Follow-up Study on Safety and Reliability of Truss-out

Follow-up Study on Safety and Reliability of Truss-out

Metal Bracket Bamboo Scaffoldings

Commissioned to

Department of Civil Engineering

The Hong Kong University of Science & Technology

by Occupational Safety & Health Council

June 2006

TABLE OF CONTENTS

1. INTRODUCTION ........................................................................................ 1

1.1. Three Types of Metal Brackets .............................................................................. 2 1.2. Research Plan and Methodology............................................................................ 5

2. STRENGTHS OF EXPANSION BOLTS................................................... 7

2.1. Pull-out Strength .................................................................................................... 7 2.2. Shear Strength ........................................................................................................ 8

3. TESTS PREPARATION............................................................................ 11

3.1. Metal Brackets...................................................................................................... 11 3.2. Concrete Panels .................................................................................................... 18 3.3. Loading Platform.................................................................................................. 20

4. LOAD-CARRYING CAPACITY OF METAL BRACKETS................. 22

4.1. Experimental Tests ............................................................................................... 22 4.1.1. Metal Brackets from Company A............................................................. 25 4.1.2. Metal Brackets from Company B............................................................. 31 4.1.3. Metal Brackets from Company C............................................................. 36 4.1.4. Remarks.................................................................................................... 41

4.2. Numerical Analysis .............................................................................................. 42 4.2.1. Estimation of Anchor Strength ................................................................. 42 4.2.2. Strength under Combined Loading .......................................................... 42 4.2.3. Finite Element Modeling.......................................................................... 44 4.2.4. Computed Load-carrying Capacity .......................................................... 45

4.3. Comparison between Experimental and Computed Results ................................ 52

5. CONCLUSIONS AND RECOMMENDATIONS ................................... 54

REFERENCES .................................................................................................. 57

APPENDICES ................................................................................................... 58

Follow-up Study on Safety and Reliability of Truss-out Metal Bracket Bamboo Scaffoldings

Occupational Safety and Health Council 1

1. INTRODUCTION Truss-out metal bracket bamboo scaffoldings have been widely used in Hong Kong for maintenance and retrofit of aged structures (Figure 1.1) due to its low cost, efficiency and flexibility. These bamboo scaffoldings are supported by metal brackets which are triangular in shape as shown in Figure 1.2. A detailed study on the safety and reliability of these truss-out metal bracket bamboo scaffoldings was performed between 1st July 2004 and 30th June 2005 [1]. Results show that the load-carrying capacity of this type of scaffolding is directly affected by the strength and the number of expansion bolts used to anchor the metal brackets. In that report, factors affecting the strength of expansion bolts, such as the drill-hole size, the anchoring tightness, the embedment depth, the anchoring locations, and the base materials, have been quantitatively studied. A typical metal bracket as shown in Figure 1.2 requires three expansion bolts to be firmly attached to structural surface. However, it has been reported from the industry that the lowest expansion bolt is quite difficult to anchor as the location is quite far to reach for the worker. Figure 1.3 shows a possible mounting arrangement for metal brackets. It is seen that a typical distance between the edge of the window and the location of the lowest anchor is more than 75 cm. This distance certainly presents a safety concern to the workers when they attempt to anchor the expansion bolt from the window. There is a need to investigate whether there are any alternative forms for the metal bracket which can be easily installed yet provide a comparable level of load-carrying capacity as the triangular-shaped one. In this report, the load-carrying capacity of three different types of metal brackets is studied. Laboratory tests are first conducted in the Structural Lab of the Hong Kong University of Science and Technology. The test results are then verified by numerical analysis using finite element program.

Figure 1.1 Truss-out metal bracket bamboo scaffoldings for maintenance/retrofit of buildings



Figure 1.2 Metal bracket currently used for truss-out bamboo scaffolding

External wall

20cm

75cm

Metal bracket

Expansion bolt

External wall

20cm

75cm

Metal bracket

Expansion bolt

Figure 1.3 Mounting of metal brackets

1.1. Three Types of Metal Brackets Three different types of metal brackets are studied in this report. They are name as Type One Cross bracket; Type Two I bracket; and Type Three T bracket (Figure 1.4). Note that the Type



Two I bracket is the type of bracket currently used by the industry. This bracket is constructed using three L-shape steel beam as shown in Figure 1.5. The horizontal component and the cross component are welded to the vertical component which is then fixed to the structural surface through three expansion bolts. Type One Cross bracket is similar to Type Two I bracket except that a short flat steel beam is welded to the back of the vertical component (Figure 1.6). There are four holes for anchoring the bracket, two on the vertical component and two on the short flat beam. Type Three T bracket also consists of four components. The horizontal component, the vertical component and a short L-shape steel beam are welded together at the top of the bracket (Figure 1.7). There are two holes on the short L beam and one hole on the vertical component for anchoring the bracket. Three sets of brackets supplied from three anonymous companies are used in this study. Detailed dimensions of these metal brackets are shown in Appendix A. Note that the all the metal brackets tested in this study are all made up of L-shape steel beams except the short horizontal flat steel beam used in the Type One Cross bracket. Although all components, including the L-shape beams and the flat beam come with a same thickness of 5.5 mm, the L-shape beams provide a stiffer and stronger load-deformation resistance than the short flat beam.

Figure 1.4 Three types of metal brackets (right: Type One Cross bracket; middle: Type Two I bracket; and left: Type Three T bracket)



Figure 1.5 Type Two I bracket

Figure 1.6 Type One Cross bracket



Figure 1.7 Type Three T bracket

1.2. Research Plan and Methodology There are two main objectives for this follow-up study: (i) to investigate whether the alternative types of brackets, Type One Cross bracket and Type Three T bracket, can have a comparable load-carrying capacity as the original bracket, Type Two I bracket; and (ii) to investigate whether two expansion bolts are sufficient to firmly fix metal brackets to structural surface. The study is conducted using combination of laboratory tests and computer analysis. Steps adopted to realize the objectives are briefly outlined in the following. 1. Strengths of expansion bolts

Metal brackets are anchored on structural surface through expansion bolts. The strengths of expansion bolts directly influence the load-carrying capacity of the metal bracket. In the previous study, factors affecting the strengths of expansion bolts have been systematically investigated [1]. The results obtained from that report is summarized briefly in this report for further processing.

2. Test preparation The brackets are to be mounted on concrete panels. These concrete panels are designed base on the code of practice BSI CP110 [2] and CP114 [3]. Also a loading platform is designed to fix the concrete panels for the test. To analysis the load-carrying capacity of metal brackets using finite element program, it is necessary to obtain the material properties of the brackets. These material properties are obtained through standard tensile test following ASTM E8-97 [4].



3. Load-carrying capacity of metal brackets The load-carrying capacity of metal brackets is studied both experimentally and numerically. Three types of metal brackets are mounted on concrete panels with different numbers of expansion bolts and tested for their load-carrying capacity. To simplify the study, the Fook Shing expansion bolt is used for the test. Numerical analyses using the finite element program SAP2000 are then performed to obtain the load-carrying capacity numerically. These two sets of results are then compared and discussed.



2. STRENGTHS OF EXPANSION BOLTS The strength of expansion bolts directly affects the load-carrying capacity of metal bracket. In the previous report [1], the strength of expansion bolts has been discussed from both the theoretical and the experimental aspect. In the following, some important findings from that report are summarized for this study. 2.1. Pull-out Strength Eligehausen et al. [5] tested the concrete cone capacity of headed anchor bolts subjected to axial tension load using 35 concrete specimens of three different sizes. An empirical equation for estimating the failure load of anchor bolts embedded in concrete was hence proposed:

6.1ef

5.0ccu hf10F = , (1)

where uF is the failure load; ccf and h are the concrete strength and the embedment depth

of the anchor bolt, respectively. They conclude that the concrete cone failure load shows a significant size effect, which is rather close to the solution according to linear fracture mechanics. Another empirical equation was proposed in [6]:

5.1ef

5.0ccu hf5.15F = . (2)

This equation was based on about 200 test results with headed anchors from different sources. It assumed the greatest possible size effect that could be obtained when applying linear fracture mechanics. The empirical formula adopted by the American Concrete Institute (ACI 349-85) [7] for the design of anchor fastenings is

)hd

1(hf96.0Fef

h25.0ccu += , (3)

where hd is the diameter of the anchor bolt. This formula however does not take into account the size effect. An anchor bolt manufacturer also proposed a formula for estimating the pull-out strength of expansion anchors according to the Guideline for European Technical Approval for metal anchors for use in concrete [8]. The formula for the anchor bolt under concrete cone failure [9]

.red/.sta,RN.red/sta,ANBNT.red/.sta,c,Rdu ffffFF = (4)

where .red/.sta,c,RdF is the design concrete cone resistance, Tf is the influence of the



anchorage depth, BNf is set to be 1.0 for this type of anchor bolt, .red/sta,ANf is the influence

of anchor spacing on standard (or reduced) anchorage depth and .red/.sta,RNf is the influence

of edge distance on standard (or reduced) anchorage depth. A safety factor of 1.2 has been incorporated into the formula to estimate the failure load of the anchor bolt. In the previous report [1], pull-out strength of expansion bolts has been obtained and validated with the formulas suggested in references [5-7] and [9] as shown in Figure 2.1. It is seen that the pull-out strengths estimated by Eqs. 3 and 4 provide a low bound when compared to the test results. The pull-out strength of the Fook Shing expansion bolts anchored in concrete panel with a mean compressive strength of 22 MPa is listed in Table 2.1. The mean of pull-strength is found to be around 10.6 kN with a standard deviation of 0.7 kN.

Figure 2.1 Pull-out strength of Fook Shing expansion bolts as compared to formulas [1]

Table 2.1 Pull-out strength of the Fook Shing expansion bolts (in kN) [1]

Base material Range Mean Std Concrete (22MPa) 9.5 - 11.8 10.6 0.7

2.2. Shear Strength Miltenberger [10] combined the concrete capacity design (CCD) model [11] and the uniform bond stress model [12] into a rational procedure for a grouted fastener strength design. The



formulas for the shear capacity of steel anchor bolts embedded in concrete under different shear failure considerations are summarized in the following. (i) Strength under steel shear failure:

euts Af6.0V = , (5)

where Vs= the shear strength of the anchor bolt in N, fut= the steel fracture strength not exceeding 1.9fy (steel yielding strength in MPa) or 862 MPa, Ae= the effective cross-sectional area of the bolt in mm2. (ii) Strength under pryout failure:

mm280h)hf5.12(kV ef5.1

ef'cNcrNeNgcpcp <ψψψ= (6a)

mm635h280)hf75.4(kV ef67.1

ef'cNcrNeNgcpcp ≤≤ψψψ= , (6b)

where Vcp= nominal concrete pryout strength in N; kcp= 1 for hef < 65 mm or kcp = 2 for hef

> 65 mm; Ngψ ≤ 1 is a group effect factor; Neψ is an edge effect factor; Ncrψ is a crack

effect factor; and 'cf is the specified concrete compressive strength in MPa.

(iii) Strength under breakout failure:

5.11

'c

2.0ef

VcrVeVgc,n cfdd

h84.0V ⎟⎠⎞

⎜⎝⎛ψψψ= . (7)

where Vn,c= the nominal concrete breakout strength in N; Vgψ ≤ 1 is a group effect factor;

Veψ is an edge effect factor; Vcrψ is a crack effect factor; hef= effective embedment depth of the anchor bolt in mm; d= diameter of the anchor bolt in mm; c1= front edge distance in the direction of the shear force in mm. Note that if an anchorage is located at a corner, the minimum value of Vn,c evaluated for both directions should be used. Also, if the shear force applied to a group of fasteners is oriented parallel to an edge, the value of Vn,c calculated with

Veψ = 1 can be multiplied by a factor of 2. Based on the above three formulas, the design strength for the shear loaded anchor bolt can be obtained from the minimum of the three values:

)V;V;Vmin(V cpc,nsn φφφ=φ . (8)

where φ is the strength reduction factor. The design procedure seems quite suitable for estimating the shear capacity of the anchor bolt embedded in concrete.



In the previous report [1], shear strength of expansion bolts anchored in concrete panel with a mean compressive strength of 22 MPa has been obtained. The mean value of shear strength is found to be about 17.5 kN with a standard deviation of 1.3 kN.

Table 2.2 Shear strength of the Fook Shing expansion bolts (in kN) [1] Base material Range Mean Std

Concrete (22 MPa) 16.0 - 18.9 17.5 1.3



3. TESTS PREPARATION 3.1. Metal Brackets Three sets of brackets supplied from three anonymous companies are used in this study. The three companies are named as A, B and C for referencing. Detailed dimensions of these metal brackets are shown in Appendix A. To obtain the material properties of these brackets, standard tensile tests outlined in ASTM E8-97 [4] are used. Each metal bracket consists of three different components: horizontal (H) component, vertical (V) component and cross (C) component (Figure 3.1). The dimensions of the test specimens are shown in Appendix B. Figure 3.2 shows the specimens and their top and side view.

Figure 3.1 Components of the metal brackets

C component

H component

V component

Welded joint

Welded joint

Welded joint



(a) Specimens from the metal brackets

(b) Top view of a specimen

(c) Side view of a specimen

Figure 3.2 Specimens for tensile test



The tensile test was carried out in the Structural Engineering Laboratory of HKUST using the tensile testing machine, MTS 810 Universal Testing Machine with hydraulic wedge grip (Figure 3.3). The specifications of the MTS 810 are:

Load range from 0 kN to 500 kN Test fixtures: tension, isosipescu shear, boeing

compression after impact, three point bending and four point bending.

Extensometer: 50 mm gage length Rate of loading: 0.15kN/sec (before yielding)

0.3kN/sec (after yielding) Gauge length: 50 mm Air conditioned at 25oC MTS 810 with hydraulic wedge grip

Figure 3.3 Setup for the tensile test The test results for Type One, Type Two and Type Three brackets are shown in Tables 3.1, 3.2 and 3.3, respectively. It is seen that the Young’s modulus of the metal brackets is on the order of 200 GPa and the ultimate stress is on the order of 450 to 500 MPa. The obtained properties are similar to that of low-carbon steels, such as AISI 1020, which are tough and ductile, easily machined, formed, and welded. The material is suitable for use as metal brackets to support a working platform.

Table 3.1(a) Properties of the Type One metal bracket from company A

Specimen Young’s Modulus (GPa)Ultimate Stress

(MPa) Yield Stress

(MPa) H component

A1-H1 210 468 324 A1-H3 216 459 335

C component A1-C5 218 499 355

V component A1-V1 208 500 357 A1-V2 214 502 372

Average 211 486 349 Std. Dev. 2% 4% 5%



Table 3.1(b) Properties of the Type One metal bracket from company B


(MPa) Yield Stress

(MPa) H component

B1-H1 216 459 337 B1-H3 207 452 335

C component B1-C1 214 460 344 B1-C4 213 457 325

V component B1-V1 207 452 326

Average 211 456 333 Std. Dev. 2% 1% 2%

Table 3.1(c) Properties of the Type One metal bracket from company C


(MPa) Yield Stress

(MPa) H component

C1-H1 214 459 340 C1-H3 219 458 320

C component C1-C1 215 453 338 C1-C4 230 458 326

V component C1-V1 218 453 330 C1-V2 219 454 328

Average 219 456 330 Std. Dev. 3% 1% 2%



Table 3.2(a) Properties of the Type Two metal bracket from company A


(MPa) Yield Stress

(MPa) H component

A2-H1 213 499 336 A2-H3 219 504 354

C component A2-C1 215 475 354 A2-C4 212 467 354

V component A2-V1 224 504 357 A2-V2 204 500 354

Average 214 491 352 Std. Dev. 3% 3% 2%

Table 3.2(b) Properties of the Type Two metal bracket from company B


(MPa) Yield Stress

(MPa) H component

B2-H1 205 455 348 B2-H3 206 454 323

C component B2-C1 211 460 339 B2-C4 201 447 341

V component B2-V1 215 450 340 B2-V2 212 451 328

Average 208 453 337 Std. Dev. 2% 1% 3%



Table 3.2(c) Properties of the Type Two metal bracket from company C


(MPa) Yield Stress

(MPa) H component

C2-H1 217 455 334 C2-H3 213 447 319

C component C2-C1 218 450 320 C2-C4 210 449 322

V component C2-V1 220 449 325 C2-V2 214 467 325

Average 215 453 324 Std. Dev. 2% 2% 2%

Table 3.3a Properties of the Type Three metal bracket from company A


(MPa) Yield Stress

(MPa) H component

A3-H1 206 453 318 A3-H3 218 454 310

C component A3-C1 208 499 356 A3-C4 214 495 350

V component A3-V1 208 491 333

Average 211 479 333 Std. Dev. 2% 5% 6%



Table 3.3(b) Properties of the Type Three metal bracket from company B


(MPa) Yield Stress

(MPa) H component

B3-H1 210 439 319 B3-H3 207 445 323

C component B3-C1 207 444 309 B3-C4 210 449 318

V component B3-V1 207 441 321

Average 208 443 318 Std. Dev. 1% 1% 2%

Table 3.3(c) Properties of the Type Three metal bracket from company C


(MPa) Yield Stress

(MPa) H component

C3-H1 215 456 327 C3-H3 217 457 333

C component C3-C1 218 449 333 C3-C4 220 451 323

V component C3-V1 207 447 322

Average 215 452 328 Std. Dev. 2% 1% 2%



3.2. Concrete Panels Three sets of concrete panels with different compressive strengths are designed for mounting of metal bracket: (I) standard strength (around 65 MPa) with 0.2% steel reinforcement; (II) standard strength without steel reinforcement and (III) low strength (around 25 MPa) without steel reinforcement. All concrete panels have a dimension of 1 m x 1 m x 0.1 m (Figure 3.4) and simulate non-structural walls of buildings. Standard compression tests were employed to determine the strength of the concrete panels (Figure 3.5). The dimension of the concrete cubes was standardized with an edge of 150 mm. The loading rate was set at 6.8 kN/s according to the standard BS1882: Part 108. [13]. Design details of these concrete panels are given in Appendices C. Table 3.4 summarizes the results of the compressive strength of the concrete panels obtained from the compressive tests. It is seen that the compressive strengths of the sets I and II concrete panels are around 50 to 70 MPa and that of the set III concrete panels are around 20 to 25 MPa. Table 3.5 summaries the means and the standard deviations of the compressive strength of the concrete panels used for the test.

Figure 3.4 Concrete panels



Figure 3.5 Compression test of concrete cubes

Table 3.4 Compressive test results of concrete panels

Concrete Type Sample Maximum Load (kN) Strength (MPa) 1 1135 50.5 2 1157 51.5 3 1160 51.6 4 1163 51.7 5 1165 51.8 6 1176 52.3 7 1187 52.8 8 1188 52.8 9 1198 53.5 10 1226 54.5 11 1243 55.3 12 1418 63 13 1435 63.8 14 1444 64.2 14 1482 65.9

I & II

16 1523 67.7 1 473 21 III 2 531 23.6



3 535 23.8 4 539 24 5 569 25.3 6 579 25.7 7 581 26.1

8 593 26.3

Table 3.5 Means and standard deviations of the compressive strength of the concrete panels Concrete Type Mean Strength (MPa) Standard Deviation (MPa)

I & II 56.4 6.1 III 24.5 1.8

3.3. Loading Platform A loading platform (Figures 3.6 and 3.7) was constructed for fixing the concrete panels during the load test. Concrete panels were lifted by a crank and carefully anchored on the loading platform using four large-size anchor bolts (Figures 3.8 and 3.9).

Figure 3.6 The loading platform



Figure 3.7 Back of the loading platform

Figure 3.8 Mounting of a concrete panel

Figure 3.9 Mounting of a concrete panel



4. LOAD-CARRYING CAPACITY OF METAL BRACKETS 4.1. Experimental Tests In this section, the load-carrying capacity of metal brackets is obtained through experimental tests. An inverted loading frame schematically shown in Figure 4.1 is used to apply uniform distributive load (UDL) to the metal bracket. The load generated from a hydraulic jack is transmitted to the metal bracket through a set of steel short pipes as seen from Figure 4.2. The load is gradually applied manually through the hydraulic jack until the brackets fail and cannot take any load. The three types of metal brackets produced by three different companies A, B and C are assigned with different types of concrete panels as shown in Table 4.1.

Figure 4.1 Schematic diagram of the loading system



Figure 4.2 Test setup to determine the load-carrying capacity of metal brackets

Table 4.1 Assignment of the metal brackets to the type of the concrete panel

Type of concrete panel Company Non-reinforced Low strength (NLC) A

Non-reinforced standard strength (NSC) B Reinforced standard strength (RSC) C

Two anchoring conditions are considered for each type of bracket: (a) anchored by three expansion bolts; and (b) anchored by two expansion bolts. Figures 4.3-4.5 show these anchoring conditions for Type One, Type Two and Type Three, respectively. To simplify the study, only Fook Shing expansion bolts are used for the test. The dimensions of the expansion bolts are shown in Fig. 4.6. The diameter of the expansion bolts is 3/8 in (9.5 mm) and the length is 2.5 in (63.5 mm). These expansion bolts are anchored properly following the recommendations summarized in the previous report.



Figure 4.3 Anchoring conditions for the Type One Cross bracket

Figure 4.4 Anchoring condition for the Type Two I bracket

(a) Three-bolt anchored (b) Two-bolt anchored




Figure 4.5 Anchoring condition for the Type Three T bracket

Figure 4.6 The Fook Shing expansion bolt used in this study

4.1.1. Metal Brackets from Company A Figure 4.7 shows the load-carrying capacity test for the Type One Cross bracket produced by Company A. Figures 4.7(a) and 4.7(b) show the photos of before and after failure of a Type


3/8 in (9.5 mm)

1/2 in (12.7 mm)

2.5 in (63.5 mm)



One Cross bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.7(c). It is seen that the maximum load that can be exerted on the bracket is about 23 kN. The failure is initiated from the pullout of the top expansion bolt. After this initial failure, it is seen that there is a residual load-carrying capacity after the initial failure and the bracket is able to take a load increase from 7.5 kN to 14 kN before the final failure. Similarly, the photos of before and after failure and the test results of a Type One Cross bracket anchored by two expansion bolts are shown in Figures 4.7(d), 4.7(e) and 4.7(f), respectively. It is seen that the maximum load applied on the bracket is about 14 kN. The failure is resulted from the fracture of the short horizontal flat beam at the weld location. As there are only two bolts to anchor the bracket, the loss of one bolt leads to a complete failure of the bracket and no residual load-carrying capacity is observed. Figure 4.8 shows the load-carrying capacity test for the Type Two I bracket produced by Company A. Figures 4.8(a) and 4.8(b) show the photos of before and after failure of a Type Two I bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.8(c). It is seen that the maximum load that can be exerted on the bracket is about 22 kN. The failure is resulted from the pullout of the top expansion bolt. There appears to be a small residual load-carrying capacity for the bracket after the initial failure and it can take a slight load increase from 8 kN to 10 kN before the final failure. Similarly, the photos of before and after failure and the test results of a Type Two I bracket anchored by two expansion bolts are shown in Figures 4.8(d), 4.8(e) and 4.8(f), respectively. It is seen that the maximum load applied on the bracket is about 20 kN. The failure is resulted from the pullout of both expansion bolts and hence no residual load-carrying capacity is seen. Figure 4.9 shows the load-carrying capacity test for the Type Three T bracket produced by Company A. Figures 4.9(a) and 4.9(b) show the photos of before and after failure of a Type Three T bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.9(c). It is seen that the maximum load that can be exerted on the bracket is about 23 kN. The failure is resulted from the pullout of one of the top expansion bolts. There appears to be a residual load-carrying capacity for the bracket after the initial failure. The bracket is able to carry a load increase from 9 kN to 16 kN before the final failure. Similarly, the photos of before and after failure and the test results of a Type Three T bracket anchored by two expansion bolts are shown in Figures 4.9(d), 4.9(e) and 4.9(f), respectively. It is seen that the maximum load applied on the bracket is about 21 kN. The failure is resulted from the pullout of both expansion bolts and hence no residual load-carrying capacity is seen. Table 4.2 summarizes the test results for the three types of metal brackets produced by Company A. The metal brackets were mounted on non-reinforced low strength concrete panels. It is seen that the load-carrying capacities of all three types of brackets are almost the



same when they are anchored by three expansion bolts. When anchored by only two expansion bolts, their load-carrying capacities are lower than when anchored by three expansion bolts. Among the three types of brackets, the load-carrying capacity of two-bolt anchored Type One Cross bracket is the smallest. Also Type Two and Type Three brackets have similar load-carrying capacities for both three-bolt and two-bolt anchored conditions. Both of these two types of brackets fail under the pullout of expansion bolts. The failure mechanism for Type One Cross bracket mainly associates with the short horizontal flat beam which is weaker in terms of flexural rigidity than the L steel beams used for the other components.

Table 4.2 Load-carrying capacity of the metal brackets produced by Company A (in kN) Type of

Concrete Panel Anchor

Condition Type One

Cross Bracket Type Two I Bracket

Type Three T Bracket

3-bolt anchored 23 22 23 NLC

2-bolt anchored 14 20 21



(a) Before failure (3-bolt anchored) (d) Before failure (2-bolt anchored)

(b) After failure (3-bolt anchored) (e) After failure (2-bolt anchored)

0 10 20 30 40 50 60 70 80 90 100-5

0

5

10

15

20

25Company A, Type 1, Bracket No. 4

Time (s)

Load

(kN

)

0 10 20 30 40 50 60 70 80-2

0

2

4

6

8

10

12

14


Time (s)

Load

(kN

)

(c) Load time history (3-bolt anchored) (f) Load time history (2-bolt anchored)

Figure 4.7 Load test for Type One Cross bracket from Company A





0 5 10 15 20 25 30-5

0

5

10

15

20


Time (s)

Load

(kN

)

0 5 10 15 20 25 30 35-5

0

5

10

15


Time (s)

Load

(kN

)


Figure 4.8 Load test for Type Two I bracket from Company A





0 10 20 30 40 50 60-5

0

5

10

15

20


Time (s)

Load

(kN

)

0 10 20 30 40 50 60-5

0

5

10

15

20


Time (s)

Load

(kN

)


Figure 4.9 Load test for Type Three T bracket from Company A



4.1.2. Metal Brackets from Company B Figure 4.10 shows the load-carrying capacity test for the Type One Cross bracket produced by Company B. Figures 4.10(a) and 4.10(b) show the photos of before and after failure of a Type One Cross bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.10(c). It is seen that the maximum load that can be exerted on the bracket is about 25 kN. The failure is initiated from the pullout of the top expansion bolt. After this initial failure, it is seen that there is a residual load-carrying capacity after the initial failure and the bracket is able to take a load increase from 7.5 kN to 18 kN before the final failure. Similarly, the photos of before and after failure and the test results of a Type One Cross bracket anchored by two expansion bolts are shown in Figures 4.10(d), 4.10(e) and 4.10(f), respectively. It is seen that the maximum load applied on the bracket is about 15 kN. The failure is resulted from the detachment between an expansion bolt and the short horizontal flat beam. As there are only two bolts to anchor the bracket, the loss of one bolt leads to a complete failure of the bracket and no residual load-carrying capacity is observed. Figure 4.11 shows the load-carrying capacity test for the Type Two I bracket produced by Company B. Figures 4.11(a) and 4.11(b) show the photos of before and after failure of a Type Two I bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.11(c). It is seen that the maximum load that can be exerted on the bracket is about 29 kN. The failure is resulted from the pullout of the top expansion bolt. There appears to be a residual load-carrying capacity for the bracket after the initial failure and it can take a slight load increase from 17.5 kN to 20 kN before the final failure. Similarly, the photos of before and after failure and the test results of a Type Two I bracket anchored by two expansion bolts are shown in Figures 4.11(d), 4.11(e) and 4.11(f), respectively. It is seen that the maximum load applied on the bracket is about 30 kN. The failure is resulted from the pullout of both expansion bolts and hence no residual load-carrying capacity is seen. Figure 4.12 shows the load-carrying capacity test for the Type Three T bracket produced by Company B. Figures 4.12(a) and 4.12(b) show the photos of before and after failure of a Type Three T bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.12(c). It is seen that the maximum load that can be exerted on the bracket is about 33 kN. The failure is resulted from the pullout of the two side expansion bolts. There appears to be a residual load-carrying capacity for the bracket after the initial failure. However as the vertical component deformed quite significantly and the horizontal component tilted forward, the test was terminated without proceeding to final failure. Similarly, the photos of before and after failure and the test results of a Type Three T bracket anchored by two expansion bolts are shown in Figures 4.12(d), 4.12(e) and 4.12(f), respectively. It is seen that the maximum load applied on the bracket is about 27 kN. The



failure is resulted from the pullout of both expansion bolts and hence no residual load-carrying capacity is seen. Table 4.3 summarizes the test results for the three types of metal brackets produced by Company B. The metal brackets were mounted on non-reinforced standard strength concrete panels. It is seen that the load-carrying capacities of Type One Cross bracket and Type Three T bracket anchored by three expansion bolts are larger than that of brackets anchored by only two bolts. For Type Two I bracket, the two anchorage conditions have a similar load-carrying capacity. Also Type Two and Type Three brackets have similar load-carrying capacities for both three-bolt and two-bolt anchored conditions. Both types of brackets fail under the pullout of expansion bolts. On the other hand, the load-carrying capacities of Type One Cross bracket are lower when compared to those of Type Two and Type Three brackets. Also the failure mechanism for Type One bracket mainly associates with the short horizontal flat beam which is weaker in terms of flexural rigidity than the L steel beams used for the other components.

Table 4.3 Load-carrying capacity of the metal brackets produced by Company B (in kN) Type of


Condition Type One



3-bolt anchored 25 29 33 NSC






0 20 40 60 80 100 120 140-5

0

5

10

15

20

25

30Company B, Type 1, Bracket No. 4

Time (s)

Load

(kN

)

0 10 20 30 40 50 60-2

0

2

4

6

8

10

12

14


Time (s)

Load

(kN

)


Figure 4.10 Load test for Type One Cross bracket from Company B





0 10 20 30 40 50 60-5

0

5

10

15

20

25


Time (s)

Load

(kN

)

0 10 20 30 40 50 60 70-5

0

5

10

15

20

25

30


Time (s)

Load

(kN

)


Figure 4.11 Load test for Type Two I bracket from Company B





0 10 20 30 40 50 60-5

0

5

10

15

20

25

30


Time (s)

Load

(kN

)

0 10 20 30 40 50 60 70 80 90-5

0

5

10

15

20

25


Time (s)

Load

(kN

)


Figure 4.12 Load test for Type Three T bracket from Company B



4.1.3. Metal Brackets from Company C Figure 4.13 shows the load-carrying capacity test for the Type One Cross bracket produced by Company C. Figures 4.13(a) and 4.13(b) show the photos of before and after failure of a Type One Cross bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.13(c). It is seen that the maximum load that can be exerted on the bracket is about 29 kN. The failure is resulted from the fracture of the short flat horizontal beam and the pullout of top expansion bolt. Similarly, the photos of before and after failure and the test results of a Type One cross bracket anchored by two expansion bolts are shown in Figures 4.13(d), 4.13(e) and 4.13(f), respectively. It is seen that the maximum load applied on the bracket is about 18 kN. The failure is resulted from the pullout of a side expansion bolt. Also, it is seen that the short flat horizontal beam underwent substantial deformation prior to the failure. Figure 4.14 shows the load-carrying capacity test for the Type Two I bracket produced by Company C. Figures 4.14(a) and 4.14(b) show the photos of before and after failure of a Type Two I bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.14(c). It is seen that the maximum load that can be exerted on the bracket is about 31 kN. The failure is resulted from the pullout of top two expansion bolts. Similarly, the photos of before and after failure and the test results of a Type Two I bracket anchored by two expansion bolts are shown in Figures 4.14(d), 4.14(e) and 4.14(f), respectively. It is seen that the maximum load applied on the bracket is about 25 kN. The failure is resulted from the pullout of expansion bolts. It is seen that the vertical L beams do not deform significantly for both conditions. This suggests that the strength of L beams is larger than the pullout strength of expansion bolts. Figure 4.15 shows the load-carrying capacity test for the Type Three T bracket produced by Company C. Figures 4.15(a) and 4.15(b) show the photos of before and after failure of a Type Three T bracket anchored by three expansion bolts. The corresponding loading time history is shown in Figure 4.15(c). It is seen that the maximum load that can be exerted on the bracket is about 30 kN. The failure is initiated from the pullout of right side expansion bolt and followed by significant bending deformation to the vertical L beam. The bracket eventually failures when the left side expansion bolt is pulled out. This failure behaviour is reflected on the loading time history plot in Figure 4.15(c). It is seen that the load steadily increases from 0 to 30 kN and drops significantly to 15 kN when the right side expansion bolt is pulled out. After this initial failure, it appears that there is a residual load-carrying capacity for the bracket as it does not failure completely. The bracket is still able to continuously support load increase from 15 to 25 kN until the left side expansion bolt is pulled out. Similarly, the photos of before and after failure and the test results of a Type Three T bracket anchored by two



expansion bolts are shown in Figures 4.15(d), 4.15(e) and 4.15(f), respectively. It is seen that the maximum load applied on the bracket is about 25 kN. The failure is resulted from the pullout of two expansion bolts which occurs almost simultaneously. Table 4.4 summarizes the test results for the three types of metal brackets produced by Company C. The metal brackets were mounted on reinforced standard strength concrete panels. It is seen that the load-carrying capacity of brackets anchored by three expansion bolts is larger than that of brackets anchored by only two bolts. Also Type Two and Type Three brackets have similar load-carrying capacities for both three-bolt and two-bolt anchored conditions. Both of these two types of brackets fail under the pullout of expansion bolts. On the other hand, the load-carrying capacities of Type One Cross bracket are lower when compared to those of Type Two and Type Three brackets. Also the failure mechanism for Type One bracket mainly associates with the short horizontal flat beam which is weaker in terms of flexural rigidity than the L steel beams used for the other components.

Table 4.4 Load-carrying capacity of the metal brackets produced by Company C (in kN) Type of


Condition Type One



3-bolt anchored 29 31 30 RSC






0 20 40 60 80 100 120 140-5

0

5

10

15

20

25

30Company C, Type 1, Bracket No. 5

Time (s)

Load

(kN

)

0 20 40 60 80 100 120-5

0

5

10

15


Time (s)

Load

(kN

)


Figure 4.13 Load test for Type One Cross bracket from Company C





0 50 100 150 200 250 300-5

0

5

10

15

20

25

30


Time (s)

Load

(kN

)

0 5 10 15 20 25 30 35 40-5

0

5

10

15

20

25


Time (s)

Load

(kN

)


Figure 4.14 Load test for Type Two I bracket from Company C





0 20 40 60 80 100 120 140 160 180-5

0

5

10

15

20

25


Time (s)

Load

(kN

)

0 5 10 15 20 25 30 35 40 45-5

0

5

10

15

20

25


Time (s)

Load

(kN

)


Figure 4.15 Load test for Type Three T bracket from Company C



4.1.4. Remarks Table 4.5 summarizes the results of load-carrying capacity test for the three types of metal brackets produced by three companies. From this table, the following findings can be concluded: (1) In general, the load-carrying capacities of brackets anchored by three bolts are larger than

those brackets anchored by only two bolts. (2) The load-carrying capacities of Type One Cross brackets are smaller than those of Type

Two I bracket and Type Three T bracket. For Type One Cross bracket, the short horizontal flat beam that welded to the vertical component appears to be the weakest component of the bracket. Tests show that failures of the Type One Cross brackets are attributed to the fracture or the deformation of this flat beam which is weaker in terms of flexural rigidity than the other components.

(3) The load-carrying capacities of Type Two I bracket and Type Three T bracket are almost the same.

(4) It appears that, when anchored by three expansion bolts, both Type One Cross bracket and Type Three T bracket have a residual load-carrying capacity. After initial failure, both types of brackets do not fail completely and can still carry some loading. On the other hand, Type Two I bracket does not seem to have this capability and its failure is normally quite sudden.

(5) When anchored by two expansion bolts, none of the three types of brackets have any residual load-carrying capacity and their failures are usually quite sudden.

Table 4.5 Summary of the load-carrying capacity test results (in kN)

Concrete Panel / Company

Anchor Condition

Type One Cross Bracket

Type Two I Bracket


3-bolt anchored 23 22 23 NLC / Company A 2-bolt anchored 14 20 21

3-bolt anchored 25 29 33 NSC / Company B 2-bolt anchored 15 30 27

3-bolt anchored 29 31 30 RSC / Company C 2-bolt anchored 18 25 25



4.2. Numerical Analysis 4.2.1. Estimation of Anchor Strength The strength of expansion bolts depends on a few factors. In Section 2, the pull-out strength and the shear strength of the Fook Shing expansion bolts anchored in concrete panels with strength of 22 MPa have been summarized (Tables 2.1 and 2.2) from the previous study [1]. As shown in Eqs. (3) and (6), the pull-out strength and the shear strength of expansion bolts are proportional to square root of the concrete strength. In this study, three types of concrete panels are used for mounting the metal brackets. These concrete panels come with strength of either 25 MPa (low strength) or 65 MPa (standard strength). The pull-out strength NRu,m and the shear strength VRu.m of Fook Shing expansion bolts anchored in these concrete panels can be easily estimated in Table 4.6.

Table 4.6 Estimated pull-out and shear strengths for the Fook Shing expansion bolts

Concrete panel Pull-out strength

NRu,m (kN)

Shear strength

VRu.m (kN) Previous study (22 MPa) 10.6 17.5 Low strength (25 MPa) 11.3 18.7

Standard strength (65 MPa) 18.2 30.1 4.2.2. Strength under Combined Loading When the expansion bolt is subjected to both pull-out force Ns and shear force Vs (Figure 4.16), its strength needs to be revised. Reference [9] gives a general formula to determine the strength of an expansion bolt under combined pull-out and shear forces:

15.1

,

5.1

,≤⎟

⎟⎠

⎞⎜⎜⎝

⎛+⎟

⎟⎠

⎞⎜⎜⎝

⎛

mRu

S

mRu

SV

VN

N (9a)

or alternatively, 3/25.1

,

5.1

,,

22 sincos−

⎟⎟⎟

⎠

⎞

⎜⎜⎜

⎝

⎛

⎟⎟⎠

⎞⎜⎜⎝

⎛ α+⎟

⎟⎠

⎞⎜⎜⎝

⎛ α=≤+=

mRumRumRuSSS VN

FVNF (9b)

where FS is the combined force acting on the anchor; α is the angle between the pull-out force and the combined force (Figure 4.15); NRu,m and VRu,m are the pull-out strength and the shear strength of the expansion bolt; and FRu,m is the strength of expansion bolt under combined

forces. When mRuS FF ,≥ , the expansion bolt is deemed as failed under the combined loading.



Figure 4.17 shows the strength of expansion bolts anchored on low-strength and standard-strength concrete under a combined loading condition. It is seen that when α increases from 0o to 90 o, the strength of expansion bolts increases from the pull-out strength to the shear strength.

Figure 4.16 An expansion bolt under combined loading

0 10 20 30 40 50 60 70 80 905

10

15

20

25

30

35

F Ru,m

Angle, α

Low strengthStandard strength

Figure 4.17 Strength of expansion bolt under combined loading

α

FS VS

NS



4.2.3. Finite Element Modeling The finite element program SAP2000 is used to analyze the behaviors of different metal brackets under external loading. Two-dimensional beam elements are used to model the metal bracket. Based on the results in Tables 3.1-3.3, the properties of the beams are assumed to be: Young’s modulus E = 210 GPa and yield stress Fy = 350 MPa. Figure 4.18 shows the finite element models for the three types of metal brackets considered in the study. Note that the anchorages fixing the metal brackets to the concrete panel are modeled by two springs, one along the perpendicular direction and the other along the panel surface direction. These springs are to model the onset initial deformation of the expansion bolts under loading. It is observed in the test that the expansion bolts undergo small deformation once they are loaded. This behavior can be approximately modeled using linear springs. Table 4.7 shows the spring coefficients for the anchorages used in the finite element models. These coefficients are obtained and estimated from the test. The contact surface between the metal bracket and the concrete panel is modeled by gap elements which respond to compression but not tension. As the upper part of the contact surface between the metal bracket and the concrete panel is not compressed during loading, so the gap elements are placed only at the lower part of the surface. The applied UDL loading is applied on the horizontal component of the metal bracket (x-direction). Two anchorage conditions, 2-bolt anchored and 3-bolt anchored, are considered for each type of metal bracket in the load-carrying capacity analysis.

Table 4.7 Spring coefficients for the anchorage modeling used in the finite element models Concrete

panel Low strength

(25 MPa) Standard strength

(65 MPa) KH (kN/cm) 15 25 KV (kN/cm) 30 50



XY X

Z

Y

Z

Anchor B

Anchor A

Anchor C

Link

XY X

Z

Y

Z

Anchor B

Anchor A

Anchor C

Link (a) Type One (b) Type Two

XY X

Z

Y

Z

Anchor B

Anchor A

Anchor C

Link (c) Type Three

Figure 4.18 Finite element models for three types of metal brackets 4.2.4. Computed Load-carrying Capacity In this numerical analysis, the load-carrying capacity of a metal bracket is defined as the load that causes the failure of any expansion bolt anchoring the metal bracket. To simplify the analysis, the metal brackets are assumed to remain linearly elastic throughout the calculation hence no failure occur to the brackets. For a given external loading that applied on a metal bracket, the finite element analysis calculates forces that act on the expansion bolts. The failure of an expansion bolt corresponds to the condition when the combined force that acts on the bolt becomes equal or larger than its strength under combined load (see Sections 4.2.1 and 4.2.2). As can be seen from Eq. (9b) in Section 4.2.2, the anchor strength under combined



loading FRu,m depends on the pull-out strength and the shear strength of the expansion bolt as well as the angle between the pull-out force and the combined force (Figure 4.17). Hence it is expected that the anchor strength under combined loading would vary from case to case. Figure 4.19 shows the anchor force versus applied loading for a Type One Cross bracket mounted on low-strength and regular-strength concrete panels. It is seen that the combined force acting on the expansion bolt is smaller for a metal bracket mounted by three anchors. The load-carrying capacities of this type of metal bracket can be calculated using this figure together with Eq. (9b). Results show that the load-carrying capacities for a 3-bolt anchored Type One Cross bracket mounted on a low-strength and a regular-strength concrete panel are 22.6 and 33.1 kN, respectively. Similarly those for a 2-bolt anchored Type One Cross bracket mounted on a low-strength and a regular-strength concrete panel are 20.8 and 33.5 kN, respectively. Figure 4.20 shows the anchor force versus applied loading for a Type Two I bracket mounted on low-strength and regular-strength concrete panels. Results show that the load-carrying capacities for a 3-bolt anchored Type Two I bracket mounted on a low-strength and a regular-strength concrete panel are 20.3 and 31.5 kN, respectively. Similarly those for a 2-bolt anchored Type Two I bracket mounted on a low-strength and a regular-strength concrete panel are 18.2 and 28.3 kN, respectively. Figure 4.21 shows the anchor force versus applied loading for a Type Three T bracket mounted on low-strength and regular-strength concrete panels. Results show that the load-carrying capacities for a 3-bolt anchored Type Three T bracket mounted on a low-strength and a regular-strength concrete panel are 24.5 and 37.8 kN, respectively. Similarly those for a 2-bolt anchored Type Three T bracket mounted on a low-strength and a regular-strength concrete panel are 20.6 and 32.3 kN, respectively. Figures 4.22 and 4.23 further show the comparisons of loading capacity among the three types of metal brackets mounted by three bolts and two bolts, respectively.



20.8 22.60

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

3-bolt anchored

2-bolt anchored

(a) Mounted on low-strength concrete panel

33.1 33.50

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

3-bolt anchored

2-bolt anchored

(b) Mounted on regular-strength concrete panel

Figure 4.19 Anchor force versus applied load for Type One Cross bracket



0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

3-bolt anchored

2-bolt anchored

18.2 20.3


0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

3-bolt anchored

2-bolt anchored

28.3 31.5


Figure 4.20 Anchor force versus applied load for Type Two I bracket



0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

3-bolt anchored

2-bolt anchored

20.6 24.5


0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

3-bolt anchored

2-bolt anchored

32.3 37.8


Figure 4.21 Anchor force versus applied load for Type Three T bracket



0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

Type One

Type Two

Type Three

20.3 24.522.6

(a) 3-bolt anchored

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

Type One

Type Two

Type Three

20.820.6

18.2

(b) 2-bolt anchored

Figure 4.22 Comparison of loading capacity for three types of brackets anchored on low-strength concrete panels



0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

Type One

Type Two

Type Three

31.5 37.833.1

(a) 3-bolt anchored

0

5

10

15

20

25

30

0 5 10 15 20 25 30 35 40Applied load (kN)

Type One

Type Two

Type Three

33.532.3

28.3

(b) 2-bolt anchored

Figure 4.23 Comparison of loading capacity for three types of brackets anchored on regular-strength concrete panels



Table 4.8 summarizes the computed load-carrying capacities for all three types of metal brackets. It is seen that the load-carrying capacities for Type Two I bracket are the smallest among the three types of brackets under all four conditions. Also, it is seen that in general the load-carrying capacities of brackets mounted by three bolts are higher than those of brackets mounted by only two bolts. In view that the truss-out metal bracket bamboo scaffolding might also be subjected to lateral load, the lateral load-carrying capacities for Type Two I bracket and Type Three T bracket are also computed and presented in Appendix E.

Table 4.8 Computed load-carrying capacities for metal brackets (in kN) Type of


Condition Type One



3-bolt anchored 22.6 20.3 24.5 Low (25 MPa) 2-bolt anchored 20.8 18.2 20.6

3-bolt anchored 33.1 31.5 37.8 Standard (65 MPa) 2-bolt anchored 33.5 28.3 32.3

4.3. Comparison between Experimental and Computed Results Table 4.9 summarizes the comparison between the experimental and the computed load-carrying capacities for metal brackets. It is seen that the experimental and the computed load-carrying capacities for Type Two I bracket and Type Three T bracket compare quite well. It is noted that the computed load-carrying capacities are generally larger than the experimental load-carrying capacities for Type One Cross bracket. This is due to the linear elastic assumption adopted for the metal brackets during the finite element analysis. As observed from the experimental tests, the failure conditions that occur to the Type One Cross brackets mostly are accompanied with excessive deformation or even fracture of short horizontal flat beams. Apparently, the linear finite element models used in the previous section are not able to emulate this nonlinear behavior, hence lead to over-estimation of load-carrying capacity. These results also suggest that if a thicker or stiffer horizontal beam is used, the load-carrying capacity of Type One Cross bracket would improve and approach those computed values. Also note that the load-carrying capacities of those metal brackets from Company A mounted on non-reinforced low strength concrete panels (NLC) are all smaller than their counterparts (metal brackets from Companies B and C anchored on non-reinforced standard strength panels (NSC) and reinforced standard strength panels (RSC), respectively). This result is in line with the result obtained in the previous report [1] that the wall property would affect the load-carrying capacity of the metal brackets.



Table 4.9 Comparison between the experimental and the computed load-carrying capacities for metal brackets (in kN)

Type One Cross Bracket

Type Two I Bracket


Concrete Panel / Company

Anchor Condition

Exp. Com. Exp. Com. Exp. Com. 3-bolt anchored 23 22.2 22 20.3 23 24.5 NLC

/ Company A 2-bolt anchored 14 20.8 20 18.2 21 20.6 3-bolt anchored 25 33.1 29 31.5 33 37.8 NSC

/ Company B 2-bolt anchored 15 33.5 30 28.3 27 32.3 3-bolt anchored 29 33.1 31 31.5 30 37.8 RSC

/ Company C 2-bolt anchored 18 33.5 25 28.3 25 32.3



5. CONCLUSIONS AND RECOMMENDATIONS The aim of the original study is on the structural safety and reliability of the truss-out metal bracket bamboo scaffolding using combination of laboratory tests and computer analysis. Conclusions and recommendations obtained from that study can be summarized as follows: (1) The truss-out metal bracket bamboo scaffolding is mounted on structures via expansion

bolts. The strength of expansion bolts directly affects the load-carry capacity and the safety of this type of scaffolding.

(2) There are two types of strength for expansion bolts: the pull-out strength and the shear strength. Some empirical formulas have been summarized in this report. Laboratory tests have been conducted to verify these formulas. In general, these empirical formulas might provide conservative or unconservative strength estimation perhaps due to inhomogeneity of the base materials. One should use these formulas with caution.

(3) The pull-out strength of expansion bolts depends on factors such as the drilled hole size, the anchoring tightness, the embedment depth, the anchoring location as well as the base material. It is found that the pull-out strength significantly reduces to close to zero if the drilled hole size is bigger than the diameter of expansion bolts. Inserting nylon strips into over-sized holes does not significantly increase the pull-out strength. Proper anchoring tightness should be applied (about two screw turns) in order to have the maximum pull-out strength. The pull-out strength is also proportional to the embedment depth of expansion bolts. The thickness of cement surfacing and tile does not seem to contribute to increase of pull-out strength. The pull-out strength for expansion bolts anchored in brick walls exhibits a larger variation than that anchored in concrete panels. The former (anchored in brick walls) is also about 30% lower than the latter (anchored in concrete panels).

(4) The shear strength of expansion bolts also depends on factors that affect their pull-out strength. In addition, the shear strength is also strongly affected by the distance of the anchorage location to the free edge. It is found that the shear strength reduced significantly if the distance to the free edge is smaller than three times of the embedment depth.

(5) Numerical analyses using finite element method and laboratory tests were conducted to determine the load-carry capacity of the truss-out metal bracket bamboo scaffolding. Three anchorage conditions have been investigated: anchored by one, two or three expansion bolts, respectively. Based on the pull-out strength and the shear strength obtained from the tests and the finite element analysis, a possible failure range can be defined for this type of scaffolding mounted on either concrete or brick walls under certain anchorage condition. Laboratory test results show that the measured failure loads basically fall within the range of UDL predicted by the possible failure range.



(6) The load-carry capacity of the truss-out metal bracket bamboo scaffolding depends on factors that affect the pull-out strength and the shear strength of expansion bolts. Test results show that when mounted on the thickness side of a concrete panel, the load-carry capacity of the scaffolding would reduce to a small fraction of that mounted on the surface. Also the load-carry capacity of the scaffolding mounted on brick walls is generally small than that mounted on concrete panels by about 30%. When over-size holes are used to anchor expansion bolts, the load-carry capacity of the scaffolding reduces quite substantially to about only one third. Numerical analysis also indicates that the scaffolding might not be able to take any load if the expansion bolts are anchored in over-sized holes, even with the insertion of nylon strips.

(7) Test results also show that the failure load for a scaffolding anchored by three expansion bolts is two times higher than that anchored by only one expansion bolt. Also when failure occurs, the metal brackets for those scaffolding anchored by one or two expansion bolts are completely detached from the wall which lead to the tilt of scaffolding. This results in a fatal situation for the workers on the scaffolding. On the other hand, it is see that only one expansion bolt is pulled out while the metal brackets are still attached to the wall when failure occurs if the scaffolding is anchored by three expansion bolts. This does not lead to an immediate fatal situation for those who work on the scaffolding.

(8) Base on this study, the following important recommendations can be made: (a) The truss-out metal bracket bamboo scaffolding should not be mounted on the

side/edge of concrete panel or canopy slab. (b) Proper drill holes should be used to anchor the expansion bolt. The hole size should

be equal to slightly smaller than the diameter of the expansion bolt. Oversized drill hole should be abandoned. No nylon, bamboo or wood strips should be inserted into the oversized drilled holes to correct the problem.

(c) Sufficient embedment depth should be provided to anchor the expansion bolt. The thickness of cement surfacing and tile does not add to the embedment depth and hence cannot increase the strength of expansion bolts. In case thick plaster is encountered on site, longer anchor should be adopted accordingly so that sufficient embedment on strong base material would be provided.

(d) Do not anchor expansion bolts near the free edge of a structural component. At least a distance of three times of the embedment depth should be provided.

(e) Proper anchoring tightness (about two screw turns after a significant torque resistance is felt from the anchor bolts) should be applied to the expansion bolt.

(f) Anchor the scaffolding with at least three expansion bolts. (g) Do not mount the scaffolding on brick walls. The load-carry capacity of a

scaffolding mounted on a brick wall has a larger variation and is smaller than that mounted on a concrete wall.



In this follow-up study, the load-carrying capacities of three different types of metal brackets manufactured by three anonymous companies are studied. These three types of brackets are named as Type One Cross bracket; Type Two I bracket; and Type Three T bracket. Among them, Type Two I bracket is currently used in the industry for truss-out bamboo scaffolding. The metal brackets are mounted on concrete panels using three expansion bolts or two expansion bolts. Laboratory tests are first conducted in the Structural Lab of the Hong Kong University of Science and Technology. The test results are then verified by numerical analysis using finite element analysis. The conclusions obtaining in this follow-up study are itemized as follows: (1) When anchored by three expansion bolts, all three types of metal brackets have roughly

the same load-carrying capacity. When anchored by only two expansion bolts, the load-carrying capacity of Type One Cross bracket is significantly lower than those of the other two types of brackets. It is observed that the failure of Type One Cross bracket is often associated with excessive deformation or fracture of the horizontal short flat beam which is welded to the vertical L component of the bracket. It appears that the flat beam is too weak and cannot provide the same level of flexural rigidity than the L-shape steel beam component. Proper care must be exercised when using this type of metal bracket.

(2) Type Two I bracket and Type Three T bracket have about the same load-carrying capacity under either 3-bolt or 2-bolt anchored condition. Both types of brackets fail under the pull-out of expansion bolts.

(3) For the same type bracket, the load-carrying capacity for the bracket mounted by three expansion bolts is higher than that mounted only by two expansion bolts.

(4) When anchored by three expansion bolts, both Type One Cross bracket and Type Three T bracket have a residual load-carrying capacity. After initial failure, both types of brackets do not fail completely and are still able to carry some loading. On the other hand, Type Two I bracket does not seem to have this capability or its residual load-carrying capacity is not significant enough. The failure of Type Two I bracket is normally quite sudden and drastic. When anchored by two expansion bolts, none of the three types of brackets have any residual load-carrying capacity and their failures are usually quite sudden and drastic.

(5) It appears that Type Three T bracket outperforms the other two types of brackets. Its load-carrying capacity is comparable to that of the Type Two I bracket currently used in the industry for supporting truss-out bamboo scaffolding. Furthermore it is recommended to mount this type of bracket using three expansion bolts which would provide a residual load-carrying capacity to the bracket under extreme loading condition. This residual load-carrying capacity is a valuable additional safety measure for this type of scaffolding.



REFERENCES 1. Chih-Chen Chang and Sze Kin Wai (2005) An engineering study for safety and reliability

of truss-out metal bracket bamboo scaffoldings, final report submitted to the Occupational Safety & Health Council.

2. BSI: CP110 (1972) Code of practice for the structural use of concrete. Part 1: Design materials and workmanship

3. BSI: CP114 (1967) The structural use of reinforced concrete in buildings 4. ASTM E8-97 ( 1997) Test methods for tension testing of metallic materials 5. R. Eligehausen, P. Bouska, V. Cervenka and R. Pukl Size effect of the concrete cone failure

load of anchor bolts. 6. R. Eigehausen, U. Ick, R. Mallee, and M. Reuter Schimmelpfenning and schmal:

tragverhalten von kopfbolzenverankerungen bei zugbeanspruchung, Bauingenierur. 7. ACI 349-85 Code requirements for nuclear safety related concrete structure. ACI journal. 8. EOTA: ETAG001 (2002) Guideline for European technical approval of metal anchors for

use in concrete. 9. Hilti (Hong Kong) Limited (2004) Fastening technology manual. 10. M. Miltenberger (2001) “Capacity design of grouted anchors,” Transaction, SMiRT 16,

Washington DC, August 2001. 11. W. Fuchs, R. Eligehausen and J. Breen (1995) “Concrete capacity design (CCD) approach

for fastening to concrete,” ACI Structural Journal, Vol. 92, 73-79. 12. R.A. Cock, J. Kunz, W. Fuchs and R.C. Konz (1998) “Behavior and design of single

adhesive anchors under tensile load in uncracked concrete,” ACI Structural Journal, Vol.95.

13. BS 1881: Part 108 (1983) Method for making test cubes from fresh concrete.



APPENDICES A: Detailing for the metal brackets

Company A Type One

Company A Type Two



Company A Type Three

Company B Type One



Company B Type Two

Company B Type Three



Company C Type One

Company C Type Two



Company C Type Three



B: Metal bracket specimens for the tensile tests



C: Details of the concrete panel (with 0.2% reinforcement)



D: Schematic diagram of the loading frame



E: Lateral load-carrying capacity of metal brackets Considering the possibility of lateral loading acting on the metal brackets, the lateral load-carrying capacities for the Type Two I bracket and Type Three T bracket are numerically studied in this section. The FEM models for these two brackets are shown in Figure E.1. Plate elements are used to model the vertical component of the Type Two I bracket. Modeling of the other components and the anchorages is the same as the vertical loading case. Table E.1 shows the results of lateral load-carrying capacities for the two types of brackets for different concrete panels and anchorage conditions. It is observed that Type Three T bracket has a much higher lateral load-carrying capacity than Type Two I bracket for all conditions considered. This can be explained easily by the fact that the moment arm for the expansion bolts of Type Two I bracket is much smaller than that of Type Three T bracket as shown in Figure E.1. It is seen that the lateral load-carrying capacity of three bolt-anchored Type Three T bracket is the same as that of the two bolt-anchored bracket. On the other hand, the lateral load-carrying capacity of two bolt-anchored Type Two I bracket is smaller than that of three bolt-anchored bracket. Roughly, the lateral load-carrying capacity of Type Three T bracket is about four times of that of Type Two I bracket when anchored by three expansion bolts. When anchored by only two expansion bolts, the ratio increases to seven times. In summary, Type Three T bracket provides a larger lateral load-carrying capacity than Type Two I bracket. Hence Type Three T bracket can provide a much stronger support to bamboo scaffolding under lateral loading condition.

Table E.1 Computed lateral load-carrying capacities for metal brackets (in kN) Type of Concrete

Panel Anchor

Condition Type Two I Bracket


3-bolt anchored 1.6 6.5 Low strength (25 MPa) 2-bolt anchored 0.9 6.4

3-bolt anchored 2.5 10.4 Standard strength (65 MPa) 2-bolt anchored 1.5 10.4



Anchor B

Anchor A

Anchor C

Link

q

Plate element

Beam element

XY XZY Z

Anchor B

Anchor A

Anchor CLink

Beam element

q

(a) Type Two (b) Type Three

Figure E.1 Finite element models for Type Two and Three metal brackets subjected to lateral loading

Documents

Follow-up Study on Safety and Reliability of Truss-out