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Vertical shores, or posts, and scaffolding are used with formwork to support concrete girders, beams, floor slabs, roof slabs, bridge decks, and other members until these members gain sufficient strength to be self-supporting. Many types and sizes of each are available from which those most suitable for a given use may be selected. They may be made from wood or steel or from a combination of the two materials. Aluminum shores and scaffolding are also available.

Shores

In general, shores are installed as single-member units that may be tied together at one or more intermediate points with horizontal and diagonal braces to give them greater stiffness and to increase their load-supporting capacities. 

 

If shores are to provide the load capacities that they are capable of providing, at least two precautions must be taken in installing them. They should be securely fastened at the bottom and top ends to prevent movement or displacement while they are in use. Because the capacities of shores are influenced by their slenderness ratios, two-way horizontal and diagonal braces should be installed with long shores at one or more intermediate points to reduce the unsupported lengths.
 

Concrete is usually pumped or placed from buckets, which permits the concrete to fall rapidly onto a limited area. The use of either method may produce a temporary uplift in the forms near the area under the load. If this should happen, it is possible that a portion of the forms will be lifted off the tops of one or more shores. Unless the tops of the shores are securely fastened to the formwork which they support, their positions may shift. For the same reason, the bottom ends of shores should be securely held in position. There are reports that forms have collapsed because the positions of shores shifted while concrete was being placed.

Table 1 and Figure 1 show the relations between the capacities of shores and their effective lengths. For example, the indicated allowable load on a 4 × 4 S4S wood shore 6 ft long is 11,076 lb for wood with an allowable unit stress in compression of 1,650 lb per sq in. and a modulus of elasticity for column stability of 580,000 lb per sq in., whereas the indicated allowable load on the same shore with an effective length of 12 ft is only 3,318 lb. The former load is approximately three times the latter load. In this instance, doubling the effective length of the shore reduces the allowable load to 30% of the allowable load for the 6-ft length. Table 6-1 gives the allowable loads on wood shores whose unsupported lengths vary from 6 to 14 ft, expressed as percentages of the allowable loads on shores that are 6 ft long. The information applies to wood with the allowable unit compressive stress parallel to grain and the modulus of elasticity as shown. The pattern of dramatic reduction in allowable load with increase in effective length applies to wood members having other physical properties and also to materials other than wood and to patented shores.

Wood Post Shores

Although the common practice today is to use patented shores, there are some instances where wood shores are fabricated for use on a particular job. Wood shores have several advantages and several disadvantages when they are compared with patented shores.
 

Among the advantages are the following:
1. The initial cost is low.
2. They are usually readily available.
3. They possess high capacity in relation to their weight.
4. It is easy to attach and remove braces.
 

Among the disadvantages are the following:
1. It is difficult to adjust their lengths.
2. The cost of labor for installing wood shores may be higher than for installing patented shores.
3. Unless they are stored carefully, they may develop permanent bows, which will reduce their load capacities.
4. They may develop rot or permanent bows.
If wood shores are too long for a given use, they must be sawed to fit the required length, which results in additional labor costs and waste of materials. If they are too short, it is necessary to splice them, which may result in a weakening of the shores when compared to the strengths of unspliced shores. Also, the cost of the labor required to make such splices may be substantial.
 

Usually the final adjustment in the height of the top of a wood shore is made with two wood wedges driven under the bottom of the shores from opposite sides. Because of the higher allowable unit stress in compression perpendicular to the grain, wedges made of hardwoods are better than those made of softwoods. Both wedges should be nailed to the mud sills, or other boards on which they rest, to prevent displacement.

Field-constructed butt or lap splices of timber shoring should not be used unless the connections are made with hardware of adequate strength and stability. To prevent buckling, splices should not be placed near mid-height of unbraced shores or midway between points of lateral support.

Patented Shores

Patented shores are more commonly used than job-fabricated wood shores for supporting formwork for concrete beams and slabs. When compared with wood shores, they have several advantages and several disadvantages.

The advantages of patented shores include the following:
1. They are available in several basic lengths.
2. They are readily adjustable over a wide range of lengths.
3. For most of them, adjustments in length can be made in small increments.
4. In general, they are rugged, which ensures a long life.
5. The shore heads are usually long enough to give large bearing areas between the shores and the stringers that rest on them.
 

Among the disadvantages of patented shores are the following:
1. The initial cost is higher than for wood shores.
2. For some, but not all, it is more difficult to attach intermediate braces than it is for wood shores.
3. Because of their slenderness, some of them are less resistant to buckling than wood shores.

Ellis Shores

The method of shoring of this company consists of two 4 × 4 S4S wood posts, fastened by two special patented clamps. The bottom of one post rests on the supporting floor, whereas the second post is moved upward along the side of the lower one. Two metal clamps, made by Ellis Construction Specialties, are installed around the two posts, as illustrated in Figure 2. The top post is raised to the desired height, and the two clamps automatically grip the two posts and hold them in position.
 

The shore specifications include lower shore members composed of two Ellis clamps with pivotal plates, permanently attached with threaded nails near the top, 12 in. apart, center to center. The upper shore member is of sufficient length to obtain the desired height. Both the lower and upper members are No. 1 grade Douglas Fir or Yellow Pine, free of heart center, stained with ends squared.

The patented Ellis clamp is designed with a solid rectangular collar with two pivotal plates, which are scorated on the flat inner surface for firm gripping. Two clamps nailed 12 in. apart near the top of a 4 × 4 make a lower shore member. The upper shore member is another 4 × 4 of desired length.
 

Figure 2 illustrates the Ellis shoring system, including the recommended spacing pattern for the indicated forming condition. The manufacturer recommends a maximum load of 4,000 lb for heights up to 12 ft, provided a horizontal lace is installed in each direction at the midpoint on the shore, attached to the lower member. For lower heights, higher loads may be obtained in accordance with the manufacturer’s recommendations. The lower shore member, with two clamps attached, is available for rent from the company.

Accessories are available for installation of the Ellis shoring system. Figure 3 shows accessories for the tops and bottoms of shores. A wire head, configured from a ½-in. rod, is used to securely hold the shore to the purlins. A metal attachment, fabricated from 4-in.-square tubing and 3⁄16-in. U-configured steel plate, can be permanently attached on the top of the upper shore member, which serves as a purlin splicer. One side is open, allowing it to be stripped from below.
 

Screw jacks can be installed under the lower shore member, allowing adjustments of the shore height to the correct position. Adjustment handles turn easily on machined threads, allowing a 6-in. standard range of adjustment, 3 in. up and 3 in. down. The screw jack has a safe working load of 10,000 lb with a 2.5:1 safety factor. A reshore spring, made of high-carbon spring steel, is available to hold the Ellis shore in place during reshoring. It is used to keep the shore tight against the slab during reshoring concrete slabs. The reshore spring is nailed to the top of each shore, eliminating the need for cutting and nailing lumber to the top and bottom of each shore. The spring returns to its original shape for many reuses. A 200-lb load will compress the reshore spring flat.

Symons Shores

The single-post steel shores of the Symons Corporation, illustrated in Figure 4, are available in three models which provide adjustable shoring heights from 5 ft 7 in. to 16 ft. Each post shore consists of two parts: a base post with a threaded collar and a staff member which fits into the base post. The assembly weight of a single-post shore varies from 67 to 80 lb, which permits a shore to be handled by one person.

Symons post shores each carry load ratings of up to 10,000 lb, depending on the shore height. The Symons post shores have a unique locking pin that, under normal use conditions, cannot be easily broken or lost. For approximate height adjustment, this pin is inserted into one of the holes spaced at 4-in. intervals along the length of the staff. A safety pin secures the locking pin and eliminates accidental slippage of the base and staff. After the post shore has been set in position, a threaded collar with handle permits fine adjustment of the post shore height over a 6-in. range.
 

To support 4-in. stringers, or 8-in. wide flange stringers, a 4-in. by 8-in. U-head is inserted into the top of the post shore staff. The Uhead is inserted through the end plate hole on the shore and attached by a ½-in.-diameter attachment pin and a ¹⁄8-in.-diameter hairpin clip. Nail holes on the side of the U-head provide a convenient means of securing lumber stringers. A steel beam clamp should be used to secure the steel stringers to the U-head.

A 5-in. by 8-in. J-head is also available for quick installment under aluminum beam stringers. The J-head must be pinned to the top of the post shore using the ½-in.-diameter attachment and hairpin clip. A steel beam clamp or aluminum attachment clip assembly should be used to secure the aluminum beam to the J-head.

The Symons timber brace nailer plate allows rapid attachment of bracing when required by the U.S. Occupational Safety and Health Administration. These single-post shores are fully compatible with Symons heavy-duty shoring.

Fresh concrete exerts pressure on vertical form surfaces, and an assessment of that pressure is needed for designing forms. In the simplest theory, fresh concrete acts as a fluid exerting pressure equally in all directions at whatever point the measurement is made essentially assuming a hydrostatic pressure effect. This is reasonable because the fresh concrete behaves much like a fluid at least briefly during vibration, or for a longer time if flowability of the mixture has been enhanced through use of admixtures or special proportioning and materials selection.

But concrete is not a true fluid, and some method of evaluating the concrete’s actual pressure is needed. Evaluating pressure has been a significant part of the work of ACI Committee 347, Formwork for Concrete. As early as 1958, Committee 347 (then Committee 622) studied available field measurements of lateral pressure on formwork and used the data to develop pressure formulas that could be safely used for form design. A report was published in 1958 and the formulas, with some modifications, were included in ACI’s first formwork standard, ACI 347-63.2 In the days before the advent of the personal computer, the committee considered it important to keep the equations simple, reasoning that this would encourage their use and minimize mathematical errors.

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Concrete formwork serves as a mold to produce concrete elements having a desired size and configuration. It is usually erected for this purpose and then removed after the concrete has cured to a satisfactory strength. In some cases, concrete forms may be left in place to become part of the permanent structure. For satisfactory performance, formwork must be adequately strong and stiff to carry the loads produced by the concrete, the workers placing and finishing the concrete, and any equipment or materials supported by the forms.

For many concrete structures, the largest single component of the cost is the formwork. To control this cost, it is important to select and use concrete forms that are well suited for the job. In addition to being economical, formwork must also be constructed with sufficient quality to produce a finished concrete element that meets job specifications for size, position, and finish. The forms must also be designed, constructed, and used so that all safety regulations are met.

Formwork costs can exceed 50% of the total cost of the concrete structure, and formwork cost savings should ideally begin with the architect and engineer. They should choose the sizes and shapes of the elements of the structure, after considering the forming requirements and formwork costs, in addition to the usual design requirements of appearance and strength. Keeping constant dimensions from floor to floor, using dimensions that match standard material sizes, and avoiding complex shapes for elements in order to save concrete are some examples of how the architect and structural engineer can reduce forming costs.

To produce concrete forms that meet all job requirements, the construction engineer must understand the characteristics, properties, and behaviors of the materials used; be able to estimate the loads applied to the forms; and be familiar with the advantages and shortcomings of various forming systems. Form economy is achieved by considering four important factors:

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The formwork operations involve a number of activities including fabricating and erecting the forms, stripping, moving, and cleaning and oiling the forms for reuse. All of these activities and the materials involved are allowed for in the pricing of the forms. The estimator measures the surface area of the concrete that comes into contact with the forms; this is known as the contact area.

Because only the area of formwork is measured, the estimator does not have to be concerned about the design of the forms at the time of the takeoff. All that needs to be established is which surfaces of the concrete require forms. In the past, estimators have agonized over such things as whether the bottom of an opening or the sloped top surface of a wall needs to be formed. If discussion with your colleagues does not provide an answer, the prudent estimator will always exercise caution and allow the forms.

Generally:

1. Formwork shall be measured in square feet of contact area; that is, the actual surface of formwork that is in contact with the concrete.

2. Formwork is classified in the same categories as those listed for concrete. As an illustration, consider a project with concrete footings, walls and columns, forms to footings; forms to walls and forms to columns would each be described and measured separately. There are, however, a number of factors which may have no effect on the price of the concreting operations but do affect the price of formwork and, therefore, should be noted. For example, the volume of concrete in all walls, whether they are straight or curved, will have the same price but the price of forms to curved walls will differ from the price of straight walls, so the forms to curved surfaces must be kept separate.

3. Bulkheads and edge forms shall be measured separately within these categories, so if there are construction joints required for long lengths of walls, the area of bulkheads to form these construction joints would be measured separately from the wall forms. Similarly, if there are pilasters projecting from the walls, the area of the pilasters would be calculated and noted separately from the wall forms. See Figure 1 for different categories of formwork in a wall system.

4. Forms to slab edges are measured separately from forms to beams and forms to walls, even where the edge forms may be extensions of beam or wall forms (see Figure 2).

5. Where there is an opening in a form system, no deduction is made from the total area of the forms if the size of the opening is less than 100 square feet (10 square meters). Examples of such openings would include openings for windows in walls, stairway openings, or elevator shaft openings in suspended slabs. The estimator must distinguish between what are openings and what are cut outs. Openings less than 100 square feet (10 m2) are not deducted but all cut outs would be deducted (see Figure 3).
 

6. Describe items of formwork that are linear in nature, stating their size and measuring their length in feet (meters). Grooves, chases, keyways, chamfers, and narrow strips of formwork less than 1 foot wide are measured in this fashion.
 

7. Describe forms to circular columns giving the diameter and measure in feet (meters) to the height of the column. Where columns widen at the top to form capitals, describe and enumerate these features. It can be useful to draw small sketches of complex items such as capitals to clarify exactly what is being measured. Include these sketches with the other takeoff notes.

Dear fellows,

After receiving numerous requests, we are providing an interactive Microsoft excel spreadsheet for design of timber formwork systems for elevated concrete slabs. The formulae, calculations and tables used in this spreadsheet are based on ACI committee 347, Formwork for Concrete, 6th Edition. Please enjoy it and keep sharing it.

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