Chapter 1 - Introduction - Rev A

Chapter 1 - Introduction

© 2009 Stephen Ferguson all rights reserved 1.1

1 Introduction 1.1 Background

1.1.1 Formwork and Falsework

Formwork is used during construction to support and mould concrete. The term "Formwork" describes both the forms directly in contact with concrete and a supporting structure. Side forms (e.g. for concrete walls, columns, slabs, beams and foundations) are usually supported by bracing and often use form ties to resist lateral concrete pressure. Where as, soffit forms (e.g. under suspended slabs and beams) are usually supported by falsework.

The term "Falsework" refers to a temporary structure used during construction to support, either:

• Formwork;

• Parts of the permanent structure until they become self-supporting;

• Loading platforms for loading or storing materials;

• Working platforms for workmen or machinery; or

• Access platforms and walkways for access and egress.

Formwork and falsework are often referred to as "temporary works"; although, some formwork, or part thereof, may remain part of the permanent structure. In any case, both formwork and falsework is important because it has a major impact on the quality, cost and time to build concrete structures. In addition, their sound structural design is essential to ensure safety during construction.

1.1.2 Causes of falsework failure

Investigations into the causes of falsework failure agree that procedural inadequacies enabled flaws in the design and/or construction (Bragg 1975) and (Hadipriono and Wang 1986).

More recently, researchers concluded that at all levels of the industry those actively involved in the design of falsework lack an understanding of the fundamentals of stability of falsework and the basic principles involved (Pallett, Burrow et al. 2001).

The "Bragg" Report

Although there are many different technical reasons for construction failure, in general, they fall into three categories, where:

1 design loads are different from those actually applied; 2 the design is inadequate for the specified loads; and 3 the works are not constructed according to the design.

Actual loads were thought to differ from design loads because of: a design error; inaccurate estimations, different application, changes on-site, unknown or unexpected behaviour. Technical faults in this category include: calculation mistakes, underestimated densities, changes to construction

Formwork and Falsework

1.2 © 2009 Stephen Ferguson all rights reserved

sequence, changes to site conditions and the effect of imperfections such as eccentricity and out-of-plumb.

In the second category, the principal design inadequacies identified were: inadequate consideration of lateral stability of the falsework, or insufficient safety factor on the falsework as a whole. In addition, collapses occurred because of designers neglecting to consider: horizontal forces; the discontinuity of unconnected elements; progressive collapse; variations in positioning and alignment inevitable even with good workmanship; eccentric or lateral loading; or the degraded strength of previously used material.

In the final category, collapse resulted from unauthorised changes to the design or substitution of materials or components. Changes to design often arise out of the economic necessity to reuse materials or unavailability of specified components. When such changes degrade the capacity of the falsework collapse can occur.

On the other hand, it might have been possible to prevent collapse had the technical faults been detected. In each case, unfortunately, coexisting procedural inadequacies enabled the technical faults to go unnoticed. The committee identified procedural inadequacies such as: communication difficulties and confusion of responsibilities amongst participants; the lack of falsework design drawings; inadequate briefing of falsework designers; unapproved modifications of the initial falsework design; inadequate checking of designs, particularly those containing novel features; and omitting to inspect falsework prior to loading.

It was the committee’s opinion that the greatest improvement in falsework safety would result from paying proper attention to the communication of information. The designer requires an adequate design brief upon which to base his design. Design documentation is required to clearly communicate the design to site personnel. Unforeseen or unexpected site conditions might require changes to the design; consequently, the designer requires notification if such conditions exist, or if changes are made on-site. All changes require authorisation.

Hadipriono and Wang

In a study of 85 major falsework collapses, Hadipriono and Wang classified the causes of failures as: triggering, enabling, or procedural. They found that half of all collapses were triggered by an event during concrete placement, e.g. impact or vibration. In each case, inadequate falsework enabled collapse; however, the inadequacies should have been detected and corrected, but for the presence of procedural inadequacies.

Inadequate falsework cross-bracing/lacing was the primary enabling cause. Other enabling causes included: poorly maintained components whose capacity was less than intended; unconnected elements relying on friction for structural stability; inadequate foundations that lead to differential settlement and overloading; and inadequate consideration of the lateral forces and stability of the temporary structures.

Hadipriono and Wang found that the most noticeable procedural cause was the lack of falsework design and construction review. In addition, some failures were the result of inadequate monitoring of erection and concrete placement; inspectors were absent or unqualified persons were given the responsibility.

Investigations into aspects of falsework

Pallett et al conducted eleven interviews of key staff actively involved in falsework design, including chief engineers and a technical director from proprietary suppliers, with full responsibility for the technical content of the majority of falsework designed in the UK, as well as formwork managers and senior design engineers from contractors, who were responsible for checking supplier's schemes. Their findings included the following key points:

• at all levels of the industry there is a lack of understanding of the fundamental and basic principles involved in achieving the stability of falsework;



• the design of faslework was almost exclusively being carried out by suppliers of proprietary falsework systems and the assumptions made by the suppliers in relation to the erection and use of the faslework was not adequately communicated to the user; and

• concerns exist in the industry about the inadequate checking of designs.

As a result of these findings, authorities became concerned that it is only a matter of time before a serious event occurred (SCOSS 2002).

Discussion

Research has shown that the most likely cause of falsework collapse is procedural inadequacies that fail to identify flaws in design and construction. Recent research suggests that some procedural inadequacies and flaws are still present.

Problems often arise because of a lack of: competence, co-ordination or communication (among designers, constructors, checkers and inspectors) and manifests as:

(a) An inadequate brief upon which to base the design;

(b) A flawed or inappropriate design;

(c) Poor design documentation, which fails to clearly communicate the design to the site;

(d) Inadequate checking of the design that fails to identify flaws and omissions;

(e) Unauthorised modifications during construction; and

(f) Discrepancies between the design and the construction (site conditions, materials, components, and arrangement) that go unreported.

It is therefore not surprising that the frequency of structural failure and the general risk of death is much higher during construction than, later, during the service life of the completed permanent structure.

There are many plausible reasons why there is lack of competence, co-ordination and communication, including:

• In the past, structural engineers have focused more on the safety of permanent structures. Researchers have neglected temporary structures; consequently, there is a relative dearth of literature and guidance on the design of formwork and falsework.

• The need to erect, dismantle and reuse formwork (all in the shortest possible time and with minimum effort) raises special design considerations often unfamiliar to designers of permanent works who, understandably, often avoid becoming involved in the design of formwork and falsework.

• Generally, the construction contractor is responsible for the design of formwork and falsework. Traditionally, contractors performed the design in-house and construction, but few still do. As a result, there has been a shift of knowledge from the contractor to specialist sub-contractors and suppliers. Today, nearly all formwork and falsework is designed by a supplier and constructed by a sub-contractor.

• The design of temporary structures is often more complex than the design of permanent structures. Temporary structures, such as falsework, are often heavily loaded tall slender structures (with additional permitted imperfections and semi-rigid connections) whose capacity is sensitive to detrimental second-order effects and difficult to determine.

• Contractors are inevitably under the pressures to save time and reduce costs. The quality workmanship and type of materials used in formwork and falsework differ from permanent structures. Materials are not new. They are often reused many times and may become



damaged. The need to erect, dismantle and reuse formwork, all in the shortest possible time and with minimum effort, means these structures: relying largely on friction to provide connectivity; have roughly spacing members, inbuilt and unintentional eccentricities albeit they fall within relaxed permitted tolerances.

• The degree of rigour of verification, validation, supervision, inspection and monitoring processes for temporary structures is much less than for permanent structures. Workers are often left to build the formwork and falsework using their knowledge and experience, without the design ever being documented or verified. Permanent structure designers are reluctant to review formwork and falsework designs, less they be deemed to have "approved" the design. Inspections are often called at the last minute and carried out by junior or inexperienced persons, at a time when: the formwork may be incomplete, it is difficult to access all parts of the formwork and under an implicit duress to compromise, approve the work and not delay concrete placement.

• AS 3610 Formwork for concrete (SA 1990) was the first national formwork Standard published in limit states format. Until all Australian Standards (structural) converted to limit states methods, designers chose between either, permissible stress or limit states, as was appropriate. Now, since nearly all permissible stress material and design Standards have been withdrawn, formwork and falsework designers need to adopt limit states design methods. Unfortunately, many of the new limit states Standards (including the limit states provisions in AS 3610) have been criticised as incomplete, difficult to understand, providing little guidance and requiring complex analysis that few designers posses the expertise to undertake.

In the past, a higher frequency of structural failure may have been tolerated because of an underlying tacit attitude in the design and construction industry that temporary structures are less important than permanent structures and therefore greater risks are acceptable (Ratay 2002). However, this is no longer the case. Construction workers should not be at any greater risk from structural failure than any other worker. To this end, temporary structures should be as equally reliable as permanent structures.

1.3 Aim

This purpose of this text is to introduce and explain the requirements, concepts and methods to design, verify and inspect formwork and falsework. It is intended for students and practitioners alike. To that end, worked examples and sample problems have been included at the end of most Chapters. In particular, many of the gaps in knowledge identified in the literature are addressed herein, including: falsework stability, effects of imperfections, horizontal forces; discontinuity; and a lack of connectivity.

The guidance herein follows the limit states design philosophy and general principles set out in ISO 2394:1998, General principles on reliability for structures (ISO 1998) and AS 1170.0 2002 Structural design actions Part 0: General principles (SA 2002). The methods and examples set out in the book (including some that may be new to the reader) fulfil the requirements of AS 3610 - 1995 Formwork for Concrete (SA 1995) and comply with the requirements, methods and, where possible, the notation set out in the latest Australian material and design Standards.

1.4 Scope

In order, the following topics are covered:

Chapter 1 — Introduction Background, aim, scope, application, definitions, terminology and notation.

Chapter 2 — Structural design concepts Principles of structural design; reliability; limit states design; robustness; design working life; formwork importance levels; and working load limit.

Chapter 3 — General requirements Occupational health and safety; project requirements;



economy; structural requirements; and documentation, verification and inspection.

Chapter 4 — Actions and action combinations Permanent, variable, accidental and notional actions; serviceability, stability and strength limit states action combinations; duration of load factor for use with AS 1720.1design situations; and design examples for wind and formwork actions.

Chapter 5 — Concrete pressure CIRIA Report No 108; factors influencing concrete pressure; concrete rate of rise; statics of concrete pressure; and design examples;

Chapter 6 — Side Formwork Form ties; double sided wall formwork; single sided wall formwork; bracing; and design examples

Chapter 7 — Soffit Formwork Loading patterns; point loads vs UDL; lateral bracing of beams; sloping soffit formwork; design aids; and design examples.

Chapter 8 — Falsework Falsework design actions and action combinations, direct and indirect actions, discontinuity, sloping soffits and differential settlement; falsework stability, free standing; top restraint, sway vs braced frames; effective bracing; bracing to reduce effective length; connector stiffness; and knee buckling; and falsework strength, initial additional imperfections, and methods of analysis.

Chapter 9 — Stripping AS 3610 minimum stripping times; AS 3600 minimum stripping times; Beeby minimum stripping times; and assessment of concrete strength at an early-age.

Chapter 10 — Multistorey shoring Undisturbed shores, simplified and modified simplified methods; reshores; and required concrete strength

Chapter 11 — Verification TBA

Chapter 12 — Inspection TBA

Appendix A — Coefficients of static friction Tables for permissible stress and limit states design.

Appendix B — Calibration of formwork action combinations

Research; the relative reliability of AS 3610; target reliability index; and calibration of new design rules.

Appendix C — Beam load tables Extracts from "Formwork: A guide to good practice"

Appendix D — Plywood and LVL properties Extracts from AS 2269:1994, AS 1720.1:1997 and manufacturers information.

1.5 Application

Despite focusing on formwork and falsework, many of the concepts presented herein equally apply, and could be adapted, to the design of other temporary structures, including: falsework for other than for formwork, scaffolding, temporary grandstands, tilt-up panels, loading platforms, etc.



1.6 Definitions and terminology

1.6.1 Formwork

Formwork is a structure, usually temporary, erected to support and mould cast-in-situ concrete until it becomes self-supporting. It consists of a form and, where appropriate falsework, form bracing and form ties.

Figures 1.1 and 1.2 show the general arrangement of simple slab formwork and wall formwork, respectively.

Figure 1.1 Simple suspended slab formwork(McAdam and Lee 1997)

Figure 1.2 Simple wall formwork(McAdam and Lee 1997)



1.6.2 Forms

Typically, forms for suspended slabs and beams comprise a three layer grillage of structural members, namely: a form face sheet, secondary beams (often called "joists") and primary beams (often referred to as "bearers" or "headers"), see Figure 1.1.

Similarly, side, wall, and column forms are often built with a form face, secondary beams [often called "studs" (vertical) or "walers" (horizontal)], and primary beams [often referred to as "soldiers" (vertical), "walers" (horizontal), or "strongbacks"].

In both cases, concrete is poured directly against the form face. The load is carried by the form face, which spans between the secondary beams. In turn, the secondary beams span between and are supported by the primary beams. The primary beams are supported by, either:

(g) for suspended slab and beam forms, usually shores (in the case shown in Figure 1.1, adjustable steel props, but often formwork frames or modular falsework); or

(h) for wall and column forms, form ties or bracing (in the case shown in Figure 1.2, form ties support the primary beams).

Instead of using an assembly of individual form face sheets, secondary and primary beams; forms might be prefabricated into panels. Usually, the panels comprise a steel or aluminium frame (secondary and primary members) with a plywood form face. Panels systems are available for wall and slab formwork. The size and weight of the panels range from small enough to man handle to large forms that are crane or mechanically handled. Typically panel systems can be connected together to form a large surface. Large wall form panels are often called "gang forms". Usually, large slab form panels that incorporate falsework are called "tableforms", see Figure 1.13.

Form face

The texture, stiffness and permeability of the form face material effects the resultant concrete surface finish. The strength and hardness of the form face effects its longevity. The most commonly used form face materials are: plastic faced plywood sheets (often called "form ply") or steel plate. Other materials used include: chipboard, sawn timber boards, non-plastic faced plywood, aluminium, expanded metal lathe, polystyrene, glass fibre (GRP) reinforced plastic, polypropylene, rubber, plastic sheet, fabric, controlled permeability fabric, concrete, glass fibre reinforced cement (GRC), and fibre reinforced cement sheet.

Secondary and primary beams

The most commonly used materials for secondary and primary beams are timber products (e.g. LVL) aluminium or steel, see Figure 1.3.

(a) LVL beams



(b) Proprietary aluminium beams

Figure 1.3 Common secondary and primary beams

1.6.3 Falsework

A temporary support structure. The part of the formwork that supports the form and transfers all the loads to a stable surface(s). Figures 1.4 to 1.7 show different types of suspended slab falsework.

Shores

The falsework members that act as columns (or struts) to transmit all or part of the loads to a lower level are called shores. Other terms for shores include: props, struts, supports, standards or legs. The term shores is also used to refer to both the shores that directly support the forms, as well as the shores used for backpropping and reshoring.

Figure 1.4 depicts the ubiquitous adjustable steel prop (prop).

(a) Adjustable steel prop ("prop")



(b) Prop falsework (MevaDec)

Figure 1.4 Adjustable steel prop

(a) Formwork frame



(b) Multi-level formwork frame falsework

Figure 1.5 Formwork frames

(a) Modular falsework components



(b) Multi-level modular falsework

Figure 1.6 Modular falsework (scaffold)

Figure 1.7 Heavy duty falsework

Screw jacks ('U' head and base)

Screw jacks are used at the base (base jacks) and top (U head jacks) of falsework shores to provide height adjustment and permit formwork stripping, see Figure 1.8.



(a) U head screw jack

(b) Base screw jack

Figure 1.8 Base and U head screw jacks

Bracing (horizontal, diagonal and plan)

Falsework bracing serves several purposes and is often critical to the strength and stability of the falsework. Horizontal members are often called "Lacing", "Ledgers" or "Transoms" and Diagonal members are often called "Braces" or "Diagonal Braces", see Figure 1.6(a).

Soleplates

The term soleplate is used to refer to timber boards used as "temporary footings" to spread the load from faslework shores and thereby reduce the bearing pressure on the foundation material, see Figure 1.1.

1.6.4 Form bracing

Side formwork usually requires bracing to maintain stability and alignment. In addition, bracing may be used to resist and transmit lateral concrete pressure and other horizontal actions to the foundation.

Figure 1.10 shows a tall single-sided form arrangement that is heavily braced to resist the lateral concrete pressure.



Figure 1.10 Vertical formwork bracing

1.6.5 Form ties

Form ties are tension members used to balance the concrete pressure on opposing forms, see Figure 1.11.



(a) Bar tie

(b) She bolt

(c) Snap tie

Figure 1.11 Different types of form ties



1.6.6 Special formwork

Panel systems

Figure 1.12 Wall and soffit panel systems

Tableform

Tableforms are suited for use in multistorey buildings where the suspended slab design repeats from floor to floor.

Figure 1.13 Tableform being moved on site

Figure 1.14 Large tableforms called "Flying forms"

Climbform



Climbform or Jumpform are used to construct the core and shear walls of multistorey buildings. They may be self-climbing or lifted by crane. Unlike slipform, concrete is placed in static forms.

Figure 1.15 Climbform

Slipform

Slipform was first used to construct concrete silo, tank and chimney structures. It is also used to construct the core and shear walls of multistorey buildings. In this system, concrete is placed in shutters that progressively move (slip) upwards.

Figure 1.16 Slipform

Wall Traveller forms

Figure 1.17 Wall traveller forms



Incremental launch forms

Figure 1.18 Incremental launch formwork

Permanent forms

Permanent forms are forms that are not stripped.

Figure 1.19 Permanent metal deck formwork supported on falsework

Figure 1.20 Permanent metal deck formwork supported on a steel structure



1.7 Notation

References

Bragg, S. L. (1975). Final report of the Advisory Committee on Falsework. London, Her Majesty's Stationery Office: 151.

Hadipriono, F. C. and H.-K. Wang (1986). "Analysis of causes of formwork failures in concrete structures." Journal of Construction Engineering and Management 112: 112 - 121.

ISO (1998). ISO 2394:1998 General principles on reliability for structures. Geneve, International Organization for Standardization.

McAdam, P. S. and G. Lee (1997). Formwork a practical approach. London, E & EF Spon.

Pallett, P. F., M. P. N. Burrow, et al. (2001). Investigation into aspects of falsework. Birmingham, University of Birmingham.

Ratay, R. T. (2002). Forensic Structural Engineering Handbook, McGraw-Hill.

SA (1990). AS 3610 - 1990 Formwork for concrete. Sydney, Standards Australia.

SA (1995). AS 3610 - 1995 Formwork for concrete. Sydney, Standards Australia.

SA (2002). AS/NZS 1170.0 - 2002 Structural design actions Part 0: General principles. Sydney, Standards Australia.

SCOSS (2002). Falsework: Full Circle. London, Standing Committee on Structural Safety: 8 pp.

Chapter 1 - Introduction - Rev A

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