review of past research on scaffold systems

School of Civil Engineering

Sydney NSW 2006

AUSTRALIA

http://www.civil.usyd.edu.au/

Centre for Advanced Structural Engineering

Review of Past Research on Scaffold

Systems

Research Report No R905

Tayakorn Chandrangsu BSc MSc

Kim JR Rasmussen MScEng PhD

October 2009

ISSN 1833-2781


Centre for Advanced Structural Engineering

http://www.civil.usyd.edu.au/

Review of Past Research on Scaffold Systems

Research Report No R905

Tayakorn Chandrangsu, BSc, MSc

Kim JR Rasmussen, MScEng, PhD

October 2009

Abstract:

This article presents an overview of scaffold research and current practice in the design of

scaffold systems. It covers brief description of scaffold systems including types of joints and

materials currently used. Also, types of analysis, loads, initial geometric imperfections, and

modelling of complex joints are described. The prediction of the ultimate load of scaffold

systems derived from simplified equations and their failure modes are shown. In addition, the

article explains the design of scaffold systems based on British and Australian standards as

well as how effective lengths and bracings commonly apply. The recommendations are

provided for modelling, analysis and design of scaffold systems.

Keywords: Scaffold systems, Falsework, Structural analysis, Ultimate load, Design, Standards

Review of Past Research on Scaffold Systems October 2009

School of Civil Engineering Research Report No R905

2

Copyright Notice

School of Civil Engineering, Research Report R905

Review of Past Research on Scaffold Systems

© 2009 Tayakorn Chandrangsu and Kim JR Rasmussen

[email protected] and [email protected]

ISSN 1833-2781

This publication may be redistributed freely in its entirety and in its original

form without the consent of the copyright owner.

Use of material contained in this publication in any other published works must

be appropriately referenced, and, if necessary, permission sought from the

author.

Published by:


The University of Sydney

Sydney NSW 2006

AUSTRALIA

October 2009

This report and other Research Reports published by the School of Civil

Engineering are available on the Internet:

http://www.civil.usyd.edu.au



3

Table of Contents

1. Scaffold Systems...............................................................................................5

1.1 Introduction.................................................................................................5

1.2 Configurations.............................................................................................5

1.3 Materials......................................................................................................9

2. Collapses of the Scaffolds.................................................................................9

2.1 Construction Stages ....................................................................................9

2.2 Method of Determining Causes of the Collapses .......................................9

2.3 Main Causes of Collapse ............................................................................9

3. Analysis and Modelling of Scaffold Systems.................................................10

3.1 Non-linear Structural Analysis .................................................................10

3.2 Three-dimensional Model vs. Two-dimensional Model ..........................10

3.3 Load Combinations and Load Paths .........................................................10

3.4 Initial Imperfections..................................................................................11

3.5 Joint Modelling and Boundary Conditions...............................................12

3.6 Suggestions ...............................................................................................15

4. Ultimate Load of Scaffold Systems ................................................................15

4.1 Parametric Studies ....................................................................................15

4.2 Failure Modes ...........................................................................................16

4.3 Simplified Equations.................................................................................18

5. Design of Scaffold Systems ............................................................................20

5.1 British Standards.......................................................................................20

5.2 Australian Standards .................................................................................21

5.3 Effective Lengths ......................................................................................23

5.4 Bracing Systems........................................................................................23

5.5 Safety in Construction of Scaffold Systems .............................................24

6. Conclusions.....................................................................................................24

References ...........................................................................................................25



4



5

1. Scaffold Systems

1.1 Introduction

Scaffolds are temporary structures generally used in construction to support various types of

loads. The vertical loads on scaffold can be from labourers, construction equipment,

formworks, and construction materials. Commonly, scaffolds must also be designed to

withstand lateral loads, including wind loads, impact loads, and earthquake loads. Depending

on the use of the scaffolds, they may be categorised as the access scaffolds or the support

scaffolds. The access scaffolds are used to support light to moderate loads from labourers,

small construction material and equipment for safe working space. They are usually attached

to buildings with ties and only one bay wide. Support scaffolds, or sometimes called

falsework, are subjected to heavy loads, for example, concrete weight in the formwork. Both

types of scaffolds can be seen in everyday construction as shown in Figure 1.

(a) (b)

Figure 1: Typical scaffold systems: (a) access scaffold; and (b) support scaffold

This report presents a review of scaffold research in the analysis and modelling, including the

design of scaffold systems. In addition, it covers a brief description of scaffold systems, types

of connections, and construction recommendations. In terms of modelling, it focuses on how

complex joints and boundary conditions have been modelled and how geometric

imperfections have been taken into account. For the design of scaffold systems, it summarises

the current procedure based on the standards of practice.

1.2 Configurations

Scaffolds are generally made up of slender framework. The configurations of scaffold units

vary from one manufacturer to another, as shown in Figure 2; however, they share common

features. Scaffolds normally consist of vertical members (standards), horizontal members

(ledgers), and braces, as illustrated in Figure 3.



6

Figure 2: Various types of scaffold unit: (a) simple (knee-braced) door type; (b)-(e) standard

door type; (f) stick construction with Cuplok joints or wedge-type joints

Figure 3: Configuration of typical stick-construction scaffold frame

The scaffold vertical members (standards) are connected to create a lift via couplers, also

known as spigot joints (Figure 4), and to connect horizontal members (ledgers) to vertical

members, Cuplok or wedge-type joints (Figure 5) are usually preferred because no bolting or

welding is required; though, in some systems manually adjusted pin-jointed couplers are still

being used. The connections for brace members are usually made of hooks for easy

assembling; however, in some systems pin-jointed couplers are used (Figure 6). The base of

scaffolds consists of adjustable jack bases (Figure 7), which can be extended up to typically

600 mm by a wing nut to accommodate irregularity of the ground. The access scaffolds

usually have ties connecting them to a permanent structure to increase the lateral stability of

the system; in contrast, the support scaffolds have adjustable shore extensions with U-head

screw jacks (Figure 8) to support timber bearers at the top to ensure the levelling of the

formwork. Scaffold systems can be from one storey (lift) up to many storeys, and can have

many bays, and rows depending on the type of construction. A scaffold unit is prefabricated

to specific dimensions, and assembled on site for the ease of construction. Moreover, scaffold

members are reused from one job to another, and for that reason, quality control program is

required to ensure that geometric imperfections, notably the crookedness of standards,

remains within stipulated tolerances.

(a) (b)

(d)

(e) (f)

(c)

Standard

Ledger

U-Head Screw Jack

Jack Base

Brace



7

Figure 4: Schematic of spigot joint

Top cup

Bottom cup

Standard

Ledger

Ledger blade

3

1

2

Locking pin

(a)

(b)

Figure 5: Schematic of (a) Cuplok joint; and (b) wedge-type joint

Top Standard

Bottom Standard

Spigot Joint

Standard

Ledger

Wedge

Pin

Clamp



8

Figure 6: Schematic of typical brace connections: (a) hook connection; and (b) pin

connection

Figure 7: Schematic of jack base

Figure 8: Schematic of U-head jack

Timber Bearer

Adjustable U-Head

Wing Nut

Standard

Standard

Ground

Jack Base

Wing Nut

Brace

Standard

Brace

Ledger

Hook

Bolt (b)

(a)



9

Normally, the total height of scaffold systems varies from 1.2 m up to 25 m. The height of

individual panel (lift) is usually between 1.0 m to 2.5 m, and the bay width ranges from 0.7 m

to 2.5 m. The plan configurations of the scaffold systems can be in different shapes, for

instance, rectangular shape, L shape, and U shape. Depending on construction requirement,

scaffold systems can be easily constructed to suit the needs because of their flexibility in

dimension and size.

1.3 Materials

Different natural materials such as timber and bamboo have been used in the past and are still

being used in Asia to construct scaffolds. In the western world, cold-formed circular hollow

steel sections are mainly used as members of scaffold system due to their high strength and

reusability. The steel tubes used for standards and ledgers commonly have an outside

diameter between 42 mm to 48 mm with thickness of approximately 3 mm. As for bracing,

various types of steel sections are currently used in scaffold construction. Some braces are

constructed with two periscopic tubes that can slide inside one another to adjust the brace

length. Following the trend of maximising the efficiency in construction, aluminium is

becoming increasingly utilised as members in scaffold construction because of its lighter

weight and ease of handling. Many aluminium scaffold manufacturers are now located in

China, Australia, New Zealand and United Kingdom.

2. Collapses of the Scaffolds

2.1 Construction Stages

In 1985, Hadipriono and Wang [1] compiled a report on the causes of failure of worldwide

support scaffold systems from 1961 until 1982. It was found that over 74% of the collapses

occur during concrete pouring operations due to the impact forces of concrete pouring. In

addition, some failures were reported to occur during the disassembly of the formwork.

2.2 Method of Determining Causes of the Collapses

Hadipriono and Wang classified the failure occurrence into three groups, representing the

triggering causes, enabling causes, and procedural causes [1]. The triggering causes are

external incidents that start scaffold collapses, for instance, heavy loads on the scaffolds. The

enabling causes are incidents that present insufficient design and deficient construction. The

procedural causes are linked with the triggering and enabling causes, and are typically faults

in communication among parties. Hadipriono [2] also introduced fuzzy set and fuzzy concept

in measuring scaffold safety. His method can be applied to determine the probability of event

combinations that lead to scaffold failure; therefore, it can be extended to control and

minimise risks in scaffold construction.

2.3 Main Causes of Collapse

Hadipriona and Wang [1] concluded that most triggering causes were due to excessive

loading on scaffolds, and impact load from concrete pouring was the major concern for

support scaffold systems. For enabling causes of failure, inadequate bracing in scaffold

systems was the main problem that led to the collapse of scaffolds during construction.

Inadequate review of scaffold design and absence of inspection during the scaffold

construction were the most important factors in procedural causes. Additionally, Hadipriona

and Wang reported some other significant causes of support scaffold collapse such as

improper or premature formwork removal, inadequate design, and vibration from equipment.



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In a study of high clearance scaffolds by Peng et al. [3], the possible causes of support

scaffold collapses were identified as overloading of the scaffold systems, instability of

shoring components, partial loading of fresh concrete in the formwork, specific concrete

placement pattern on the formwork, and load concentration from concrete placement. The

cause of the collapses due to load patterns notified by Peng et al. were presented in detail by

the method of influence surfaces in a separate study [4].

Milojkovic et al. [5] presented an inspection by the HSE in the UK on typical faults in access

scaffold systems. The most common cause of collapse was insufficient tying to a permanent

structure. Some other structural faults included in the report were a settlement of support, out-

of-plumb and out-of-straightness of standards, and inadequate bracing.

3. Analysis and Modelling of Scaffold Systems

3.1 Non-linear Structural Analysis

With the ready availability of powerful computers and sophisticated structural analysis

software packages, non-linear structural analysis has become feasible and practical. Non-

linear analysis allows researchers and practitioners to more accurately predict the failure load

and deformation of scaffold systems. Non-linear analysis involves the modelling of changes

of the geometry of structures as a result of loading and/or inelastic material properties. In

research by Gylltoft and Mroz [6], and Chan et al. [7], the models were analysed considering

both non-linear material and geometric modelling. However, in many cases research on

scaffold systems focuses on non-linear geometric modelling associated with second-order

effects since scaffold members are slender and sensitive to stability effects. For example,

elastic geometric non-linear analyses were reported by Peng et al. [8], Prabhakaran et al. [9],

Yu et al. [10], Chu et al. [11], and Weesner and Jones [12]. Geometric non-linear analysis is

also a common practice in design offices, whereas the use of inelastic analysis is rare.

3.2 Three-dimensional Model vs. Two-dimensional Model

Access scaffold systems usually fail in complex three-dimensional modes locally or globally,

and require the use of three-dimensional analysis models to accurately predict their behaviour

and strength. Support scaffold systems are often more regular in geometry, and can then be

analysed and designed using two-dimensional models. Particular attention needs to be paid to

local eccentricity and member imperfections in the non-linear analysis of scaffold systems.

By means of available commercial finite element softwares such as ANSYS, GMNAF, and

NIDA, many new studies on scaffold behaviour were carried out through three-dimensional

models such as those presented by Prabhakaran et al. [9], Milojkovic et al. [5], and Godley

and Beale [13] . Some past models proposed by Huang et al. [14], and Peng et al. [15] were

two-dimensional for simplicity and less demanding computability.

3.3 Load Combinations and Load Paths

Scaffold systems usually require consideration of different types of load patterns, load

sequences, and load combinations. As a result of concrete operation on support scaffold

systems, several load patterns and sequences normally occur in considering the load

combination of gravity loads and lateral loads. The research by Peng et al. [16] presents three

different sequential loading patterns on 3-storey scaffold systems described as model R

(rectangle scaffold plan), model L (L-shape scaffold plan), and model U (U-shape scaffold

plan), as shown in Figure 9. In all models, sequential paths were investigated and compared

with uniform loads. It was shown that the critical loads of the scaffold system under different

sequential paths and uniform loads were about the same. This finding was in good agreement



11

with the analysis of concrete placement load effects using influence surfaces from earlier

research [4]. Thus, designer can safely assume uniform loads in practical design of the

scaffold system. In terms of load combinations, self weight and imposed (working) load are

usually considered critical in predicting the behaviour and strength of scaffold systems.

Figure 9: Top view of rectangular, L, and U shapes of scaffold systems

Some researchers have considered wind loads in perpendicular and parallel directions in their

access scaffold model [5]. Godley and Beale [17] considered the combinations of dead,

imposed and wind loads with different magnitudes for both in-use and out-of-service

conditions of scaffold in construction practice. For design purposes, the magnitude of

imposed (working) load and wind load applied are usually taken from international design

codes, such as British Standards [18] and Australian Standards [19].

3.4 Initial Imperfections

Scaffold structures are slender by nature; therefore, small initial imperfections producing

member P-δ and frame P-∆ second order effects must be considered in the model to

accurately predict the behaviour and load carrying capacity of the system (Figure 10). There

are many efficient ways of taking geometric imperfection effects into account. For instance,

Chan et al. [7] considered two types of geometrical imperfections in portal frames, i.e.

imperfections from initial sway and initial member distortion. The same imperfections were

considered in the modelling of scaffold systems [20]. Three methods of modelling

imperfections were trialled, including the scaling of eigenbuckling modes (EBM), the

application of notional horizontal forces (NHF), and the direct modelling of initial geometric

imperfections (IGI) [7]. EBM was performed by carrying out eigenbuckling analysis on the

structural model, and then scaling and superimposing the lowest eigenmode onto the perfect

geometry to create an initial imperfect structural frame for the second-order structural

analysis. In the NHF approach, additional lateral point loads were applied at the top of each

column in one direction of the frame and initial member out-of-straightness could be

represented by lateral distributed forces along each member. The IGI method consisted of

applying an initial sway of the frame and an out-of-straightness to each column in the frame.

Rectangular

Shape

U Shape

L Shape



12

Figure 10: P-δ and P-∆ effects

For scaffold systems, these same approaches can be applied to model the effects of initial

imperfections in the analysis. For example, Yu et al. [10] integrated EBM with the magnitude

of the column out-of-straightness of 0.001 of the height of the scaffold units into the model.

Moreover, Yu and Chung [20] investigated a method called critical load approach where

initial imperfections were integrated directly into a Perry-Robertson interaction formula to

determine the failure loads of the scaffolds in the analysis. In other research on scaffold

systems by Chu et al. [11], the notional horizontal force approach was incorporated in the

model by applying a horizontal notional force of 1% of the vertical loads on the scaffold.

Godley and Beale [17] adopted an initial geometric imperfection approach by imposing a

sinusoidal bow to the members and angular out-of-plumb to the frame. In all these proposed

methods, careful calibration against test results or numerical reference values is required.

3.5 Joint Modelling and Boundary Conditions

Scaffold joints are complex in nature due to need for rapid assembly and reassembly in

construction. The Cuplok connections behave as semi-rigid joints, and show looseness with

small rotational stiffness at the beginning of loading. Once the joints lock into place under

applied load, the joints become stiffer [13]. Wedge-type joints are generally more flexible

and closer to pinned connections. They also often display substantial looseness at small

rotations [17]. Figure 11 shows typical moment-rotation curves for cuplock [13] and wedge-

type [17] joints. As to spigot joints, out-of-plumb of the standards can occur due to the space

between the standard and the spigot and the lack of fit in the joints can create complexity in

modelling [21]. Various scaffold researchers devised ways in modelling joints; moreover, the

study of boundary conditions of scaffold systems is crucial because the top and bottom

restraints can highly influence the stability and strength of the systems [22].



13

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.00 0.05 0.10 0.15 0.20

Rotation (radian)

Mo

me

nt

(kN

m)

Cuplok joint

Wedge-type joint

Figure 11: Typical moment-rotation curves for Cuplok and wedge-type joints

In recent research by Peng et al. [8], analysis models of wedge-type jointed, 3-storey, 3-bay,

and 5-row scaffold system were presented. Experimental test on scaffold joints showed that

the joint stiffness was between 4.903 kNm/rad (50 ton cm/rad) and 8.826 kNm/rad (90 ton

cm/rad) with the average of 6.865 kNm/rad (70 ton cm/rad) being adopted for all joints into

their model. Godley and Beale [17] found that scaffold connections were frequently made of

wedge-type joints, for which the joint stiffness exhibited different response under clockwise

and counter-clockwise rotations, and occasionally exhibited looseness in connections with

low stiffness. Consequently, Prabhakaran et al. [9] modified the stiffness matrix for the end

points of the beam to include connection flexibility, using a piecewise linear curve to model

the moment-rotation response.

Yu [22] studied the boundary conditions of the scaffold system, and categorised them into

four cases, i.e. Pinned-Fixed, Pinned-Pinned, Free-Fixed, and Free-Pinned, with the first term

being translational restraint at the top of the scaffold, and the second term being the rotational

restraint at the base of the scaffold. In all analyses, the rotation at the top was assumed to be

free. These conditions were incorporated into the models of one bay of one-storey modular

steel scaffolds (MSS1), and two-storey modular steel scaffolds (MSS2). Yu found that for

MSS1 the failure load results for Free-Fixed and Pinned-Pinned conditions are reasonably

close to test results; however, for MSS2 the model results are considerably higher than the

test results. Subsequently, Yu suggested that since the top of the scaffolds normally has

lateral restraints then joints at the top can be modelled as translational springs, and for the

bottom rotational spring can be applied. A stiffness of 100 kN/m for the top translational

spring and stiffness of 100 kNm/rad for the bottom rotational spring gave very comparable

results to the tests.

In single storey double bay scaffold research by Chu et al. [11], in the presence of restraints

in the loading beam and the jack bases, the top and the base were modelled with various

boundary conditions, and the scaffold connections were assumed to be rigid. The researchers

found that both Pinned-Pinned and Pinned-Fixed conditions gave higher load carrying

capacities than the experimental results; on the other hand, the Free-Fixed condition gave

satisfactory result compared to the tests. Research on the stability of single storey scaffold

system by Vaux et al. [23] found that Cuplok connections represented by pin joints, and

connections of the top and bottom jacks to the standards assumed as rigid with the top-bottom



14

boundary conditions taken as Pinned-Pinned gave good agreement between numerical and

experimental failure loads.

Weesner and Jones [12] studied the load carrying capacity of three-storey scaffolds assuming

rigid joints between the stories, and pin joints for the top and the bottom boundary conditions.

The results of their elastic buckling analysis came out to be rather larger than the test values

with the percentage differences ranging from 6% to 17%. In the analysis of large access

scaffold systems by Godley and Beale [17], cantilever arm tests were done on scaffold

wedge-type joints. The non-linear moment-rotation curve from the tests showed joint

looseness and different values of rotational stiffness under positive (counter-clockwise)

rotation and negative (clockwise) rotation. The authors suggested the use of a multi-linear or

non-linear moment-rotation curve for scaffold joint modelling.

In the work by Enright et al. [21], the spigot joints were studied for the stability analysis of

scaffold systems. The spigot inserts were considered to have bending resistance, but not to

transmit axial load; therefore, the model adopted two vertical members connected by pin

joints representing the standards, and on the side, the entirely rigid spigot member was

connected at the top, centre, and bottom to the standard via short and axially stiff members

capable of transferring only lateral forces, as shown in Figure 12. Due to the axial load in the

out-of-plumb standards, the spigot would be in bending, and the amount of bending would

depend on the amount of axial load and the degree of out of plumb. From research of Harung

et al. [24], it was found that if the spigot joints were modelled as fully continuous joints, the

analysis would overestimate the load carrying capacity of the system.

Figure 12: Spigot joint model

Milojkovic et al. [25] studied eccentricity in the modelling of scaffold connections. Given

that the neutral axes of the connections were offset by 50 mm, the authors modelled the

eccentric joint with a finite spring of length equal to the eccentricity of 50 mm. The spring

had specific rotational stiffness, and was assumed to be axially stiff. The authors concluded

that for large frames, unless torsion failures can occur, then the effects of joint eccentricity

are insignificant. In the scaffold study by Gylltoft and Mroz [6] the braces were represented

as truss members with pinned joints connected to the standards, and the connections between

other members were modelled as short finite elements with non-linear stiffness in all

directions. To model the shores of the scaffold system, Peng et al. [15] applied rigid links

with pinned supports at both ends, given that actual shores were connected loosely by nails at

the top and bottom.

Axial load

Standards

Spigot

Pin joint



15

3.6 Suggestions

To accurately study the behaviour and strength of scaffold systems, geometric and material,

three-dimensional non-linear analyses are efficient tools. Geometric imperfections have to be

incorporated into the model, in order to consider the second order effects that exist in the

structures. Further research into joint modelling of scaffold systems is needed since scaffold

joints exhibit non-linear behaviour and present a lack of fit at early stage. To model the

systems accurately, these factors must be taken into account. Moreover, boundary conditions

must be considered carefully since top and bottom jacks can have eccentricities, which can

greatly affect the overall stability of the system. The degree of rotational and translational

fixity over the top and bottom has to be calibrated correctly to achieve accurate results.

4. Ultimate Load of Scaffold Systems

4.1 Parametric Studies

Yu et al. [10] investigated the influence of the number of storeys and boundary conditions on

the load carrying capacity. They analysed one, two, and three storey steel scaffolds, and

found that two-storey and three-storey scaffolds had only 85% and 80% of the load carrying

capacity of the single storey steel scaffold respectively because the different numbers of

storey presented considerable variation in buckling behaviour. Moreover, through different

boundary conditions applied at the top and the bottom, the analytical load carrying capacity

varied in the range from 50% to 120% of those of the experimental tests.

The comprehensive study on wood shoring of double-layer systems by Peng [26] showed the

effects of the length of horizontal stringers (horizontal timbers to connect uprights) and

vertical shores, stiffness of stringers, and positions of strong shores on the load carrying

capacities of the shoring systems. Peng found that adding strong shores (vertical shores with

horizontal bracing in a closed pattern) to the systems could increase the ultimate loads. In

contrast, when the stiffness of the horizontal stringer decreased or the length of the stringers

increased for the cases of unsymmetrical arrangement of strong shores that are at least one

combination of strong shore and leaning column (pinned-ended column) in a vertical

direction as shown in Figure 13(c) and 13(d), the system ultimate loads would be reduced;

however, for symmetrical cases as shown in Figure 13(a) and 13(b) the ultimate loads were

unaffected by the change in stringer stiffness. The varied lengths of vertical shores had

different effects depending on the strong shore arrangement in the system. In addition, strong

shores were not as effective when applied at the outmost location in the system as to apply to

the inner. The same author concluded that the system ultimate load only increased by adding

the strong shores, but not the leaning columns [27].



16

Figure 13: Model of 2-bay shoring system

Research on the correlation between the load carrying capacity and the number of storeys of

shoring scaffold system [28] showed that the critical loads of the system reduced rapidly from

the range of two to eight storeys, followed by a gradual decrease thereafter. Furthermore, it

was found by Peng et al.[15] that when the initial imperfection of 1.50% by notional

horizontal force approach was applied at mid-height, the reduction in critical load of simple

door-type scaffold systems was found to be near 16%, which was conservative based on

experimental results, and the relationship between the initial imperfection and reduction in

ultimate load of the scaffold system was nearly linear. With the imperfection of 0.1% applied

to the model, the predicted critical load showed good agreement with test results. Also, Peng

et al. [15] found that with long shores installed, the ultimate load of the scaffold system could

be as little as 25% of that of the system without shores. From the analyses of high clearance

steel scaffolds by Peng et al. [3], the optimum load carrying capacity for steel scaffolds with

shoring occurred in the range of three to six storeys. In addition, scaffolds of more than eight

storeys were not recommended due to high reduction in strength.

Other factors influencing the ultimate load of scaffold systems are bracing arrangement, load

eccentricity, and ground irregularity. Investigating of bracing arrangement can help designers

to achieve the optimum design for the scaffold system. Load eccentricity and ground

irregularity exist in any construction; therefore, knowing the extent of those parameters can

reduce the risks in scaffold construction.

4.2 Failure Modes

Due to the high slenderness of members in scaffold systems, failure usually occurs by

buckling. The two common types of buckling in scaffold systems are the out-of-plane mode

perpendicular to the plane of scaffold unit and the in-plane mode. The critical mode depends

on the relative stiffness of the connecting members in each direction. The standards can

buckle in single or double curvatures, depending on the configuration of the scaffolds and

support conditions. Figure 14 shows common out-of-plane failure mode of single storey

door-type scaffold.

Strong shore Strong shore

Leaning column

Strong shore

Strong shore Strong shore

Leaning column

Strong shore

Stringer Stringer

Stringer Stringer

(a) (b)

(c) (d)



17

Figure 14: Typical failure mode of single storey door-type scaffold

Yu et al. [10] performed vertical load tests on multi-storey door-type steel scaffolds. The

researchers found that single storey and double storey scaffolds both buckled out-of-plane,

and deflected in single and double curvatures respectively. In addition, Yu and co-workers

noticed that there were large displacements of the standards in the plane of the cross-bracings

at failure, suggesting that the door-type systems were stiffer in the in-plane direction. From

the three-storey scaffold test results by Weesner and Jones [12] on four different door-type

frame, most of the scaffolds failed by buckling out-of-plane. Only one of the tests failed in

torsion.

Huang et al. [14] carried out tests on one, two and three storey scaffolds test, as shown in

Figure 15. The tested scaffold unit was a portal frame with knee braces at the top. The one-

storey scaffolds failed by out-of-plane buckling, whereas the two-storey and three-storey

scaffolds displayed in-plane buckling at failure, and the highest lateral displacement was

found to be at the top of first story, as shown in Figure 15.

Figure 15: Schematic failure modes of one-to-three storey knee-braced scaffolds

From three-dimensional analyses of high clearance steel scaffolds, Peng et al. [3] observed

that the deformation modes of the steel scaffolds were dependent on the relative strength

between the steel scaffold units and the cross-braces providing lateral support. If the cross-

braces offered stiff lateral support, then the plain scaffold units would deform in-plane;

In-plane Out-of-plane



18

conversely, the scaffold units would deform in the out-of-plane direction, in case of flexible

cross-braces.

4.3 Simplified Equations

Several researchers have proposed simplified equations to determine the ultimate load of the

scaffold system based on structural analysis models and experimental tests. Huang et al. [28]

used a two dimensional model to derive a closed-form solution for the critical load of

scaffold systems with knee-braced units. The solution was based on a bifurcation

(eigenvalue) method to the elastic-buckling condition, and the critical loads were calculated

as functions of the material properties, the number of storeys, and the section properties of

the scaffolds. The assumptions for the derivation were as follows: all members behaved

elastically, and the frame buckled in-plane at the lowest storey (Figure 16).

Figure 16: Assumption of proposed analytical model

The analytical solution was given as:

0tantan)sec1)(1(2 2 =−+−− kLkLNkLkLN (1)

where N = number of storeys; k = effective length factor; and L = one-storey height of the

scaffold unit. From Eq. (1), kL could be solved and applied as the effective length to compute

the critical load.

In other research of scaffolds by Huang et al. [14], the critical loads were calibrated and

modified based on failure modes and critical loads from the computational critical loads to

the experimental values, then the modified values could be taken as the critical loads of the

scaffold systems for any number of storeys shown in the published graph of critical loads

versus number of storeys (Figure 17). In case the scaffold units were different from the ones

used by the authors (portal frames with knee braces), the critical load should be based on

computational results of different section properties that were functions of the slenderness

( 2/ iii LIE ) of the uprights, and adjusted as:



19

3830

/)(

2

,iii

graphcricr

LIEPP ×= (2)

where icrP )( = critical load of the scaffold in concern; graphcrP , = critical load from the

published graph of Huang et al. [14]; iE = Young’s modulus in N/cm2; iI = moment of inertia

in cm4; and iL = one-storey height of the scaffold in cm.

Figure 17: Computational critical loads based on two-dimensional model

Peng et al. [29] proposed simple formulae for finding critical buckling loads of scaffold

systems using a sway frame concept. The following equation could be used to calculate the

critical loads:

2

2

)( hK

EIPcr β

π= (3)

where

βαβα

π3

5

61+

=K (4)

and

t

approxE

hHI

∆=

3

)( 3β (5)

where crP = critical load of the scaffold; E = Young’s modulus of the scaffold; I = equivalent

moment of inertia of the equivalent column of the scaffold; K = effective length coefficient;

α = ratio of the height of shore extension to the height of the scaffold unit h ; β = the number

of storeys of the system; and t∆ = top horizontal-sway displacement under a unit horizontal

0

10

20

30

40

50

60

70

0 2 4 6 8 10 12 14 16 18 20

Number of stories

Co

mp

uta

tio

nal

cri

tica

l lo

ads

(kN

)



20

load H at the top from a linear analysis of in-plane, two-dimensional frame (Figure 18). Eq.

(4) allows for the effect of shoring. In the case of scaffolds with no shore extensions, K = 0.7

as α = 0. By substituting approxI from Eq. (5) into I of Eq. (3), crP was found to be close and

fairly conservative compared with the accurate three-dimensional non-linear analysis of the

model.

Figure 18: Model for approximating moment of inertia of scaffold

Based on the analysis results of the high clearance steel scaffold systems, Peng et al. [30] also

suggested that the critical loads of the scaffold systems could be quickly estimated by using a

“set concept.” The set concept utilised the relationship between the number of steel scaffold

sets and the critical load of the systems. For example, the one-bay-two-row-two-storey

scaffold would consist of four sets of scaffold unit, thus by multiplying the critical load of

one unit scaffold with the number of sets (four in this case), the critical load of the scaffold in

interest could be approximated. This method could be applied to scaffolds with shores to

estimate the critical load since the ratio of critical loads between the scaffolds with and

without shores was found to be constant for a given number of storeys. If the critical load of

scaffolds without shores is known, the critical load of scaffolds with shores by the same

number of bays, rows, and storeys can be computed by multiplying the former value by the

proposed ratio. These ratios were presented in Peng et al. research [30].

5. Design of Scaffold Systems

5.1 British Standards

BS 5975 [18] provides guidelines for the loads and load combinations to be applied in the

design of falsework. Recommended applied loads given in this code of practice consist of

self-weights, imposed loads, and environmental loads. The practical design of steel scaffold

systems follows the steel column buckling design method given in BS 5950 [31] to assess the

load carrying capacities based on modified slenderness ratios of the column members. The

summarized design procedure is as follows:



21

(a) First, the area and the second moment of area of the circular tube member are calculated

respectively as:

)(4

22

ie ddA −=π

(6)

and

)(64

44

ie ddI −=π

(7)

where ed = external diameter of the tube; and id = internal diameter of the tube.

(b) The slenderness ratio of the column member is then computed based on the ratio of the

effective height of the scaffolds, eh , and the radius of gyration, r , as given by:

r

he=λ (8)

where hkh ee ×= ; ek = effective length factor (discuss later on); h = height of the column

member between restraints; and AIr /= .

(c) The elastic buckling strength of the column member, Ep , is then computed by:

2

2

λπ E

pE = (9)

where E is the Young’s modulus.

(d) Finally, the compressive strength of the column member, cp , can be obtained as follows:

5.02 )( yE

yE

cpp

ppp

−+=

φφ (10)

in which:

2

)1( Ey pp ++=

ηφ (11)

where yp = yield strength of the steel tube; the Perry factor, η , is calculated as

1000)( 0λλ −a ; the Robertson constant, 5.5=a for cold-formed steel tubes; and the limiting

slenderness, 5.02

0 )/(2.0 ypEπλ = .

5.2 Australian Standards

AS 3610 [19] specifies the loads and load combinations to be applied in the design of

formwork assemblies, which can also be adopted for load calculations in support scaffolds

since this type of scaffolds is generally used to carry loads from concrete construction. The

loads are considered in three stages: before, during, and after concrete placement. These loads

consist of vertical loads such as dead load, concrete load, live load, and material loads, as

well as horizontal loads such as wind loads, and earthquake loads. AS 4100 [32] is commonly

applied to the structural design of steel scaffold systems. The simple design procedure for

load capacities of the standard is described as follows:



22

(a) The area and the second moment of area, A and I , are calculated as in Eq. (6) and (7)

respectively.

(b) The radius of gyration can be obtained from AIr /= , and the form factor ( fk ) is taken

as 1 when the slenderness, 82250

≤

= yoe

f

t

dλ for circular tubular members;

otherwiseg

ef

A

Ak = where od = outside diameter of the section, t = wall thickness of the

section, yf is the yield strength of the column tube, eA = effective area of the section

specified in Clause 6.2.4 of AS 4100 [32], and gA = gross area of the section. The

compression member constant, bα is taken as -0.5 for cold-formed steel tubes.

(c) The effective length, el , is computed as lke where ek = member effective length factor

determined from Clause 4.6.3 in AS 4100 [32], and l = actual length of the standard between

restraints.

(d) The modified compression member slenderness, nλ , is then computed by:

250

yen

f

r

l=λ (12)

(e) The compression member factor, aα , is defined as:

20503.15

)5.13(21002 +−

−=

nn

na λλ

λα (13)

(f) The elastic buckling load factor, λ , is given by:

ban ααλλ += (14)

(g) To account for member imperfection, the compression member imperfection factor, η , is

calculated as;

0)5.13(00326.0 ≥−= λη (15)

(h) The modified compression member factor, ξ , is defined as:

2

90

290

)(2

1)(

λ

λ ηξ

++= (16)

(i) The slenderness reduction factor, cα , is determined as:

−−=

2

9011

ξλξαc (17)



23

(j) The nominal member capacity, cN , is then computed by:

ycc fAN α= (18)

To obtain the design member capacity, the capacity reduction factor, ϕ = 0.9 is applied

to cN .

5.3 Effective Lengths

The design approaches described in sections 5.1 and 5.2 rely on the determination of the

column effective length. Since levels of end restraints of the standards in scaffold systems are

difficult to determine, researchers have proposed values of column effective length based on

buckling analysis of their models. Yu et al. [10] found that the effective length coefficients of

the door-type steel scaffolds up to three storeys could be conservatively assumed to be 1.6 for

any idealised boundary conditions. Also, they pointed out that cross-bracings effectively

reduced the effective lengths of scaffold columns. In a separate investigation into the

behaviour of door-type steel scaffolds, Yu [22] back-calculated the effective lengths from the

finite element results of the load carrying capacities based on various boundary conditions.

The effective length factors were found to be in the range of 1.06 to 1.40. In addition, Harung

et al. [24] proposed that an effective length of the steel scaffolds should be about 1.2 times

the height of each storey based on the measurement of the largest distance between closest

points of contra-flexure or zero bending moment on the buckled columns in the analysis

model.

5.4 Bracing Systems

Bracings are important in terms of increasing the stability and the load carrying capacity of

scaffold systems. Peng [26] studied two different types of bracings for two-layer shoring

system, as shown in Figure 19. The V-type bracing was found to be stiffer than the N-type

bracing. The load carrying capacity of the system with V-type bracing was twice as much as

that of the N-type bracing. The study showed that the diagonal braces offered a very efficient

sideway restraint to the system, as confirmed by very small lateral displacements compared to

the shoring system without bracing. Moreover, Peng et al. [30] investigated the effect of

bamboo cross-braces on the exterior in-plane surface of the high-clearance steel scaffold

system, and noticed that the critical load of the scaffold was improved by about 20%; on the

other hand, if the braces were fitted to the exterior out-of-plane surface, there was no

significant improvement. Further research on bracing configuration will be useful in

determining the optimum design of scaffold systems.



24

Figure 19: Two layer shoring system with V-type and N-type bracings

5.5 Safety in Construction of Scaffold Systems

A monitoring method for support scaffolds was proposed by Huang et al. [33] to prevent

collapse. In order to avoid the buckling failure of the standards, the procedure in construction

was to observe axial forces and lateral displacements. From the analyses and the site tests, the

critical locations in the scaffold systems to be monitored for axial forces were the standards

next to the outmost standards along the perimeter and any locations where heavy loads were

expected. As for lateral displacements, the top of the lowest storey should be observed. The

allowable lateral displacement suggested in the literature was 10 mm [33].

Strain gauges and linear variable differential transducers (LVDTs) were recommended for

monitoring axial forces and lateral displacements respectively [33]. These devices could be

connected to a computer to collect real-time data and send off warning signals when the

allowable limits for axial forces and lateral displacement were approached. Moreover, as

suggested by Yu and Chung [20], erection tolerances based on construction practice such as

out-of-plumb between any two storeys of the scaffolds and maximum out-of-straightness of

each beam or standard should be limited at 5 mm. Also, the overall out-of-plumb of scaffold

structure should be within a tolerance of 25 mm.

6. Conclusions

This review provides guidelines for modelling, analysis and design of scaffold systems based

on past research. In modelling, initial geometric imperfections that include sway of the frame

and out-of-straightness of the uprights need to be incorporated so that second-order effects

are considered in the non-linear analysis. The magnitudes of imperfections applied to the

model are usually taken from the available codes of practice or in some cases by scaling the

critical buckling mode to amplitude equal to the maximum tolerance. In many cases,

modelling of semi-rigid joints between ledger and standard based on initial rotational

stiffness from joint test is adequate; however, the top and bottom boundary conditions applied

for the model are significant in determining the ultimate load; therefore, careful calibration

has to be done.

The most common analyses used in practice are linear elastic buckling and geometric non-

linear. When linear elastic buckling analysis is applied, the member buckling load is used in

the determination of the moment amplification factor, and this factor is then applied to the

corresponding moment to be used in the design. If geometric non-linear analysis is used, the

internal axial forces and moments can be applied directly in the design. Besides, some of the



25

codes of practice allow the use of advanced analysis that takes into account of material

properties and geometric imperfections, provided that a structure has sufficient section

capacity.

To ensure safety during construction, support scaffolds should be monitored by their axial

forces and displacements of the standards especially during concrete placement, and

inspected if bracings are applied correctly and adequately. For access scaffolds, sufficient ties

to permanent structure must be provided to prevent excessive lateral movement.

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26

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review of past research on scaffold systems

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