. and width > 2 in.

[13] Beams and stringers: thickness > 5 in. and width > thickness + 2 in.

[14] Southern Pine Inspection Bureau

[15] Check local suppliers for availability before specifying Plyform Class II grade, as it is rarely manufactured.

[16] Type: APA B-B plyform class I with species group of face

ply = 2.

• Dry conditions.

• Thickness: 11/8 in.

[17] Consider 1-ft strip. It carries a load of 180 x 1 =

180 lb/ft.

[18] Fabrication of the flying formwork is normally performed on the ground, which yields higher productivity. Strip­ping flying formwork as one integral unit reduces the stripping costs to approximately 50 percent of the strip­ping costs for hand-set formwork systems such as con­ventional wood and conventional metal systems. Strip­ping of hand-set systems is performed by removing small pieces, which results in rather high labor costs.

[19] Loads are transformed by telescoping extension legs lo­cated underneath the aluminum trusses and thus giving enough working space below the formwork to allow other

[20] I = 5.358 in.4

• S = 3.063 in.3

• d = 3.5 in.

• A = 5.25 in.2

Bending

Size factor Cf = 1.5

F’ = allowable stress = F^s) (Ct)(CD)(Cf)

[21] The forms may be designed for almost any plan, shape, and size structure.

• The forms can be easily adjusted during construction to accommodate changes in wall thickness.

• The application of form liners provides numerous archi­tectural finishes.

• Forms can be reused approximately 50 times or more.

• The formwork system is preassembled, therefore make­up area is not necessary.

The Tabor Center is a 1.3 million ft2 (120,000 m2) [20,000 ft2 (1860 m2) per floor] multi-use facility in the center of downtown Denver. It consists of twin office towers of 32 and 40 stories. The tower structure is composed of three elements: the exterior wall, the cen-

tral core, and the interior deck area that ties the two together (tube in tube). The core is structural steel with a concrete diaphragm or infill walls up to floor 12 to resist part of the lateral loading. The deck area is also structural steel with a metal deck and concrete fill. The exterior wall consists of closely spaced columns (tube sys­tem). A study showed that the crane time is not adequate for hoisting formwork and it will only be used for material handling and concrete placing. Architectural concrete is required for the exterior walls.

Table 9.2 is presented to help the formwork designer/selector choose the appropriate vertical formwork system. These tables show the relationship between the factors affecting the selection of formwork systems and the different forming systems available for vertical concrete work. The user must first list all the known major components of the project and then compare them to the characteristics listed in the table under each forming system. The best formwork system can then be identified when the project fea­tures agree with most of the characteristics of a particular system. These tables can also be used by architects to make some minor adjustments in their design to accommodate the use of an efficient formwork system.

When deciding to use a special forming technique, the contractor has to evaluate his or her own in-house expertise, which includes

trouble-shooting experience and safety management. For exam­ple, in vertical forming systems such as slipform and self-raising forms, the in-house experts have to deal with special problems such as leaking hydraulic equipment, leveling of the hoist jacks, keeping the forms plumb within specified tolerance, and placing inserts and openings under the fast rate of placement.

Safety management is another area of in-house expertise that should be available to support a specific forming technique. For example, the availability of fire protection expertise is necessary in slipforming to prevent a fire several hundred feet in the air re­sulting from the flammable oil used in the hydraulic jacks. The availability of such expertise may be a factor which determines if a special forming technique is or is not used.

An important factor that influences the selection of the formwork system is the availability of crane time. Crane time is defined as the time in which the crane is engaged in raising and lowering construction materials and tools. In congested site conditions where installing more than one crane is difficult, the limited crane time available for formwork erection to meet the project comple­tion date becomes a major factor that may lead the formwork de­signer to choose crane-independent systems such as self-raising formwork or slipform alternatives (which requires no crane time).

As previously indicated in Chapter 5, most of the ganged formwork systems (i. e., jump form, slipform, and self-raising formwork) re­quire high initial investment and intensive crane involvement. However, high repetitive reuse can make these systems economi­cally competitive. Availability of capital investment is a must for utilizing these systems.

Vertical forming systems are sensitive to weather conditions. Typi­cally, in vertical forming systems, the newly placed concrete is sup­ported by the wall already cast below it. The lower wall section must gain sufficient strength to support the fresh concrete above. The rate of strength gain for the lower wall is influenced by ambi­ent temperature, moisture content, and freezing and thawing cy­cles.

Another factor that affects the economy of the selected sys­tem is the effect of stopping forming and concreting because of extreme weather conditions. For example, in slipforming, the work is usually continuous, 24 hours around the clock, with a minimum crew requirement of 50 to 75 laborers for a medium size shear wall. If the slipforming stops because of the weather conditions, the contractor has to pay the workers a show-up time, plus the cost of inactive cranes and their operators. As a result, if severe weather conditions are expected, some vertical formwork systems, such as slip forms, should be avoided.

Construction sites are generally classified into downtown re­stricted sites and open, suburban, or unrestricted site condition. Gang and jump formwork require good crane service. As a result, it is difficult to use these formwork systems in restricted site condi­tions. On the other hand, slipform and self-raising formwork are crane-independent systems and can be used in restricted site con­ditions.

The nature of the job, including local conditions, is one of the pri­mary factors in formwork selection. Some of the factors that should be considered are explained below.

Area Practice

Area practice has an important impact on the selected formwork system. This influence is evident in slipform operations which are highly labor intensive and usually subject to premium pay. Re­search indicates that slipforms are most prevalent in the northeast, southeast, and Hawaii.

The most important advantage of using a sophisticated formwork system is the speed of construction. The speed of construction affects cost because it determines the time when the building will be available for use and also reduces the financial charges. The major factors that determine the speed of construction follow.

Floor Cycle. Faster floor cycle is always desirable for con­tractors and owners. For contractors, faster floor cycle allow the contractor to finish on schedule or earlier which reduces the over­head cost. For the owner, faster floor cycle reduces his/her short­term financial charges and allows early utilization of the con­structed facility.

The use of efficient vertical formwork systems such as slip formwork and self-raising formwork allow the contractor to com­plete the casting of one story every two to three days. Also, vertical formwork systems control the pace of progress for horizontal con­crete work.

Rate of Placement. The speed at which concrete is placed in vertical formwork has the largest influence on the lateral pres­sure that is imposed on the formwork. The lateral pressure is in­creased by increasing the rate of placement, up to a limit equal to the full fluid pressure. Higher rates of placement influence the size, material, and tie pattern of the selected system.

Construction Sequence. Tall buildings typically have an in­side core to resist lateral loads. One alternative is to completely construct the inside core in order to create a ‘‘closed’’ area for the other trades to start. This alternative has proven to be faster than constructing and finishing floor by floor. This can be accomplished by using slipforms or self-raising forms.

Surface quality and appearance are always referred to as the con­crete finish. Concrete surfaces can be classified as rough finish ‘‘as-cast surface,’’ smooth exposed, covered (with a special clad­ding material or painted), or textured (with an architectural sur­face texture and treatment).

As-cast concrete finish typically shows some irregularity on the surfaces and may contain some concrete surface defects. An as-cast concrete finish is usually found in concrete buildings where appearance is not important, such as warehouses or silos.

Exposed concrete finish is characterized by smooth regular concrete surfaces, and regular positions of form ties. Exposed con­crete surfaces are typically found in columns and bearing walls.

Architectural concrete is favored in vertical concrete ele­ments (columns and walls). As a result, concrete finish must be considered as one of the major factors in the selection of a form­
work system. Architectural concrete requires a careful selection of a formwork system which includes stiffer form liners, tighter joints, smoother finishes, and more care in implementing cham­fers and justifications.