Design methods of integral-lift tubular steel scaffolds ...naoce.sjtu.edu.cn/upload/1543935727253201.pdf · In this paper, we discuss the design and calculation methods of integral-lift

Design methods of integral-lift tubular steel scaffolds for high-rise building construction

F. Yue1*,†,‡, G. Q. Li2 and Y. Yuan3

1 Department of Civil Engineering, Zhejiang A&F University, Lin’an 311300, China2 Department of Building Engineering, Tongji University, Shanghai 200092, China

3 Department of Geotechnical Engineering, Tongji University, Shanghai 200092, China

SUMMARY

In this paper, we discuss the design and calculation methods of integral-lift scaffolds under condition of operating, climbing up/down and falling. Dead loads, construction live loads and wind loads for integral-lift scaffolds are given in this paper. The key factors such as additional load factor κ are introduced and defi ned for recommendation as well. Load and resistance factor design method based on probability theory is introduced for the design of the main load-bearing structures. Herein, the additional partial safety factor of material strength γ ′m is introduced specifi cally for scaffolding frames which are often frequently reused under poor outdoor conditions. Some important checking methods proposed specifi cally for integral-lift scaffolds such as checking of anti-slide strength of couplers and anti-overturning analyses are also pre-sented. Meanwhile, allowable strength design method is suggested in the design of the lifting equipments, suspending rods and slings. The proposed methods in this paper will be useful guidelines for safe design of integral-lift scaffolds. Copyright © 2010 John Wiley & Sons, Ltd.

1. INTRODUCTION

Integral-lift scaffolds were developed based on the development of outrigger scaffold, swinging scaf-fold and suspending scaffold during the late 1980s and early 1990s in the 20th century, and several technical fi elds are involved, they are scaffolds, steel structures, mechanicals, electrics and automa-tions. So, scaffolding is a combination of comprehensive and complex construction equipment and a specifi c technique. The scaffolding frame is about only 10 m in height. It is installed around the exterior wall of a building when the top layer of the fi rst story is fi nished. The frame body is installed completely after a three- to four-storey structure construction. With the construction progress layer by layer, the frame can be lifted integrally up or down by its own lifting device. Compare with regular down-to-ground scaffoldings, the big advantage is that it uses fewer steel tubes and it is easier to use for high-rise buildings specifi cally for ultra-high-rise buildings. It meets the requirements of safe operation for workers and the needs of construction technology at specifi c stages, such as at the stage of construction (see Figure 1), installation and fi nishing (see Figure 2). Because of these advantages, the application of this kind of scaffold has become widespread since the beginning of the 1990s in China (Yue et al., 2005). It was classifi ed as one of the 10 most promoted new technologies by the Ministry of Construction of P. R. China in 1994.

There are a large number of unknown uncertainties during construction phase which is a short and important phase in the whole life cycle of building structures. The vast majority of accidents occur during the construction phase. The collapse of construction facilities for temporary supports such as scaffolding and formwork shoring system is the main reason why a lot of accidents happen during this phase.

As scaffold structures are prone to damage, most related articles at home and abroad, such as Harung et al. (1975), Peng et al. (1997, 2001), Chan et al. (1995), Xu et al. (1989) and Ao (2000),

* Correspondence to: Feng Yue, Department of Civil Engineering, Zhejiang A&F University, Lin’an 311300, China† E-mail: [email protected]‡ Present address: School of Naval Architecture, Ocean and Civil Engineering, Shanghai Jiao Tong University, Shanghai 200240, China

Copyright © 2010 John Wiley & Sons, Ltd.

THE STRUCTURAL DESIGN OF TALL AND SPECIAL BUILDINGSStruct. Design Tall Spec. Build. 21, 46–56 (2012)Published online 1 December 2010 in Wiley Online Library (wileyonlinelibrary.com/journal/tal). DOI: 10.1002/tal.635

DESIGN METHODS OF INTEGRAL-LIFT SCAFFOLDS 47

Copyright © 2010 John Wiley & Sons, Ltd. Struct. Design Tall Spec. Build. 21, 46–56 (2012) DOI: 10.1002/tal

are mainly focused on the ultimate bearing capacity problem from the regular down-to-ground scaf-folding system or load-bearing system. The safety design and research of integral-lift scaffolds are still lagging. We have almost not seen any comprehensive research papers overseas on integral-lift scaffolds. Related research in domestic universities and institutes is relatively limited as well. Cur-rently we do not have formal technical specifi cations for the design and construction of integral-lift scaffolds although a couple of national and local organizations have unveiled some regulations in China in recent years (The Ministry of Construction of P. R. China, 2000, 2001a; The Construction and Management Commission of the Shanghai Municipal Government, P. R. China, 1999; China State Technique Superision Breau, 2006). Due to the lack of reliable theoretical proof or experimental evidence, regulations for wind load calculation and design method etc. are mostly based on the speci-fi cations for normal constructions.

Integral-lift scaffold

Building in structure construction

Figure 1. Integral-lift scaffold climbing up in structure construction of Europe Square, Beijing, China.

Integral-lift scaffold

Building in finishingconstruction

Figure 2. Integral-lift scaffold climbing down in fi nishing construction of Lijing Building, Shanghai, China.

48 F. YUE, G. Q. LI AND Y. YUAN


Du (2000a, 2000b) has done a relatively comprehensive research focused on safety design of integral-lift scaffolds by proposing six key topics, he also compared and discussed the power factor γ1 and load coeffi cient of variation γ2 under lifting and falling conditions. Back in 2002, 2005 and 2007, Yue and Li (Yue, 2002; Yue et al., 2001; Li et al., 2004; Yue et al., 2005; Yue and Li, 2007), the authors of this paper, aimed at exploring the wind effects on integral-lift scaffolds attached to high-rise buildings during construction. Their studies are focused on shape coeffi cient of wind pres-sure and wind-induced vibration of the scaffolds. The shape coeffi cients of wind pressure on the typical scaffolds under various conditions are obtained through wind tunnel tests (Yue et al., 2001). They established a set of wind load calculation methods, by using random vibration theory, the dif-ferential equations of wind-induced vibration of the scaffolds were established and the wind vibration coeffi cients of the scaffolds were obtained through the solution of the equations (Li et al., 2004). Yue also did some research on the ultimate bearing capacity calculation methods (Yue and Li, 2007).

In this paper, based on the above research work in recent years, some design methods proposed for integral-lift scaffolds are comprehensively discussed and summarized. As a document covers almost every respect of the design and construction, it will be useful guidelines for safe design of integral-lift scaffolds.

2. BASIC DESIGN REQUIREMENTS

2.1. Components introduction

Integral-lift scaffold includes the main load-bearing structure, lifting equipment, suspending rod and sling.

The main load-bearing structure includes scaffolding frame, horizontal frame beam (also called as horizontal supporting structure), vertical supporting structure (see Figure 3), and the connection between the scaffold and the building structure (see Figure 4). The integral-lift scaffolds are set up by many similar scaffolding sets in length of about 6–8 m, which are connected at the joints of verti-cal supporting structures. In China, connected normally with couplers or cuploks (cup lock), steel tubes (φ48 × 3.5 mm) construct a set of scaffolding frame with two rows, about 4–6 columns and 6–8 layers. The horizontal frame beam, supported by neighboring vertical supporting structure at the bottom of the scaffolding frame, can be generally simplifi ed as a plane truss. The vertical supporting structure can be generally simplifi ed as a plane rigid frame with rigid joint.

Load and resistance factor design (LRFD) method based on probability theory is introduced for the design of the main load-bearing structures. Meanwhile, as for the scaffolding frame, it is also required

Building in construction

Safety net

Horizontal supporting structure

Vertical supporting structureScaffold frame

Suspending rod

Suspending rod

Figure 3. A set of scaffolds being tested.



to do safety checking, the safety factor should satisfy the following requirement: K1 ≥ 1.5 for strength calculation and K2 ≥ 2.0 for stability calculation.

The allowable strength design (ASD) method is suggested for the design and calculation of lifting equipments, suspending rod (see Figure 4) and sling, which belong to the discipline of mechanical engineering.

Elastic analysis should be used for integral-lift scaffolds.

2.2. Loads

2.2.1. Standard value of dead load Gk

The dead loads Gk include the weight of the scaffolding structure itself, the safety net and baffl er wrapped outside scaffold body, operating fl oor, the lifting equipments, which fi xed on the scaffolds and other equipments.

The values of dead loads Gk of standardized products are available in CSBSL (The Ministry of Construction of P. R. China, 2001b). For nonstandardized products, characteristic values of dead loads Gk may be obtained through representative spot test by calculating the sum of mean value plus two times of mean square deviation.

In addition, Gk of the 50-mm-thick wood or bamboo scaffold board is generally specifi ed as 0.35 kN/m2 in China considering the weight augments due to water absorption, lapping between neighboring pieces and conglutinated cement slurry during construction.

2.2.2. Standard value of construction live load Qk

Standard value Qk of construction live load includes the weight of workers, tools, facilities and tem-porary construction materials on the operating layer of horizontal projection area.

In condition of operating for integral-lift scaffolds, normally two-storey operation layers are used during the stage of building’s structural construction, so Qk is generally taken as 3 kN/m2 for each layer. In addition, normally three-storey operation layers are used during the stage of building’s fi n-ishing construction, in that case Qk is often taken as 2 kN/m2 for each layer.

In condition of climbing up/down or falling, due to the fact that nothing is allowed to be loaded on the operation layers for integral-lift scaffolds, Qk is generally taken as 0.5 kN/m2 for the operating layers no matter what the construction stage is.

2.2.3. Standard value of wind load Wk

Scaffolds may collapse occasionally by high wind, which not only leads to work delays and property damage but also causes numerous worker deaths or injuries. The wind load on scaffolds may be much

Connection betweenscaffold and building structure

Suspending rod

Figure 4. Connection between scaffold and building structure.



greater than dead load and it could be the dominant effect when the scaffolds climb over 150 m in height. In 2001 (Yue et al., 2001), 2004 (Li et al., 2004), 2005 (Yue et al., 2005) and 2007 (Yue and Li, 2007), Yue, Li and Yuan, the authors of this paper, came up with the following standard values of the wind force on integral-lift scaffolds.

W k Wk ss z zs= μ μ β 0 (1)

where Wk is the standard values of wind pressure; k is the wind pressure reduction factor during a fi ve-year return period (k should not be less than 0.619 according to Yue and Li, 2007); μss is the shape coeffi cient of wind pressure (μss of the typical scaffolds under various conditions are obtained through wind tunnel tests in form of diagram (Yue et al., 2005), and in table (Yue, 2002; Yue and Li, 2007); μz is the coeffi cient of wind pressure variation with height (The Ministry of Construction of P. R. China, 2001b); and βzs is the wind vibration coeffi cient of integral-lift scaffold (by using random vibration theory, two applicable equations of the wind vibration coeffi cients at the two stages of climbing up and going down are obtained (Li et al., 2004; Yue et al., 2005), and Yue (2002) cal-culated their values in form of tables; W0 is the reference wind pressure.

2.3. Three conditions for integral-lift scaffolds

When calculating each part of the integral-lift scaffolds, the most unfavorable condition should be decided among three conditions of operating, moving up/down and falling. The condition of operating refers to the phase when the scaffold is fi xed on the building structure and ready to use, the condition of moving up/down refers to the phase during which the scaffold is being lifted up (or climbing down) normally, and the condition of falling refers to the phase that the scaffold is dropping down freely because of an accident occurred during climbing up or down.

2.4. Additional load coeffi cients κ

Additional load coeffi cients κ, specifi cally proposed only for integral-lift scaffolds, may be expressed as eccentric load effect coeffi cient κe, load variation coeffi cient κj1, κj2 and load impact coeffi cient κc depending on different conditions and components for integral-lift scaffolds. Two kinds of recom-mended values for κ are listed in Tables 1(a) and 1(b), respectively (The Ministry of Construction of P. R. China, 2000; The Construction and Management Commission of the Shanghai Municipal Government, P. R. China, 1999).

Table 1(a). One of recommended values of additional load coeffi cients κ.

No. Items

Values of κ in each condition

Design methodOperating Moving Falling

1 Frame structure Scaffolding frame 1.0 1.0 – LRFD2 Horizontal supporting structure, node κj1 κj2 κj2

3 Vertical supporting structure κj1 κj2 κj2

4 Connection between scaffold and building structure κj1 κj2 κj2

5 Anti-overturning and anti-falling set κj1 κj2 κj2

6 Lifting equipment – κj2 – ASD7 Suspending rod and sling in operating condition κj1 – –8 Suspending rod and sling connected with lifting

equipment– κj2 –

9 Suspending rod and sling connected with anti-falling set

– – κj2

κj1 shall be 1.3, κj2 shall be 2.0. For single-set integral-lift scaffold, κj1 and κj2 shall be 1.0.



Load eccentricity factor κe is proposed specifi cally for poles of the scaffolding frame due to the existence of eccentric node of coupler (Figure 5) with which the horizontal and vertical tubular poles are connected.

Load variation coeffi cient κj1, κj2 shall be considered because of the loading redistribution between the connected neighboring sets of the integral-lift scaffolds.

Load impact coeffi cient κc shall be used in condition of falling due to the effect of inertia.

3. DESIGN METHOD FOR MAIN LOAD-BEARING STRUCTURES

As normal building structural members, checking of strength and stability of the basic members of integral-lift scaffold such as bending elements, axial compression members, axial tension members, compression fl exure members and tension fl exure members, shall still comply with current China

Table 1(b). The other recommended values of additional load coeffi cients κ.

No. Items

Values of κ in each condition

Design methodOperating Moving Falling

1 Frame structure Compression poles connected with coupler in scaffolding frame

κe κe – LRFD

2 Any other poles or nodes excepting item no. 1 in scaffolding frame

1.0 1.0 –

3 Horizontal supporting structure, node κj1 κj2 κc

4 Vertical supporting structure κj1 κj2 κc

5 Connection between scaffold and building structure κj1 κj2 κc

6 Anti-overturning and anti-falling set κj1 κj2 κc

7 Lifting equipment – κj2 – ASD 8 Suspending rod and sling in operating condition κj1 – – 9 Suspending rod and sling connected with lifting equipment – κj2 –10 Suspending rod and sling connected with anti-falling set – – κc

κe shall be 1.15; κj1 shall be 1.3, κj2 shall be 1.8. For single-set integral-lift scaffold, κj1 and κj2 shall be 1.0; κc, determined normally by experiment, shall be specifi ed as 1.2 times of the test value depending on the capability of anti-falling equipment, but not less than 1.8.

Figure 5. Right-angle coupler (mm). 1—screw nut; 2—washer; 3—cover plate; 4—bolt; 5—longitudinal horizontal pole; 6—vertical pole.



national code TCCTSS (The Ministry of Construction of P. R. China, 2002) or CDSS (The Ministry of Construction of P. R. China, 2003). Some useful checking guidelines for integral-lift scaffolds are as follows.

3.1. Load effect and combination equations

In case of ultimate limit states, the structural design of the main load-bearing structures in condition of operating, lifting and falling should be based on load effect combinations which include variable load effect and permanent load effect as showed in Equation (2) and (3). The most unfavorable result of the design values of internal force between the components from one of the above combinations will be taken.

If the combination is controlled by the construction live load effects, then

S S S SG Gk Q Q k W W Wk= +( ) +κ γ γ γ ψ1 (2)

If the combination is controlled by the wind load effects, then

S S S SG Gk W Wk Q Q Qk= + +κ γ γ κ γ ψ (3)

In case of serviceability limit states, the main load-bearing structure of integral-lift scaffolds should be designed based on standard combination of load effect, as depicted in Equ.4.

S S S SGk Q k ci Qik

i

n

= + +⎛⎝⎜

⎞⎠⎟=

∑κ ψ12

(4)

In Equations (2)–(4), κ = additional load factors; γG = partial safety factor for dead loads (its value is 1.35 when the combination is controlled by dead load effect, and its value is 1.2 while by variable load effects); γQi = partial safety factor for the ith variable load (its value is 1.4 for both construction live load and wind load, herein, γQ1 represents the partial safety factor for the controlling live load effect); SGk = the load effects value that is calculated in accordance with the dead load standard value Gk; SQik

= the load effects value that is calculated in accordance with the characteristic value of ith live load Qik, herein, the SQ1k represents the controlling one among all live load effects, SWk represents the characteristic wind load effect; n = number of live loads that are participated in load effect com-binations; ψci = oeffi cient of combined value of live loads. The value of ψci is 0.7 for construction live load and 0.6 for wind load.

3.2. Resistance expressions

In the structural resistance expression as follows (Equation (5)), the factor γ ′m shall be considered specifi cally for scaffolding frames, which are often frequently reused under poor outdoor conditions.

R Rf

a Rf

ak

R mk

mk=

′⎛⎝⎜

⎞⎠⎟

=′

⎛⎝⎜

⎞⎠⎟γ γ γ

, , , ,� � (5)

where fk = the standard value for the strength of the materials; f = the design value for the strength of the materials; γR = the partial factor for the strength of the materials; γ ′m = the additional partial safety factor of material strength, which is introduced specifi cally for scaffolding frames. γ ′m is 1.111 in strength calculation, and 1.481 in stability calculation; ak = the standard value of geometric param-eters for structural elements

However, for integral-lift scaffold, it is worth mentioning that the design value of materials strength f shall not be replaced by f/γ ′m when calculating the main structures except the scaffolding frame.



3.3. Checking of anti-slide strength of couplers

Anti-slide strength of couplers (see Figure 5) shall be checked as follows in Equation (8).

R Rc≤ (8)

where R = design values of vertical forces at the couplers to which the horizontal poles transmitted; Rc = design value of anti-slide strength of couplers. It shall meet the requirements of current China national code TCSSTSCC (The Ministry of Construction of P. R. China, 2001) as shown in Table 2.

3.4. Anti-overturning analysis

Except high wind discussed earlier, scaffolding overturning that occurs especially in condition of moving up/down is another root cause that may lead to serious scaffolding collapses.

Anti-overturning analysis is based on the comparison of overturning moments with anti-overturning moments, as shown in Equation (9).

M Mo r≤ (9)

where Mo = the design value of overturning moment around the lifting point caused by the vertical load and horizontal wind load; Mr = the design value of anti-overturning moments around the lifting point provided by special anti-overturning equipments or other restrictions (see Figure 6). For safety reasons it is recommended in this paper that the partial safety factors of anti-overturning resistance load shall be adopted by 0.80.

During the lifting process, the relative position of the gravity center of the frame body, the con-necting point of the anti-overturning equipments and the hanging points constantly change during the process of climbing up or down. So the most unfavorable position should be chosen for overturning resistance analysis. Generally speaking, the most unfavorable position is defi ned as follows: when the scaffold just fi nishes climbing up and the frame body has not been restored back to working condi-

Table 2. Design value of anti-slide strength of couplers (kN).

Items Design value of anti-slide strength Rc (kN)

Butt coupler 3.20Right-angle or rotary coupler 8.00

Guide rail (part of overturningresistance equipments)

Pulley (part of overturningresistance equipments)

Figure 6. Overturning resistance equipment.



tions, or when the scaffold just starts to climb down, and, the horizontal wind load should take the deviated direction of the outer face of the building structure. So, anti-overturning analysis must be performed in the condition of moving up/down. However in condition of operating, overturning resistance analysis is not necessary if a guaranteed reliable connection exist between the scaffold and the building structure.

4. STRENGTH CALCULATION OF SUSPENDING ROD AND SLING

Suspending rods (see Figure 4) are generally made of Q235 or Q345 steel bars, whereas suspending slings are generally made of steel wire ropes, the quality of which shall meet the requirements of current national code SWRGU (The Ministry of Machinery Industry of P. R. China, 2006) in China. The allowable stress method is suggested for design and calculation of lifting equipments, suspending rod and sling as shown below in Equation (10).

σ σ≤ [ ] (10)

where σ stands for the design value of stresses; [σ] stands for allowable material stresses, i.e. standard value of material strength divided by safety factor.

The standard value of load shall be used in Equation (10) and additional load factor κ depicted in Tables 1 and 2 shall also be considered. According to CTPIS (The Ministry of Construction of P. R. China, 2000) and STCIS (The Construction and Management Commission of the Shanghai Municipal Government, P. R. China, 1999), safety factor of suspending rod and sling should not be less than 6.0, and hand chain blocks of 5t should not be less than 4.0. The safety factors of machinery equip-ments should comply with current corresponding standards, however, if no provisions are available, it is suggested in the paper that the safety factor should not be less than 3.0 for lifting equipments and 2.0 for other mechanical facilities.

The design and calculation of the suspending rod and sling shall also take each of the conditions into consideration. It is worth mentioning that the computing model, the maximum load and additional load safety factors are different in each condition.

5. SUMMARY AND CONCLUSIONS

Some important basic design requirements and methods of design and calculation proposed specifi -cally for integral-lift tubular steel scaffolds are comprehensively discussed in this paper. The proposed methods will be useful guidelines for safe design of integral-lift scaffolds.

(1) Dead loads, construction live loads and wind loads for integral-lift scaffolds are given in this paper.

(2) Three different conditions shall be taken into consideration (condition of operating, moving up or down and falling) in the design and calculation of the main structures which are the lifting equipments, the suspending rods and slings.

(3) The additional load factor κ , listed for recommendation in the paper, is introduced as a key factor specifi cally for integral-lift scaffolds.

(4) This paper suggests LRFD method for the design of the main structures, and introduces in detail the combination equations of action-effects both in ultimate and serviceability limit states and resistance expressions in ultimate limit states for integral-lift scaffolds. Herein, the addi-tional partial safety factor of material strength γ ′m is introduced specifi cally for scaffolding frames, which are often frequently reused under poor outdoor conditions.

(5) The checking methods for anti-slide strength of couplers and anti-overturning of the major components are also discussed in this paper.

(6) The ASD is also suggested for design and calculation of lifting equipments, suspending rod and sling, which belong to the fi eld of mechanical engineering. The safety factors are also introduced in the paper.



ACKNOWLEDGMENTS

The authors would like to thank the fi nancial support from Shanghai Construction Technology Foun-dation. Specifi c thanks are given to Ke-Ming Ye, member of The Chinese Academy of Engineering, and chief engineer of Shanghai Construction Group Co., for providing valuable advice and expertise. The authors also extend thanks to Mr. Fei Zeng, Jie Liu, a manager and an engineer of Shanghai Lao’an Scaffolds Co. Ltd., respectively, for providing valuable resources, assistance and information about scaffolds, which are critical to the successful completion of the presented work.

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NOMENCLATURE

The following symbols are used in this paper:

ak—characteristic value of structural geometrical parameterC—allowable values that meet serviceability request of structures or structure members such as

allowable value of deformation, crack width, vibrating magnitude, acceleration or stress.fk—characteristic value of materials strength



f—design value of materials strengthk—wind pressure reduction factor during a fi ve-year return periodκ—additional load factorsκc—load impact coeffi cientκe—eccentric load effect coeffi cientκj1, κj2—Load variation coeffi cientMo—design value of overturning momentsMr—design value of anti-overturning momentsn—number of live loads that participated in load effect combinationsR—design values of vertical forces at the couplers which the horizontal poles transmitted toRc—design value of anti-slide strength of couplersS—design value of combination of action-effectsSGk—the load effects value that is calculated in accordance with the characteristic value of dead load

Gk

SQik—the load effects value that is calculated in accordance with the characteristic value of ith live load Qik

SQ1k—the controlling one among all live load effectsSWk—characteristic wind load effectWo—reference wind pressureWk—standard wind pressureμss—shape coeffi cient of wind pressureμz—coeffi cient of wind pressure variation with heightβzs—wind vibration coeffi cient of integral-lift scaffoldψci—coeffi cient of combination value of live loadsγG—partial safety factor for dead loadsγQi—partial safety factor for the ith live loadγQ1—partial safety factor for the controlling live load effectγ0—importance factor of structuresγR—partial safety factor of material strengthγ ′m—additional partial safety factor of material strengthσ—design values of stresses[σ]—allowable material stresses

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