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International Journal of Earth Sciences and Engineering ISSN 0974-5904, Volume 04, No 06 SPL, October 2011, pp 642-644
Structural Scrutiny of the Existing Billboards C.B. Joshi Post Graduate Student, Applied Mechanics Department, L D College of Engineering, Ahmedabad, 380015 Gujarat (India) Email:
[email protected]
N.K.Arora Associate Professor, Applied Mechanics Department, L D College of Engineering, Ahmedabad, 380015 Gujarat (India ) Email:
[email protected]
ABSTRACT: Looking to the number of failures of billboards in near past in Ahmedabad city, a need was felt to refer back the structural design design of existing billboards. billboards. In present paper, a total of 12 billboards billboards existing existing in various parts of city are analysed and designed as per IS: 800-1984 as well as IS: 800-2007. All these structures were found unsafe for the design wind loads. Most of the failed members exceeded either structural capacity or slenderness ratio limit. KEY WORDS: Billboards, wind, slenderness ratio, capacity, analysis INTRODUCTION Loads Billboards are large outdoor signboards kept on ground The billboards are liable to fail under combined action of and terraces of buildings and are vulnerable to wind self weight, earthquake loads and wind forces. forces. The failures of these structures have resulted in Self weight fatalities in the past. As these are not considered as The self weight of members is given in –Y direction. important structures, very less attention has been given to their collapse. In India, civic body clearance is required Wind load before placing a billboard which requires submission The wind loads are calculated as per IS: 875 Part 3-1987. structural design and drawing from a registered structural Wind Zone - Zone III (Ahmedabad City). Following engineer. However, looking to the loss of properties and parameters are considered: considered: lives in various parts of world due to the collapse of Basic Wind Speed that depends upon the city is taken billboards has forced to re-examine re-examine the structural safety of as per Cl. 5.2. existing billboards. A total of twelve billboards were Terrain Category 3 is considered for city area as per modelled and designed as per their structural drawing Cl. 5.3.2.1. submitted to the local civic body. Risk Coefficient factor k1 is taken as 0.76 (considering 5 years design life) as per Cl. 5.3.1. GEOMETRICAL DESCRIPTION OF BILLBOARDS For k2 factor class A is considered for the structure. The geometrical description including board dimensions, k3 factor is taken as 1 since the billboards are located height from ground, total number of members, plan within the city. dimensions etc. of the billboards accounted in present Pressure is calculated as per Cl. 6.3.2.3 according to study is given in Table.1. All the billboards have almost a which a Cf factor is to be applied. similar configuration consisting of horizontal members, It is obtained as follows: vertical members and a trestle system at back. All of these Design wind speed (Vz) members generally have angular sections. A sample Vz = Vb x k1 x k2 x k3 configuration is as shown below: Design wind pressure (pz) pz = 0.6 Vz2 Design force: F = Cf x A x pz Where, Cf = Force coefficient that depends upon b/h ratio. The wind load is applied by generating floor loads in +Z and –Z directions.
Earthquake load load The design seismic base shear is computed by STAAD in accordance with the IS: 1893 (Part 1) -2002 equation 7.5.3. Design seismic base shear (Vb) = Ah x W
Fig. 1 Sample Configuration ANALYSIS The analysis of billboards is carried out in STAAD-Pro. The geometry of all the billboards is generated as per their structural drawings. The section properties of each of the members are assigned as per the drawing. The supports of billboards are assumed to be fixed at the locations of the pedestal.
Where, Ah = Design horizontal acceleration spectrum value as per 6.4.2 using the fundamental natural period T as per 7.6 in the considered direction of vibration W = seismic weight Where, Ah = (Z/2) (I/R) (Sa/g) Different parameters that are required to be given for computation of earthquake are as follows: Zone factor Z = 0.16,
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Structural Scrutiny of the Existing Billboards
Response reduction factor R = 3 (for Ordinary RC moment-resisting frame), Importance factor I = 1 Rock and soil sites factor = 2 for medium soil sites. Billboard Designation
Table 1 Geometrical Description Size Height Total Plan (mxm) of members dimensions bottom at pedestal member level from G.L L B (m) (m) (m)
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range of 1.1-8.9. Since the design is carried out as per both old and new code it is possible to compare the results which are obtained as per both. Such a comparison is shown in Fig. 2 and Fig 3. Load Combinations Load combinations given are as follows: Table 2 Load combinations
Sr. No.
Combinations
Sr. No.
Combinations
1
1.5 DL
14
0.9 DL + 1.5 WL (+Z)
1
12x6
12
144
10
2.4
2
1.2 DL + 0.6 WL (+Z)
15
0.9 DL + 1.5 WL (-Z)
2
6x6
3
64
4
2.4
3
1.2 DL + 0.6 WL (-Z)
16
0.9 DL + 1.5 EQX
3
9x6
3
90
7
2.4
4
1.2 DL + 0.6 EQX
17
0.9 DL + 1.5 EQZ
4
9x4.5
9
68
7
2.5
5
1.2 DL + 0.6 EQZ
18
DL + 0.8 WL (+Z)
5
6.1x6.1
12
85
5
4
6
1.2 DL + 1.2 WL (+Z)
19
DL + 0.8 WL (-Z)
6
9x6
4
132
8
3
7
1.2 DL + 1.2 WL (-Z)
20
DL + 0.8 EQX
7
12x6
12
138
11
3
8
1.2 DL + 1.2 EQX
21
DL + 0.8 EQZ
8
6x6
12
108
5
2.7
9
1.2 DL + 1.2 EQZ
22
DL + WL (+Z)
9
12x6
6
101
12
2.3
10
1.5 DL + 1.5 WL (+Z)
23
DL + WL (-Z)
10
6x6
6
101
6
2.3
11
1.5 DL + 1.5 WL (-Z)
24
DL + EQX
11
6x6
8
396
6
3
12
1.5 DL + 1.5 EQX
25
DL + EQZ
13
1.5 DL + 1.5 EQZ
12
12x6
8
678
12
4.5
RESULTS Design is carried out as per IS: 800-1984 as well as IS: 800-2007. The failed members are indicated whenever the section of the member is not sufficient to carry the loads. The failed members are as indicated in Table III. The failure is either in slenderness or capacity. The table also indicates maximum deflection observed under loads. Even though there are no specifications for the deflection limit in case of billboards the value of deflection is very high for billboard 5. The capacity ratio which is the ratio of allowable stress to permissible stress is found in the
DISCUSSION OF RESULTS Wind loads are the predominant loads except for a few cases where earthquake loads are also dominating. But that is for only a few members. For each of the case considered all the panel members are failing. Members failing in slenderness and capacity are same for a few cases. However for most of the cases members failing in slenderness are much higher than members which are failing in capacity. Considering the design in both codes, members failing in slenderness are exactly same while the members failing in capacity in are slightly different.
Table 3: Summary of results
Designation 1 2 3 4 5 6 7 8 9 10 11 12
Total members 144 63 90 68 85 138 138 108 101 101 396 678
AS PER IS 800: 1984 Members Total failure due failed to members slenderness 114 54 39 23 59 38 53 32 74 36 60 52 88 72 64 53 41 35 35 35 92 92 222 222
Member failure due to capacity 60 16 21 21 38 8 16 11 6 0 0 0
Maximum Deflection (mm) 96.56 54.42 40.11 36.21 544.32 5.302 55.63 14.02 2.76 1.37 9.65 7.67
AS PER IS 800: 2007 Members Total failure due failed to member slenderness 108 54 39 23 59 38 53 32 72 36 58 52 88 72 63 53 39 35 35 35 92 92 222 222
International Journal of Earth Sciences and Engineering ISSN 0974-5904, Volume 04, No 06 SPL, October 2011, pp 642-644
Member failure due to capacity 54 16 21 21 36 6 16 10 4 0 0 0
Maximum Deflection (mm) 96.56 51.42 40.11 36.21 544.32 5.302 55.63 14.02 2.76 1.37 9.65 7.67
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C. B. Joshi, N. K. Arora
Fig. 2 Comparison between percentage members failing in slenderness due to IS 800:1984 and IS 800:2007
1) None of the billboards are satisfying the design criteria and hence they are unsafe. 2) Members failing in slenderness are much higher than members which are failing in capacity. Thus sslenderness of the member is majorly contributing to its failure. Hence there is a need to reconsider the configurations that are currently 3) Popular and suitably change them so that the structure becomes safe. 4) Wind loads are governing for most of the cases. However in few members, the earthquake loads are found to be governing. 5) The design results as obtained by the use of old and new IS 800 code are similar in most of the cases. REFERENCES
Fig. 3 Comparison between percentage members failing in capacity due to IS 800:1984 and IS 800:2007 CONCLUSION In present paper, a total of 12 billboards existing in various parts of city are analysed and designed as per IS: 800-1984 as well as IS: 800-2007. The following conclusions are observed:
[1] An Explanatory Handbook on IS: 875 Part 3-1987Wind Loads on buildings and structures. [2] A.P. Robertson, R.P. Hoxey, J.L. Short, W.A. Ferguson, S. Osmond, Wind Loads on Fences and Hoardings, James Cook University, 1998. [3] C.W. Letchford, Wind loads on rectangular signboards and hoardings, Department of Civil Engineering, University of Queensland, Brisbane, Australia, and Received 16 January 1998; accepted 7 July 1999. [4] IS 1893 (Part 1): 2002: Criteria for Earthquake Resistant Design of Structures. [5] IS: 875 (Part 3): Wind Loads on Buildings and Structures-Proposed Draft and Commentary. [6] Kobchai Poemsantitham, Interference effects from adjacent structures on wind-induced forces in large Billboard, Asian Institute of Technology.
International Journal of Earth Sciences and Engineering ISSN 0974-5904, Volume 04, No 06 SPL, October 2011, pp 642-644
