A Case Study of Using Metal Scaffold System for Demountable Grandstand: The Opening Ceremony of Hong Kong 2009 East Asian Games

Information Paper

A Case Study of Using Metal Scaffold System for Demountable Grandstand: The Opening Ceremony of Hong Kong 2009 East Asian Games

STRUCTURAL ENGINEERING BRANCH ARCHITECTURAL SERVICES DEPARTMENT December 2011

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Table of Contents

1.

Introduction Introduction .................................................... ................................................................................................... ............................................... 1

2.

Structural Structural Behaviour Behaviour and Components Components .................................................. ....... 4

3.

Design Loading ........................................ .................................................... ...................................................... 23

4.

Dynamic Effects ............................................. ............................................... 28

5.

Foundation Foundation ............................................. ....................................................... 30

6.

Construction Construction Supervision Supervision ...................................................... .............................................................................. ........................ 30

7.

Case Study ............................................... ................................................. .... 31

8.

References References ............................................. ........................................................ 52

Annex A Sample Checking Certificate

Structural Structur al Engineering Branch, ArchSD Information Informatio n Paper on Demountable Grandstand Issue No./Revision No. :1/-

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1

Introduction

1.1

A grandstand is a structure which provides seating for spectators at entertainment or sporting events. Grandstands are typically classified into three distinct types: permanent, demountable and retractable. Structural Engineering Branch (SEB) has promulgated a set of guidelines in September 2011 - SEB Guidelines SEBGL OTH5: Guidelines on the Design for Floor Vibration Due to Human Actions Part   III: Vibration Effect to Grandstands, Sensitive Equipment and Facilities (available: http://asdiis/sebiis/2k/resource_centre/ ))  –  providing guidance the effect of human http://asdiis/sebiis/2k/resource_centre/  induced vibration on permanent grandstands. This paper will focus on the analysis, design and construction of demountable grandstands by sharing the experience on the demountable grandstands erected for the Opening Ceremony of Hong Kong 2009 East Asian Games held on 5 December 2009.

1.2

Demountable stands ( Photo 1(a)) are lightweight temporary structures whose trussed appearances are reminiscent of scaffolding systems. These stands are typically erected for a single specific event (e.g. parade, sports, and show) and therefore left in place for a short duration to house the large number of spectators. However, in some events (e.g. in the Opening Ceremony of Hong Kong 2009 East Asian Games), such demountable grandstands may have occupancies of up to thousands of people. Demountable grandstands were widely used in the Sydney 2000 and Beijing 2008 Olympics Games, and in the recent Auckland 2011 Ruby World Cup to increase the seating capacity of the competition venues. Photo 1(b) shows a large-scale example of demountable grandstands used in the softball centre of the Sydney 2000 Olympics Games, where 7,000 additional seats were provided by such demountable grandstands, and Photo 1(c) shows the scaffold system of  another large-scale example of demountable grandstands used in Eden Park  Stadium of the Auckland 2011 Ruby World Cup, where 10,000 additional seats were provided by such demountable grandstands.

Photo 1(a) Typical Demountable Grandstand (Bellinzona, Switzerland)

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Photo 1(b) Demountable Grandstand in Softball Softball Centre at the Sydney 2000 Olympics Games (Source: www.austseat.com.au/ )

Photo 1(c) Scaffold System System of the Demountable Demountable Grandstand in Eden Park Stadium at the Auckland 2011 Ruby World Club (Source: www.zimbio.com/pictures/-sUagbFggBA/  www.zimbio.com/pictures/-sUagbFggBA/ )) PH

1.3

Unlike permanent structures, demountable grandstands are usually designed to be repeatedly assembled and disassembled with the use of lightweight components such as slender steel tubes. The supporting structure and the member connections connections for such grandstands are also designed to make the assembly easily, rapidly and usually with the use of various types of proprietary scaffold system. Usually,

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demountable stands are proprietary products designed, supplied in modular units and installed by specialist contractor employed by the event organizers. Moreover, to ease installation, the supporting scaffold structures consist of slender tubular members with short spans between supports, rather than having larger steel sections with longer spans in permanent grandstands. Ellis and Ji (2000) further note that because of the short spans and slender tubular scaffolds, sway and front-to-back  vibration in horizontal direction are often the most important modes for demountable grandstands for human-induced dynamic crowd loads, while vertical modes are usually not a significant problem. 1.4

Because of the limited time for installation and the incentive to save cost, the structure of such demountable grandstand will just be able to achieve the minimum factor of safety. A number of accidents involving the collapse of such demountable grandstands have occurred overseas resulting in a number of casualties. Typical causes of these collapses are: overloading, lack of bracing, failure in support, problems with connections, and synchronized movements of audience (de Brito and Pimentel 2009). Two serious incidents of collapse of demountable stands occurred in the UK during 1993 and 1994. The UK Department of the Environment therefore appointed the Institution of Structural Engineers, who in collaboration with the Steel Construction Institute, published a guide for clients, contractors, engineers and suppliers of demountable structures. This guide guide has then been updated with latest technological and regulatory changes, and its latest version is published as Temporary Demountable Structures: Guidance on Procurement,  Design and Use (IStructE 2007).

1.5

The performance of demountable grandstand is the subject of this paper. First, structural forms and structural components of common types of grandstand are presented and discussed. Next, the design loading (including the dynamic loads) for the analysis and design of such structures will be detailed. Finally, the analysis, design, erection, and inspection process for the demountable grandstands erected for the Opening Ceremony of Hong Kong 2009 East Asian Games grandstands is presented and discussed.

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2.

Structural Behaviour and Components

2.1

Demountable grandstands can be assembled in a variety of shapes and sizes depending on the client requirement, nature of the event, weather, type of spectator and terrain. These structures ( Photo 2(a) and Photo 2(b)) typically have seats or benches arranged in tiered rows with access to the seats from aisles that run perpendicular to the rows of seating ( Figure 1). These structures will usually be dismantled once the event is completed. Most common demountable grandstands are one that has a scaffold structure with bracing to provide lateral stability to which a modular floor and seating system is fixed at the top.

Photo 2(a) Demountable Grandstand (Front Elevation) (Source: www.layher.com www.layher.com))

Photo 2(b) Demountable Grandstand in Australia (Rear Elevation) (Source: http://www.austseat.com.au/  http://www.austseat.com.au/ ))

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Figure 1 Typical Demountable Grandstand (Plan) (Source: Crick and Grondin 2008)

2.2

The structural behaviour of these structures is complex due to the presence of  countless components connected by clamps or simply inserted into each other. The structural system is such that spans are reduced due to a significant number of  vertical supports, and flexural stiffness in a vertical direction would benefit from that. On the other hand, along the line of seats (or sometimes called “sway” sway”) direction, stiffness is mainly due to bracing, whereas perpendicular to the line of  seats (or sometimes called “front to back ”) direction, apart from bracing, there is also the presence of frames to support the seats then contributing to stiffen the structure in this direction. It is thus expected that the flexibility of the structure in each direction varies significantly, with implications on its static as well as dynamic behaviour.

2.3

Components of Grandstand

2.3.1 Demountable grandstands are structures generally made of steel and consist of  members, connectors, and planks erected on site. The structural system is a modular three-dimensional frame, in which height and length of the structure are adjusted during design to accommodate a specified number of users. Such 3-D frame consists of proprietary scaffolds (instead of conventional structural steel sections as the supporting structure for demountable grandstands due to the relatively faster speed of assemble and lower material and erection costs of scaffold. Structurally, these scaffolds serves as “support scaffold” scaffold ” (instead of as “access scaffold” scaffold ”), and are required to carry heavy imposed load similar to falsework used in concreting. Structural Engineering Branch, ArchSD Information Informatio n Paper on Demountable Grandstand Issue No./Revision No. : 1/-

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However, those scaffolds used in falsework for concreting are sometimes manufactured as planar moment-resisting frame s (or called the “door -type” scaffold Figure 2(a)). For the proprietary scaffold used in demountable grandstands (Figure 2(b)), the joints are usually assumed to be pinned-connection, and its stability must rely on the brace members. Figure 2(c) shows the structural components of a typical scaffold used for demountable grandstands. Most proprietary demountable grandstands have similar components in their structures, and the various common types of connector and bracing members in the scaffold will be discussed in the following paragraphs.

Figure 2(a) Frame or Door-Type Scaffold

Figure 2(b) Proprietary Scaffold Demountable Grandstand

Figure 2(c) Structural Components of Typical Scaffold (Source: Rasmussen and Chandrangsu 2009)

2.3.2 Tubes To ease erection, all proprietary scaffolds are supplied as a modular system with tubes and connectors. Structural steel tubes are used to make up of the three elements of a modular unit: standard (the vertical element), ledger (the horizontal element), and brace (the diagonal element). The standards are connected to create a lift via connection tubes in sleeve joints ( Photo 3), and are connected to the ledgers Structural Engineering Branch, ArchSD Information Informatio n Paper on Demountable Grandstand Issue No./Revision No. : 1/-

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via connectors ( Figure 4). Figure 3 shows an example on the size of the connection tubes, sizes and dimensions of the pins for connecting the connection tube and standards as extracted from a supplier’s catalogue. catalogue. Table 1 shows typical sizes of these three elements in a modular unit (Crick (Crick and Grondin 2008). Crick  and Grondin (2008) note that the steel tubes are generally of Canadian Standard Grade 40.21 300W with minimum yield yield strength of 44ksi (300MPa). However, this paper has reservation on the applicability of such general statement, especially to those tubes used in Hong Kong. Hence, project officer should check the country of origin of the proprietary scaffold and refer to the catalogues for scaffold to determine the yield strength of the steel tubes.

Pin Hole

Photo 3 Connection tube between upper and lower standards

Connection Tube Pin Hole

Details of  Connection Tube

Figure 3 Connection Tube between Upper and Lower Standards (Source: http://www.scaffoldgold.com ) Structural Engineering Branch, ArchSD Information Informatio n Paper on Demountable Grandstand Issue No./Revision No. : 1/-

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Table 1 Typical Sizes of Tubes of a Modular Unit for Scaffold Elements Length Diameter (mm) Thickness (mm) Standard 0.5m, 1.0m, 1.5m, 2m or 3.2 49 3m Ledger US: 2.13m or 3.05m 3.2 49 Europe 2.25m or 1.35m Brace Length to suit standard 2.3 45 and ledger

2.3.3 Connector There are several systems of connector that can connect the tubes together. In this paragraph, three common systems, namely couplers, wedge-based connectors and spigot ( Figure 4), will be described.

Right-angle Swivel (a) Coupler/Clamp

(b)

Wedge-based connector

(c) Spigot Figure 4 Common systems systems of connector (Source: De Brito and Pimentel 2009, and Labour Department 2001)

2.3.1.1 Coupler/Clamp Standards are connected to ledgers via right angle couplers ( Photo 4(a)), and braces are connected to the scaffold via swivel couplers ( Photo 4(b)) to form tube and couple (or tube and clamp) scaffold.

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Photo 4(a) Right-Angle Couplers/Clamps (Source: www.tubeandclampscaffold.info )

Photo 4(b) Swivel Couplers/Clamps (Source: www.aptsuspensions.co.uk  www.aptsuspensions.co.uk ))

Tube and clamp scaffold is commonly used in construction. The ledgers and thus walking-decks can be placed at any height along the standard, and standards can be spaced at any distance apart up to the maximum distance allowed by engineering constraints. Tube and clamp scaffold is also the simplest and versatile system; but is among the most labour-intensive of all scaffolding applications, and is therefore generally used only when high capacity, unlimited adaptability and versatility are required. 2.3.1.2 Wedge-based connector Kwikform (or KwikStage) scaffold ( Photo 5(a)) and allround scaffold are using wedge-based connector. A distinct feature for such connection system is that the wedge pin can provide some moment carrying capacity, and hence, unlike the other systems, braces are sometimes not provided for such scaffold system in light loading (e.g. access scaffold). Further discussion on the effectiveness of such connection in carrying moment will be given in Section 2.5. Kwikform scaffold has metal loops attached to the standard at fixed intervals. The ledger has a hooked head that fits into the loop and a wedge pin to tighten the connection (Photo 5(b)). A hammer blow is used to drive the wedge pin between the ledger head and the loop creating a secure connection. The wedge fixing of the ledgers gives a simple and fast means of erecting access scaffolding without loose parts, its rigid 4-way fixing giving a positive location without movement, and wedge fitting on the standard giving guaranteed vertical alignment. To install braces, steel tubes ( Photo 5(c), 5(d) and 5(e)) with pivoted wedge devices at each end fitting onto the standards.

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(a)

(b)

(c)

(d) (e) Photo 5 Kwikform System (Source: www.scaffoldingcn.com and www.rmdkwikform.com )

Allround scaffold ( Photo 6(a)) has a rosette (i.e. circular plate with slots) attached at fixed intervals along the length of the standard. The ledger has a ledger head at each end that has a horizontal slot that mate with a wedge pin drops down into the slot on the rosette. The rosettes have 8 slots that allow up to eight members at one connection. To make a connection, the wedge head is slid over the perforated rosette. A harmer blow is then used to force the wedge pin into the slot securing the ledger to standard. Typical connection procedure of the system is shown in Photo 6(b).

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Photo 6(a) Allround System (Source: www.layher.com www.layher.com))

Photo 6(b) Connection Procedure of the Allround Scaffold Scaffold (Source: www.layher.com www.layher.com))

2.3.1.3 Spigot Cuplock (or cuplok) scaffold ( Photo 7(a)) uses spigot to connect ledger and standard together. Spigot is a cuplike element fixed to the standard at set intervals along its length. It allows four members to be connected at one place.

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Photo 7(a) Cuplock Scaffold (Source: www.scaffoldgold.com and www.indiamart.com www.indiamart.com))

To make a connection, the ledger end is placed into the bottom cup and the top cup is screwed down on to the top of the ledger end locking it into place. A hammer blow is used on the top cup to tighten the connection. Thus the top and bottom of the ledger head is secured against the standard. Typical connection procedure of the system is shown in Photo 7(b). Holes are intentionally left in the upper and lower standards so that pins can be inserted so that the standard is of the correct plumb and to transmit tensile force along the standard.

Photo 7(b) Connection Procedure of Cuplock Scaffold (Source: www.scaffoldingasia.com )

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Photo 7(c) Special Braces for Cuplock Scaffold (Source: www.scaffoldgold.com and http://scaffoldsales.com )

A distinct feature for such connection system (like wedge-based connector) is that it can provide some moment carrying capacity, and hence braces are sometimes not provided for such scaffold system (especially as access scaffold). Again, further discussion on the effectiveness of such connection will be given in Section 2.5. To install braces, steel tubes ( Photo 7(c)) with couplers at each end for fitting onto the standards. 2.3.1.4 For demountable temporary temporar y grandstands, modular mod ular system scaffold scaff old such as the th e cuplock, Kwikform and allround scaffolds are used most frequently. Scaffold in the form of tube and clamp scaffold are considered too slow in construction and labour intensive. 2.4

Bracing

2.4.1 One of the main reasons r easons for the collapse col lapse of demountable demountabl e grandstands is an insufficient number of bracing members provided (Bolton 1992; Ji and Ellis 1997). Demountable grandstands must therefore be provided with sufficient bracing members to resist horizontal loads and wind loads. It is essential that diagonal bracing be installed at all times. Free-standing individual support towers, and the start and end bays must have diagonal bracing installed. Moreover, the bracing elements also have effects on the permissible loadings on the standards. The following figure shows 5 arrangements of bracing element in a scaffold system, nd rd th namely (0) every bay, (A) every 2 bay, (B) every 3 bay, (C) every 4 bay, and th (D) every 5 bay, in the descending order in their permissible loadings:

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(Source: www.layher.com www.layher.com))

2.4.2 Theoretical study Extensive study of the effect of the bracing on such lightweight scaffold system has been carried out by Ji and Ellis (1997). The governing principles in providing bracing are: 1) the load shall take the shortest path to the supports (the “direct force path  principle”); and 2) the internal forces shall be uniformly distribu ted (the “uniform force distribution principle”). Based on these two principles, they listed out the following five criteria for arranging bracings in an efficient way in order to achieve a larger lateral stiffness: (a) (b) (c) (d) (e)

Bracing members in different storeys should be provided from the top to the support of the structure. Bracing members in different storeys should be directly linked where possible. Bracing members should be linked in a straight line where possible. Bracing members at the top adjacent bays should be directly linked where possible. If extra bracing members are required, they should be used following the above four criteria.

2.4.3 Table 2 shows six examples of typical bracing arrangement with descriptions on which criteria as listed above can be fulfilled. Those systems fulfilling all the above criteria would perform a higher static stiffness when subjected to horizontal loading.

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Table 2 Typical Bracing Arrangement for Scaffold Type

Bracing Arrangement

1

2

3

4

Descriptions

-

Satisfy criteria (a) Traditional bracing form Load transfer from top through all members

-

Satisfy criteria (a) and (b) Shorter load path than type 1, higher static stiffness

-

Satisfy criteria (a), (b) and (c) More straightforward force path Higher stiffness than type 1 & 2

-

Satisfy criteria (a), (b), (c) and (d) Highest stiffness amongst the first 4 types

-

More bracing members used, but not fully follows the criteria Lower stiffness than type 4

5 -

6

-

Satisfy all 5 criteria More uniform inner force distribution The highest stiffness among all the above types

(Source: Ji and Ellis 1997)

2.4.3 Among the six types of bracing arrangement, Type 6, which consists of a pair of  straight cross-bracing from the top to bottom, is the most effective bracing system. Type 6 in Table 2 only shows the ideal arrangement for a scaffold of two storeys height. Figure 5(a) shows how to modify Type 6 arrangement for scaffold with more than two storeys. Such bracing system can satisfy the first three criteria, and has small number of bracing members. Ji (2003) carried out tests on three models (frames A, B and C) of bracing system ( Figure 5(b)(i)), which were made up of  aluminium members same cross-section of 25 mm by 3 mm with an overall dimension of 1.025 m×1.025m. The frames were fixed at their supports and a hydraulic jack was used to apply a horizontal force at the top right-hand joint of the frame. At a load of 1.07kN, the horizontal displacement were respectively 3.0mm for frame A, 0.73mm for frame B and 2.2mm for frame C ( Figure 5(b)(ii)). They Structural Engineering Branch, ArchSD Information Informatio n Paper on Demountable Grandstand Issue No./Revision No. : 1/-

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therefore verified that Type 6 arrangement, which satisfies all five criteria, is the stiffest.

Figure 5(a) Type 6 Bracing for Multi-Storey Scaffold

Figure 5(b)(i) Models of Bracing Systems (frames A, B and C being placed from the left to right) (Source: Ji 2003)

Figure 5(b)(ii) Deflection Curves of Frames A, B and C (Source: Ji 2003)

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2.4.4 Similar remarks have been made earlier in Grant (1975). Grant (1975) notes that Type 6 bracing arrangement can result in the vertical loads in the standards to resist the couple created by the horizontal loads to vary proportionately with their distance from the centre line ( Figure 5(c)), whilst Type 1 or 2 bracing arrangement will create large vertical forces in the two legs adjacent to the standards in the bracing bay ( Figure 5(d)). Grant (1975) further notes that Type 6 bracing arrangement can effectively redistribute a concentrated vertical load onto the other standards ( Figure 5(e)).

Figure 5(c) Induced vertical loads on standards standards using Type 6 bracing (Source: Grant 1975)

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Figure 5(d) Induced vertical loads on standards using Type 2 bracing (Source: Grant 1975)

Figure 5(e) Redistribution of concentrated concentrated vertical load using using Type 6 bracing (Source: Grant 1975)

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2.4.5 Type 6 bracing arrangement, however, has the following disadvantages: a) there is no the bracing to the scaffold until its installation is completed, and hence cannot provide the required stability during erection and dismantle works; b) the erection of such bracing is much more difficult than the other types, as it is difficult to align the bracing straight, especially with in the jungle of scaffolds underneath the seating deck, and Bolton (1997) also raised concerns on the potential instability of the scaffold system during dismantle when such bracing has been removed; c) under the effect of lateral force, the bracing will induce a large tensile force onto the edge scaffold, which may not be of adequate strength, and anchor or kentledge may be required at the foundation level to counteract the tensile force; and d) such bracing may be required to tie to the scaffold at intermittent levels ( Figure 5) by swivel couplers, producing a moment on the thin tubes of the scaffold. Grant (1975) further comments that in the case that it is not possible to tie the bracing member to a standard at a node, a ledger is preferred to a standard for such coupling, as the former is not already heavily loaded; Hence, in reality, such bracing system will not be adopted by most specialist contractors, although theoretically such bracing system provides the highest stiffness. Instead, the common arrangement of bracing system adopted is Type I (Photo 1(c) and Photo 8).

Photo 8 Typical Bracing System (Source: http://www.austseat.com.au http://www.austseat.com.au))

2.4.6 However, even though Type 1 bracing system is usually adopted as bracing system in such modular scaffold, this paper still recommends that global Type 6 crossbracing should be provided around the scaffold system in addition to Type 1 so as to increase the overall stiffness of the scaffold, especially when the demountable grandstand is tall. The number of Type 1 bracing may also be reduced. A suggested arrangement is shown in Figure 6. With such global global bracing, there is the potential for eight braces to interest at one connection, exceeding the maximum of  Structural Engineering Branch, ArchSD Information Informatio n Paper on Demountable Grandstand Issue No./Revision No. : 1/-

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4 braces even for allround scaffold system. Hence, it is necessary to attach the braces to the standard or ledger using swivel couplers.

Figure 6 Preferred Bracing Arrangement at End Bays 2.5

Moment Carrying Capacity of Joint

2.5.1 It is generally assumed in the analysis and design that the joints in these scaffolds are modelled to be pinned connection (i.e. with no moment carrying capacity). In reality, they have some degree of fixity, especially the cuplike element in cuplock  scaffold and wedge-based connector in Figure 7(a). The cuplock 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 (Godley and Beale 1997). Wedge-type joints are generally more flexible and closer to pinned connections. They also often display substantial looseness at small rotations (Godley and Beale 2001). As to spigot joints in the cuplock system, the spigot can create out-of-straightness of the standards, and the possibility of the joint to open up due to the gap between the standard and the spigot can produce complexity in modelling (Enright et al 2000). Figure 7(b) shows typical moment –  moment – rotation rotation curves for cuplock and wedge-type joints. It should further be noted that the relationship for all types of joint is not generally the same for positive and negative rotations (Godley and Beale 2001), and that the curves do not show linear relationship. 2.5.2 Although there is moment carrying capacity of the connections (and indeed, the catalogues of many proprietary scaffold systems also provide their recommended moment carrying capacity), project officer should note that the uncertainty and limitations of such connections in carrying moment as discussed in the above paragraph. This paper still maintains that in the design of demountable grandstand, the joints should be modelled as pinned connection, and bracing members are required to provide lateral stability for the scaffold. The moment carrying capacity of the connections only serves as additional safety margin, especially during the

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erection and dismantle processes, where the bracing members have not yet been installed or has been dismantled.

(i) Wedge-based connector (ii) Cuplock connector Figure 7(a) Moment carrying capacity in wedge-based and cuplock connector (Source: Godley and Beale 1997, 2001 )

Figure 7(b) Typical moment against rotation graph of joint joint in proprietary scaffold (Source: Chandrangsu and Rasmussen 2006 ) 2.6

Elastic Critical Load Factor λ cr cr

2.6.1  Eurocode 3 defines the the elastic critical load factor λ cr cr as the value of the load factor by which the loads are to be multiplied to check of buildings for “sway mode” failures. λ cr cr is therefore an important parameter to classify the scaffold frame into non-sway, sway or ultra-sway sensitive frame. For both sway and ultra-sway ultra-swa y sensitive frames, the load-carrying capacity of the steel tubes in the scaffold system decreases with the height of the scaffold, as the effective length of tubes increases Structural Engineering Branch, ArchSD Information Informatio n Paper on Demountable Grandstand Issue No./Revision No. : 1/-

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due to the P- P-- effects. The Code of Practice for the Structural Use of Steel 2005 (the “  HK Steel Code”) issued by Buildings Department (as modified by SEI  08/2009: Design 08/2009:  Design Code for Structural Steel ) classifies frame using λ cr cr as follows: a) when λ cr cr 10, the frame is non-sway and the P- - effects can be ignored; b) when 5≤ λ cr P- -δ effects can be included by cr