CP 23: 2000

1.3.7 Engineer

A person who is registered with the Professional Engineers Board in Singapore.

1.3.8 Formwork

The surface, support and framing used to define the shape of concrete until it is self-supporting.

1.3.9 Formwork assembly

An assembly of formwork components including footings which together constitute a structure.

1.3.10 Form lining (also called 'form liners')

Non-structural material placed on, or part of, the form face to achieve a desired surface finish.

1.3.11 Form tie (also called 'wall tie' or 'tie rod')

A device which penetrates a form, extends through the permanent structure and retains the form from movement due to concrete pressure.

1.3, 12 Permissible stress

The stress that can be sustained with acceptable safety by structural component under the particular condition of service loading.

1.3.13 Prop

A compression member used as a temporary support and incorporating a means for varying and fixing its length.

1.3.14 Proprietary item

An item made in quantity production for general use in formwork assemblies, and whose load capacity has been proven by analysis or test.

1.3.15 Repropping

A process used during the construction operation in which the original props are removed and replaced in a sequence planned to avoid any damage to partially cure concrete.

1.3.16 Soffit form work

Formwork to the undersides of slabs, beams and the like.

1.3.17 Sole plate

A timber, concrete or metal spreader used to distribute the load from a standard or baseplate to the ground or supporting structure.

1.3.18 Stacked materials

Construction materials such as formwork components, bricks, reinforcement, which temporarily loads either the formwork assembly or the previously placed concrete.

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1,3,19 Stripping (also called 'striking')

The removal of forms from the surface of the hardened concrete. -... 1,3,20 Statutory Authority

A body having statutory powers to control the design and erection of buildings or structures in the area in which the building or structure concerned is to be erected.

1,3,21 Strut

A member in compression.

1.3.22 Tie

A member in tension.

1,3,23 Tolerance

Acceptable limits for deviation.

Section Two - Materials

2.1 General

2,1,1 Responsibility for materials

The selection of materials for formwork and shoring is primarily the responsibility of the Contractor to meet the safety and the specified finish of the concrete. If approval of any material by the Engineer is specified, such approval shall be based only upon considerations of safety and the quality of the finished work. -

For proprietary formwork the manufacturer shall provide documentary evidence to substantiate the design strength of the formwork material. If approval of any formwork material by the Engineer is specified, such approval shall be based only upon considerations of safety and the quality of the finished concrete work.

2.2 Materials

2.2.1 Proprietary formwork material

Manufacturers' data should be supported by tests at an accredited or a recognised laboratory before they are accepted. Load factors applied to ultimate strengths claimed by manufacturers shall conform to the requirements of this code.

2.2.2 Non-proprietary formwork material

2.2.2.1 Timber materials

It is recommended that timber material used for the formwork should comply with Singapore Standards SS 1, "Plywood" and other timber relevant codes.

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2.2.2.2 Scaffolding tubes and fittings

Tubes and fittings shall conform to the recommendations given in the current Singapore Standard CP 14, "Code of practice for scaffolds".

2.2.3 Other formwork materials

Other materials may be u::;ed for formwork pro~ided these materials meet the requirements of this Code.- --

2.2.4 Form coatings

The choice of form coatings to give the specified quality of finished concrete surface shall be the responsibility of the Contractor and approved by the Engineer/Architect.

The purposes of form coatings are:

(a) to facilitate the clean separation of the contact surface of the form and the concrete during stripping;

(b) to facilitate the production of a specified texture of concrete surface;

(c) to improve the durability of the contact surface of the form; and

(d) to seal the form surfaces from the intrusion of moisture.

Care shall be taken to ensure compatibility of coatings with form surfaces and plastic concrete, including admixtures in the concrete, and with the finished surface requirements of the hardened concrete, taking into account the application of further materials such as paint and tile' adhesive to the concrete surface.

Form coatings on form surfaces shall be maintained so that they retain the required properties up to the time of stripping of forms. When coatings are applied to forms in the field, care shall be exercised to prevent contact of the coating material with construction joint surfaces or reinforcing steel.

The recommendations of manufacturers of form coating material should be followed, but an independent investigation of their performance is recommended before use.

Section Three - Design

3.1 Scope

This section specifies minimum structural design requirements applicable to formwork assemblies and components. It provides information on the loading to be considered in the design of the formwork assembly (see Clause 3.4) and sets out analysis and design by:

(a) ultimate strength methods (see 3.5.4);

(b) permissible stress methods (see 3.5.5); and

(c) testing (see 3.5.3).

Minimum requirements for the documentation of the formwork design are also provided in Annex C.

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3.2 Application of section

The loads given in Clause 3.4, as appropriate, shall be used for formwork design except for the following situations where the requirements of this section l'nay not be sufficient; the loading should then be assessed from the best available information:

(a) Where the formwork assembly is of unusual configuration, materials or geometry;

(b) Where the formwork assembly is subject to special loads; or

(c) Where the formwork assembly is to be used under conditions of loading that are more adverse than those given in this section.

The technique of formwork fabrication, timing, erection, use and removal can affect the structural integrity, accuracy, surface finish and long term deformation of both the concrete elements it is used to produce and the previously cast portions of the permanent structure. It is therefore essential that the matters listed in Annex C be known prior to commencement of the formwork activity and that, where applicable, they are to be included in the formwork documentation.

3.3 Design requirements

3.3.1 General

The design of the formwork shall consider the structural stability adequacy, restraint system, footings and construction techniques.

3.3.2 Structural requirements

The structural requirements are as follows:

(a) Stability

The formwork assembly shall resist overturning, uplift, sliding and sidesway under the action of all appropriate load combinations.

(b) Strength

The formwork assembly and its component members shall withstand the effects of all appropriate load combinations.

(c) Stiffness

The stiffness shall be such that the deformation under the appropriate loading on the formwork assembly and its component members does not exceed the limits specified in this Code.

3.3.3 Restraint systems

Lateral restraint may be introduced to reduce the effective length of a compression member provided the restraint member is designed to support the load. It is more effective for the restraint member to be deSigned-for the transfer of forces to the fo"U"ndation or permanent structure.

3.3.4 Construction

The detailed design of the formwork assembly shall permit the components to be erected and dismantled without conflict with other structures or the newly cast structure.

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3.3.5 Foundations and footings

The foundation material beneath the formwork assembly shall be investigated to determine its bearing capacity and settlement characteristics.

A footing system shall be designed to support the formwork assembly on the foundation material so that the whole support system satisfies the requirements for stability, strength and stiffness.

3.4 Loads

3.4.1 General

Formwork shall be designed to withstand all forces, both vertical and lateral, resulting from dead, superimposed, wind and any other loads which could occur simultaneously during the period the formwork is used. The magnitude of all such forces, individually and collectively shall be taken into account in the design. .

Provision for wind loading shall comply with requirements of the Building Control Regulations of the Building Control Act and Singapore Standard CP 14.

3.4.2 Vertical loads

3.4.2.1 Dead loads.

In computing the vertical loads, the dead load shall include the weight of formwork, freshly placed concrete, reinforcement and other embedded materials.

3.4.2.2 Superimposed loads

3.4.2.2.1 Working areas

Allowance should be made for access and working area loading on the formwork.

A loading of 1.5 kN/m2 is recommended to allow for the following situations:

(a) construction operatives;

(b) hand tools and small equipment commonly used in reinforced concrete construction;

(c) materials required for immediate use; and

(d) the common situations of impact and heaping of concrete occurring during placing operations.

For this loading allowance to be valid, the concrete should not be dropped into the reception arrangements or on to horizontal surfaces from a free height greater than 1 m, nor should the concrete be allowed to heap and accumulate on the formwork to a height more than 3 times the depth of the slab, with a limit in area for any such accumulation to this height of 1 m2 (see figure 1). Should it be necessary to exceed these limitations, allowance for the additional loading should be made in the design.

Where allowance has only to be made for access and inspection purposes, a loading of 0.75 kN/m2 should be adequate.

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3.4.3.2 Other lateral loads

Braces and shores shall be designed to resist all foreseeable lateral loads arising from forces such as wind, cable tensions, inclined supports, dumping of concrete, and starting and stopping of equipment. In no case shall the assumed value of lateral load due to wind, dumping of concrete, and equipment acting in any direction at each floor line be less than 1.5 kN/m of floor edge or 2.5 percent of total load on the form, whichever is greater.

Wihd loads on any attachments to the forms shall be considered, as well as wind loads on the forms themselves.

3.4.3.3 Special loads

The formwork shall be designed for any special conditions of construction likely to occur, such as asymmetrical placement of concrete, impact of machine-delivered concrete, uplift, concentrated loads of reinforcement and storage of construction materials. Formwork should be designed for special loading conditions such as walls constructed over slabs or beams which exert a loading pattern before hardening of concrete or brickwork, mortar, etc, different from that for which the supporting structure is designed.

3.5 Analysis and design

3.5.1 General

Formwork components or assemblies shall be analysed and designed in accordance with 3.5.2 or tested in accordance with 3.5.3. Design information shall be provided in accordance with 3.5.8 and formwork documentation shall be in accordance with Annex C.

For all members or assemblies loaded in compression, eccentricity shall be taken into account.

Where differences exist between requirements of the material design standards and this standard, the requirements of this standard shall take precedence.

Where the whole formwork assembly is composed of members of different materials, a combination of the methods described in 3.5.2(a) and (b) may be used.

3.5.2 Theoretical analysis and design

Formwork components or assemblies shall be analysed and designed in accordance with one of the following methods:

(a) ultimate strength design, in accordance with the appropriate material structural design code and 3.5.4; or

(b) permissible stress design, in accordance with the appropriate material structural design code and 3.5.5.

3.5.3 Testing data and testing

Manufacturers' data should be supported by tests at an accredited or a recognised laboratory before they are accepted. Load factors applied to ultim'ate strengths claimed by manufacturers shall conform to the requirements.

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Determining and proving the structural capacity of formwork components and assemblies shall be either by destructive or non-destructive evaluation. Interpretation of results from testing, presentation of design data and use of this data shall be in accordance with Annex B.

3.5.4 Ultimate strength design

3.5.4.1 load combinations for ultimate strength approach Factored load combinations for use in ultimate strength design shall be one of the followings which produces the most critical design condition:

(a) 1.4 D + 1.6l;

(b) 1.2 D + 1.2 L + 1.2 W; or

(c) 1.4D+1.4W.

NOTE - The above load factors are obtained from BS 5950 : Part 1 : 1990.

3.5.5 Permissible stress design

3.5.5.1 load combinations for permissible stress approach

Unfactored load combinations for use in permissible stress design shall be one of the following:

(a)

(b)

D+L

D+ L+W

3.5.6 Stability

Design for 1.0 times allowable stress; or

Design for 1.25 times allowable stress with 25% overstress allowed for wind effect.

The formwork assembly shall be designed for safety to ensure stability against overturning, uplift, sliding and sidesway taking into account the sequence of construction under the action of the most adverse load combination. This is to ensure that a safe and stable formwork assembly can be erected. A minimum 1.5 factor of safety against overturning should be provided.

3.5.7 Deflection and movement

Deflection and movement of the formwork shall be such that the permitted tolerances given in Clause 4.4 are not exceeded.

3.5.8 Design Information

3.5.8.1 General

For materials given in Clause 2.2 the design information and procedures shall be in accordance with the listed Singapore Standard. For unidentified materials, the additional requirements of 3.5.8.2 shall apply. For components the requirements of 3.5.8.3 shall apply.

3.5.8.2 Unidentified materials

Where it is not possible to identify materials as being of the type and grade which is specified in the formwork documentation, these materials shall not be used in the co"nstruction of formwork. The following requirements shall apply to the identification of materials:

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(a)

.. (b)

Aluminium

No assumptions shall be made in respect of alloy or temper. A representative sample of the material shall be submitted to an appropriate testing authority for identification .

Steel

Unless the steel can be positively identified by brand marks or other distinguishing features as being of a particular grade, the yield strength shall be assumed to be not more than 200 N/mm2 (mild steel).

(c) Timber

Unless the stress grading of the timber is clearly marked on the members no assumptions shall be made in respect of grading. The visual grading of timber shall be made in accordance with the following standards:

(i) Singapore Standard CP 1; and

(ii) Singapore Standard CP 7.

A representative sample of timber may also be submitted to an appropriate testing authority for identification to enable visual stress grading to be achieved in accordance with the above standards.

(d) Other materials

Appropriate means of identification, by established visual grading methods, non-destructive evaluation or laboratory tests, shall be undertaken before the material is used for formwork construction.

3.5.8.3 Components

Components for formwork assemblies shall comply with and be used in accordance with the relevant Singapore Standard or, in its absence, other acceptable standards may be used. The following additional requirements apply:

(a) Proprietary items

The design structural capacity of proprietary items used in formwork assemblies and recommendation on the application shall be derived from manufacturer's information.

(b) Scaffold tubes and fittings

Scaffold tubes and fittings forming part of formwork assemblies shall comply with this Standard and Singapore Standard CP 14.

(c) Anchors

This standard does not make recommendations relating to anchors into concrete or masonry. Consequently requirements for limit state or permissible stress loads are not provided. If anchors are to be used, the relevant Singapore Standards or manufacturers' information should be referred.

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3.6 Shores

3.6.1 Proprietary shores

When proprietary shores, splices in shoring or methods of shoring are used, manufactures' recommendations on load-carrying capacities may be used if supported by test reports by an accredited or recognised testing organisation provided the load factors are in accordance with the requirements of this code. Particular attention shall be given to unsupported shore lengths. Bracing shall be provided if necessary.

3.6.2 Special shoring conditions

Where the floor to ceiling height exceeds 9.14 metres or where the formwork deck is supported by shores constructed in two tiers, or where the dead, live and impact loads on the formwork exceed 732.3 kgf per square metre, the formwork structure shall be designed by an Engineer. The specification and drawings shall be kept on the job site.

3.7 Diagonal bracing

The formwork system shall be designed to transfer all lateral loads to the foundation or to construction of adequate strength and stiffness already in place. 3.4.3.2 specifies the minimum intensity of lateral loading to be carried by the bracing.

Diagonal bracing shall be provided in vertical and horizontal planes where required to provide stability and to prevent buckling of individual members. Where the only bracing requirement is to prevent buckling of individual members, lateral br'acing shall be provided in whatever directions are necessary to ensure a slenderness ratio less than the maximum permissible for the load to be supported. Such a laterally braced system shall be able to resist 2.5% of the axial compression in the main member and secured in such a manner to ensure stability of the whole formwork system.

3.8 Foundations for formwork

Foundations of sufficient capacity to carry the maximum loads imposed by formwork during construction shall be provided on ground., soleplates, spread footings or pile footings.

Differential settlements between elements of formwork and between formwork and permanent structure shall be limited to avoid damage to or unacceptable deformation of the permanent structure.

3.9 Shoring and screwjacks

Shoring shall be designed to allow vertical adjustments to be made to facilitate erection and stripping of forms and to compensate for movements. Screwjacks may be used at both top and bottom of tubular shores and frame-type shoring.

3.10 Deflection limits

The deflection of unsupported areas of form facing shall not exceed 1/270 of the clear span between supports or 3 mm whichever is the greater, unless otherwise specified. . .

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Care should be taken to ensure that the calculated deflections are less than the tolerances given in Clause 4.2. These tolerances apply to the finished structure, and the calculated structural deflection of the formwork is only one of many factors contributing to the final tolerance . .. 3.11 Drawings

3.11.1 Contract drawings and specifications

The contract drawings and specifications for the permanent works shall include and cover all information necessary for formwork design and for the preparation of formwork drawings.

NOTE - Annex C lists information which should be included in contract plans and specification, if applicable.

3.11.2 Formwork drawing

If the Engineer requires formwork drawings to be submitted for his approval, he shall so state in the specification.

Formwork drawings shall include the following information:

(a) Design assumptions

All major formwork design values and loading conditions shall be shown on the formwork drawings. These include assumed values of superimposed load, rate of placement, temperature of concrete, height of drop, weight of moving equipment which may be operated on formwork, foundation pressures, design stresses, camber diagrams, and other pertinent information.

(b) Other items

In addition to specifying types of materials, sizes, lengths and connection details, formwork drawings shall provide all relevant details.

NOTE - Annex C lists details which should be shown if applicable.

Section Four - Construction

4.1 Safety precautions

4.1.1 General

Attention to safety is particularly significant in formwork construction as these structures support the concrete during its plastic state and as it is developing its strength, at which time it may be unstable. To ensure the safe performance of formwork, it is essential that the design requirements contained in this code should be observed. All formwork members and their connections shall be designed so that sound determination of loads thereon is accurately made and allowed for.

NOTE - In addition to fulfilling the very real moral and legal responsibility to maintain safe conditions for workmen and the public, safe construction is the final goal, irrespective of any short-term cost savings from cutting· corners on safety provisions.

Multi-storey work requires, in addition to consideration of the adequacy of forms, further consideration of the shoring of new work onto recently completed work which may not have attained adequate strength, or which may not itself be adequately shored. The advice of an Engineer shall be sought where any doubt exists as to the adequacy of the newly-completed work to support following-on loads.

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4.1.2 Safe formwork construction practice

Some good common construction practices to prevent formwork failure are:

(a) Adequate shoring or re-shoring;

(b) Adequate lateral and diagonal bracing of shores;

(c) Avoid using a prop on top of another prop;

(d) Adequate bracing at joints in shores;

(e) Plumbed shoring;

(f) Locking devices on metal shoring secured, adequate and not improvised;

(g) Shore soleplates resting of stable soil;

(h) Sufficient nailing;

(i) Adequate provision for lateral pressures on forms;

(j) Adequately tightened or secured form ties;

(k) Adequately secured wedges for forms and for shoring;

(I) Ensure tightness of reshores on floors below;

(m) Inspect formwork during and after concrete placement to detect abnormal deflections or other signs of incipient failure which could be corrected;

(n) Control vertical rate of placement of concrete, and therefore the lateral pressure on the forms, with regard to slump, type of vibration used, effect of admixtures, and effect of low concrete temperatures;

(0) Regulate properly the horizontal rate and sequence of placing concrete to avoid unbalanced loading on the formwork;

(p) Control of vibrations and shock arising from impact loading;

(q) Prevent premature removal of supports, especially under cantilevers;

(r) Comply with recommendations of manufacturers of standard components and to keep within limits required by the designer;

(s) Protect cardboard forms (usually tubes) from the weather or water before concrete is placed into or around them;

(t) Proper field inspection by Supervisor to see that form design has been properly interpreted by the formwork erectors;

(u) Avoid use of faulty timbers or other materials;

(v) Provide adequate shoring or re-shoring;

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(w) Ensure proper positioning of shores from floor to floor. This is to avoid reverse bending in floor slabs which are not designed for such stresses; and

(x) Ensure proper splices in shores, struts and walling.

4.1.3 Planning

Planning of formwork construction and placement of concrete is to ensure safety of all personnel involved and the quality of the finish structure. Particular consideration shall be given to the following:

(a) Unsupported areas of plywood forms shall be arranged so that deflections and bending stresses due to the pressure of plastic concrete are within allowable limits.

(b) Where the bottom of the form is inaccessible, access shall be provided to permit thorough removal of extraneous material before placing concrete.

(c) For expansion joints or construction joints, provision shall be made for stripping of the form without applying undue force to projecting reinforcement, which might damage the concrete.

(d) Placing of building materials shall be planned and located in advance so as not to cause damage or overloading to the formwork.

4.1.4 Inspection of materials and forms

All materials and components used for the formwork shall be inspected before each use and particularly for those which will be re-used.

Forms shall be inspected and checked to ensure that the concrete will have the dimensions and be in the location shown on the drawings.

The erection of the formwork and its supporting and bracing system shall be supervised by designated person. Formwork and its supporting and bracing system shall be inspected to ensure their rigidity, adequacy, stability and that they are erected according to the approved design and drawings by Engineer, where applicable before concreting.

Forms shall be sufficiently tight to prevent excessive leakage of cement grout. Gaskets and tapes may be used to seal joints.

Forms shall be thoroughly cleaned before placing of concrete.

4.2 Tolerances

4.2.1 General

Formwork shall be so constructed that the finished concrete is within the required tolerances as specified in Table 2. Cumulative tolerances shall be considered as well as tolerances on single members. In specifying tolerances for concrete work, recognition should be given to the needs of other trades whose work adjoins the concrete construction.

No tolerances specified for horizontal or vertical building lines or footings shall be construed to permit encroachment beyond the legal boundaries.

The Engineer/Architect may specify alternative tolerances, either greater or less, for a particular project or part of a project.

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4.5.2 Recommended minimum stripping and propping times

The recommended minimum stripping times for reinforced concrete structures are shown in Table 4 taking into consideration variations due to types of member and temperature. The use of accelerators or retarders have not been considered.

Table 4 - Minimum stripping and propping times

Minimum period for reinforced concrete members or structures using Ordinary Portland Cement

Location Duration after last pour

Sides of columns, walls and beams 12 hrs

Soffits of slabs and beams (props left under) 3 days

Soffits of cantilevers (with reshoring) 7 days

Re-props to slabs and beams 14 days

Re-props to cantilever slabs and beams 21 days

Minimum stripping times may be reduced provided evidence is furnished to the satisfaction of the Engineer that the concrete to be stripped has acquired sufficient strength to support safely its own mass and any superimposed load.

In cases where there is a possibility of increased loading, such as multi-storey work, the times given in Table 4 may be lengthened.

4.5.2.1 Cube Tests Results

When standard cube tests are used to determine stripping times, sampling and testing should be carried out in accordance with SS 78 Parts1, 3 and 4.

4.5.3 Removal of shores

No reshores or undisturbed shores shall be removed until the structural member has attained sufficient strength to support all superimposed loads. Removal operations shall be carried out in accordance with planned sequence so that the structure is not subject to impact, excessive load or eccentric load.

4.6 Shoring and reshoring for multi-storey structures

4.6.1 General

Multi-storey work presents special conditions, particularly in relations to removal of forms and shores. Re-use of form material and shores is an obvious economy. Furthermore, the speed of construction customary in this type of work provides the additional advantage of permitting other trades to follow concreting operations from floor to floor as closely as possible. However, the shoring which supports green concrete is necessarily supported by lower floors which may not be designed for these loads. For this reason shoring shall be provided for a sufficient number of floors to develop the necessary capacity· to support the imposed loads without excessive stress or deflection ....

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4.6.2 Systems of shoring and reshoring

4.6.2.1 Summary of systems - ... Two systems of shoring and two systems of reshoring are described below:

(a) Undisturbed shores

It permits stripping of forms at an earlier age than is possible with any other system because . large areas of concrete are not required to carry their own mass. Use of undisturbed shores eliminates the cost of a second placing of shores, avoids the special attentiqn required to en?ure· uniform tightness of reshores under the slab, and provides better assurance that shores are placed in the same pattern on each floor.

(b) Secondary shoring

Where undisturbed shoring is not used, the use of secondary shores is the preferred alternative. Secondary shores are additional shores placed before any formwork or falsework is disturbed.

(c) Partial reshoring

Where a soffit is stripped bay by bay and reshores are placed in the stripped bay before further stripping is undertaken.

(d) Total reshoring

Complete stripping of whole soffit and the subsequent placing of reshores.

4.6.2.2 Procedures

In either method (c) or (d) of 4.8.2.1 reshoring should be carried out only under proper supervision and shall comply with the following requirements:

(a)

(b)

(c)

(d)

Operations shall be performed so that at no time will areas of new construction be required to support combined dead and construction loads in excess of their capabilities as determined by design load and developed concrete strength at the time of stripping and reshoring . .. In no case shall reshores be so located as to alter significantly the pattern of stress determined in the structural analysis. The reshore is simply a strut, not a jack, and shall be tightened only to the extent that it is firmly in place against the concrete.

The reshoring system shall be such that it contains a sufficient number of shores of adequate size to carry any loads that may be imposed on the system. The bearing pressure on the cap and base of reshores shall not exceed the permissible bearing stress of the material. Reshores should be located in the same position on each floor so that they will be continuous in their support from floor to floor. Where the number of reshores on a floor is reduced, such reshores as remain shall be located directly under shores on the floor above. Where shores above are not directly over reshores, an analysis shall be made to determine whether or not detrimental bending and shear stresses are produced in the slab.

When reshoring flat slab constructions one row of reshores shall be placed along each common boundary between column and middle strips in both directions, with additional rows of reshores being provided as required.

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