laminated composite stiffened panels for bridge deck

International Conference on “Design & Construction of Urban Transport Structure”, 23rd – 25th April, 2010, Hydrabad, India , pp 487-506.

LAMINATED COMPOSITE FRP STIFFENED PANELS FOR BRIDGE DECK

Hemendra Kumar Jain1, Akhil Upadhyay2

1: M.Tech. Student, Computr Aided Design, Deptt. of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee – 247667. Email: [email protected]: Associate Professor, Deptt. of Civil Engineering, Indian Institute of Technology Roorkee, Roorkee – 247667.

ABSTRACT

Concrete bridge decks are subjected to severe environmental conditions and heavy traffic loads. They sometimes account for a major percentage of a bridge structure’s dead load. Alaminated composite FRP deck weighs approximately 80% less than a concrete deck. Reducing the dead load will increase the allowable live load capacity of the bridge without significant repair to the existing superstructure, thus lengthening its service life. The high strength, high fatique resistance, low density, and excellent corrosion resistance of composite panels are desirable characteristics for bridge application, especially for decks. Being a thin walled structure, their behaviour is governed by stability criteria. Accurate knowledge of critical buckling load and mode shapes are essential for reliable and lightweight structural design. In this paper, parametric studies have been carried out to understand the buckling characteristics of graphite epoxy hat and multicellular laminated composite stiffened panels for bridge deck when subjected to bridge loading actions. The effect of variation in fiber orientation and pitch length (number of stiffener) on buckling response has been examined in some detail using ANSYS 11. It is shown that variation in fiber orientation and pitch length can substantially improve the buckling capability of the bridge deck stiffened panels sections.

Key words: Bridge deck, Tracked loading, Multicellular, Hat stiffened panels, FE analysis.


1. INTRODUCTION

Since the early dawn of civilization, the strong and light material has always fascinated mankind for typical applications. Over the past decade, FRP is gaining popularity in civil engineering and many structural applications because of their high specific stiffness and specific strength. The idea of composite materials for structural member are formed by the combination of two or more materials that retain their respective characteristics when combined together to achieve desired properties (physical, chemical, etc.) that are superior to those of individual constituents. Composites are light in weight, possess high strength-to-weight ratio and high stiffness-to-weight ratio as compared to conventional materials. The high strength, high fatique resistance, low density, and excellent corrosion resistance of composite panels are desirable characteristics for bridge application, especially for bridge decks. Hence, lightweight and durable FRPs can be an excellent candidate for replacing concrete decks. Laminated composites are a special form of FRP which belongs to the new generation of energy efficient materials, almost dominating over the metallicmaterials. The potential of laminated composites offer several possibilities but on the other hand the mechanical characterization of a composite structure is more complex than that of metal structures.

In all cases of buckling of plates, critical loads are proportional to the flexural rigidity of the plates. Stability of the plate increases with the increase in the thickness of the plate but a more economical solution is obtained by keeping the thickness of the plate as small as possible and increasing the stability by introducing stiffeners. Being a thin walled structure the design of stiffened plates is governed both by stability and strength criterion. These panels are becoming increasingly used in structural applications because of their high specific stiffness and specific strength. Laminated composite stiffened panels, which are non homogeneous and anisotropic are gaining popularity in structural applications such as long span bridge decks, ship deck hulls and superstructure of offshore oil platforms. The use of laminated composite provides flexibility to tailor different properties of the structural elements to achieve strength and stiffness requirements. Laminated composite stiffened panels are generic structural elements in weight sensitive structure applications. Some various shaped stiffening members commonly used for panel structural concepts are “T”, “Z”, “I”, “C”, “J”, and hat. Blade stiffened panels which actually is a plate perpendicularly attached to the composite plate.

Many composite structure are thin-walled in nature, the assessment of their buckling behaviour is a predominant aspect of research. Mallela U. K. and Upadhyay A. (2006) presentedparametric studies on simply supported laminated composite blade-stiffened panels subject to in-plane shear loading. Knippers J. and Gabler M.(2007) studied a new option in the form of hybrid structures, i.e. steel girders combined with a pultruded FRP bridge deck. Based on this technology the design of a highway flyover was developed as a first major FRP road bridge in Germany. The innovative technology, its economical aspects and the design of the bridge are highlighted in this paper. Upadhyay A. and Kalyanaraman V. (2003) discussed the behaviour of FRP box girders and proposed a simplified computationally efficient method for the analysis of single cell FRP box –girder bridges made of blade, angle or T stiffened panels.

In this paper, parametric studies have been carried out to understand the stability analysis of graphite epoxy multicellular and hat stiffened panels for bridge deck subjected to IRC class AA tracked loading. All study carried out using finite element package ANSYS 11. The effects of variation in fiber orientation and increase in number of stiffener or cell on deflection and buckling response are also examined in some detail. An attempt has been made to achieve optimum panel configuration for bridge deck having practical dimension and subject to realistic loading.


2. MODELLING FEATURES OF STIFFENED PANELS

In the present work Eigen-buckling analysis is performed for the laminated composite multi-cellular and hat-stiffened panels for deck by using a finite element package ANSYS 11. Modelling laminated stiffened panels needs care in defining the properties of the plate and stiffener, number of layers, pitch length thickness and fiber orientations of each layer.

Out of the several elements available in the ANSYS library for modelling laminates, Shell91, which is designed to model thin plates and shell structures, is taken for the analysis. It is an eight-noded, quadrilateral nonlinear layered shell element, which has both bending and membrane capabilities and capable of taking up to 16 layers. It can sustain in-plane loads, normal loads and is suitable for large deformation effects. The element has six degrees of freedom at each node. Numerical studies are carried out on laminated composite stiffened panels made of graphite epoxy. Material properties are defined in Table 1. In table 1, subscript 1 denotes a direction parallel to the fiber orientation and subscript 2 denotes a direction transverse to the fiber orientation in plane of plates. Numbers of panels are modelled with the desired pitch length and the required number of stiffeners. All the panels are simply supported and at the supports out of plane deformations in the plate and the stiffener are restricted. Equivalent load corresponding to given tracked load is applied on all centre nodes of the panel. The buckling factor obtained by the analysis is multiplied with the intensity of loading to get the buckling load.

Longitudinally stiffened rectangular panels, simply supported along two opposite transverse edges or all the four edges, are dealt with. The panel may be either multicellular or hat stiffened panels (Figure 1). Each element in the panel is tailored out of a number layers of unidirectional lay-ups oriented at different angles to the longitudinal direction. In the present work three practical fiber orientations (0o, ±45o and 90o) are considered wherein the + and – 45 degree plies are always kept together and the other layers are arranged so as to maintain a balanced and symmetric lay-up configuration and thus avoid membrane bending coupling. Uniform pressure loading in thetransverse direction of panels are applied.

3. VALIDATION STUDIES

Deflection of simply supported plate is model in ANSYS11 compared with the analytical deflection as shown in table 2 and table 3. A close match in the values can be seen thus, validating the present approach.

Dimension of plate = a×2a Maximum deflection = (0.0101×p×a4) /D D = (E×h3)/[12(1-ν2)]

Now hare symbols are a=width of plate, p=pressure intensity, h=thickness of plate, E=modulus of elasticity of plate material, ν=poisson’s ratio of material, D=flexural rigidity of the plate.

EXAMPLE 1A square simply supported plate as shown in Figure 2. This problem was analyzed analytical

and in ansys. The geometrical properties are: Plate: side length (a) = 200 mm, thickness (t) = 15 mm. Plate is made of the isotropic material with modulus of elasticity (E) = 20,000N/mm2 and poisson’s (ν) = 0.3. The load is uniformly distributed over the plate with intensity (p) of 0.115N/mm2. The results are given in Table 2.

EXAMPLE 2


This example is identical to Example 1 by changing the geometric property of plate. The geometrical properties are:Plate: side length (a) = 100 mm, thickness (t) = 10 mm. The loading is uniformly distributed of 0.1N/mm2 intensity. The results for mesh 4×4mm for plate are shown in Table 3.

4. PARAMETRIC STUDY FOR DECKS

A stiffened panel may buckle in local or global (general instability) mode. Generally, for panels the local mode means buckling of the plate element between the stiffeners, while the global mode extends over more than one plate element between the stiffeners.

4.1 BRIDGE DECK PARAMETERS USED FOR ANALYSIS

Multi-cellular and hat stiffened panels have been modelled using ANSYS software. The dimensions of the panels have been kept constant as 3000 mm length, 1500 mm width and 200 mm depth. Only the pitch lengths are varied. Pressure is applied on the top surface to simulate the Class AA tracked vehicle loading. This loading consists of a tracked vehicle of 700kN. The load of one track is 350kN and equivalent intensity of load applied is 350/(0.85x3.6)=115 kN/sqm. Thesestudies are carried out on symmetric laminates of 0, ±45 and 90 degree fiber orientations for plate1–stiffener1, plate2–stiffener2 and plate3–stiffener3 combinations as defined in Table4. Some of the lay-up sequence as simulated in ANSYS 11 for plate and stiffener elements are shown in Figure 3.The numbering of layers starts from the far end as one to eight at the front. Weights of various stiffened panel used for the study are given in Table 5.

4.2 DEFLECTION CRITERIA OF FRP DECK PANELS

Figure 4(a)–(c) shows the variation of deflection with pitch lengths. From these figures, it can be observed that with the increase in number of stiffeners the deflection decreases in all the cases and for different fiber orientation this phenomenon is valid. Deflection of stiffened panels, it is the least for 0 degree fibre orientation and maximum being for 90 degree fibre orientations.Generally the acceptable limit of deflection is span/300 according to serviceability criteria so that up to the 10mm deflection is considered in safe limit. Deformation of FRP stiffened panels are within the acceptable limit for 0 degree fiber orientation and pitch length from 187.5 to 125mm. In 0 degree fiber orientation load is transferred in the longitudinal direction so deflection value is less as compared to other.

Thus it can be concluded that any configuration of pitch ranging from 187.5 to 125 mm and fiber orientation 0 degree is meeting the requirements of deflection. Hence, less pitch length and 0 degree fiber orientation are preferable from deflection point of view. In practical situation a combination of 0, ±45, 90 degree fiber orientation will be utilised and to control deflection 0 degree fiber layer should be in sufficient number.

4.3 BUCKLING LOAD FOR FRP DECK PANELS

Figure 5(a)–(i) shows the variation of buckling load/kg weight with type of stiffeners, fiber orientation and pitch length. From these figures, it is evident that the value of buckling load per kg for hat stiffened panels are greater than multicellular stiffened panels in all cases. However, since the panel width is 1500 mm, spacing less then 125 is not a practical option. The configurations 187.5, 150 and 125 mm are more practical in nature since the spacing is within acceptable limits from both manufacturing, workability and strength point of view.


90 degree fiber orientation are giving the slightly better buckling load per kg value. This happens because 90 degree fiber orientation tries to balance stiffness in both X and Z direction in presence of ribs.All pitch length show high values of buckling load per kg for 90 and ±45 degree orientations. 0 degree orientation is having the buckling load per kg less than half of ±45, 90 degree orientation for both multicellular and hat stiffened panels. with the increase in number of stiffeners it is noted that the buckling is more predominant in the webs due to reduction of width to thickness (b/t) ratio in the flange. Figure 6 and 7 shows the buckling mode shapes for the composite stiffened panel, subjected to class AA tracked vehicle loading.

5 CONCLUSION

Parametric studies on buckling behaviour of simply supported laminated composite multicellular and hat-stiffened panels subjected to class AA tracked loading are carried out by changing pitch length and fiber orientation the following conclusions are drawn:

The deformation of the FRP stiffened panel is within acceptable limits for 0 degree fiber orientation for pitch length 187.5 to 125 mm. Deflection point of view 0 degree fiber orientation is better then ±45 and 90 orientation.

185.5 to 125mm pitch of stiffners show high values of buckling load per kg for 90 and ±45 degree orientations but not for 0 degree fiber orientation.

For a box and hat stiffened panels pitch length 187.5mm to 125mm, the increase in weight is just 14 to 15kg whereas, the increase of buckling factor is almost double.Increase in weight of stiffened panels are negligible when compared increase in load carrying capacity.

In general we required both less deflection and high stiffness so mixed fiber orientation can be used to to achieve both. It is evident that there is immense scope for stiffened FRP decks to reduce self weight of a bridge. Also the utility of these types of materials is enhanced owing to theirhigh stiffness, strength and stability properties.

REFERENCES

1. Akhil Upadhyay and V. Kalyanaraman, “Simplified analysis of FRP box-girders,” Composite Structures, vol. 59, 2003, pp217–225.

2. C. B. York, F. W. Williams and D. Kennedy, “A Parametric study of optimum designs for benchmark stiffened wing panels”, Composites Engineering, Vol. 3, No. 7-8, 1993, pp. 619-632.

3. Dr.-Ing. Rudolph Szilard P.E, “Theory and Application of Plate analysis Classical Numerical and Engineering Methods”, John Willey & Sons, Inc., Hoboken, New Jersey, 2004.

4. Edward A. Sadek and Samer A. Tawfik, “A finite element model for the analysis of stiffened laminated plates”, Computers and Structures, vol. 75, 2000, pp 369-383.

5. Hwai-Chung Wu, Gongkang Fu, Ronald F Gibson, An Yan, Kraig Warnemuende, and Vijay Anumandla, “Durability of FRP Composite Bridge Deck Materials under Freeze-Thaw and Low Temperature Conditions”, J. Bridge Engg., ASCE, 11(4), 2006, pp 443-451.

6. Jan Knippers and Markus Gabler, “New Design Concepts for Advanced Composite Bridges-The Friedberg Bridge in Germany”, 2007.

7. Klaus-Jurgen Bathe, “Finite Element Procedures”, Prentice Hall of India Pvt Ltd, New Delhi, 2006.

8. R. M. Jones, “Mechanics of Composite Materials”, Scripta Book Co., Washington, D.C., 1975.


9. R. Thapliyal, “Stability analysis of multi-cellular FRP panels”, M.Tech Thesis, department of civil engineering, Indian Institute of Technology Roorkee, India, 2009.

10. Tang Benjamin, “A Successful Beginning for FRP Composite Materials in Bridge Applications”, FHWA Proceedings, International Conference, December 7-11, 1998, Orlando, FL.

11. Tang Benjamin, “Fiber Reinforced Polymer Composites Applications in USA”, U.S.A. Road Workshop Proceedings, USA, 1997.

12. Upendra K. Mallela, Akhil Upadhyay, “Buckling of laminated composited stiffened panels subject to in plane shear: A parametric study”, Thin-walled structure, vol. 44, 2006, pp 354-361.

13. Upendra K. Mallela, Rajeev Chandak And Akhil Upadhyay, “Laminated Composites for Structural Engineering – Perspective Application and Challenges”, International conference, Indian Institute of Technology guwahati, 2006.

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Table 1 Lamina material properties

E1 (MPa) E2 (Mpa) G12 (MPa) ρ (kg/m3) υ12 υ21

145000 16500 4480 1520 0.314 0.037

Table 2 Maximum deflection

Analytical Present approach a (ANSYS11) % error0.03006mm 0.03014mm 0.266

a 2×2 mm Meshes for plate.

Table 3 Maximum deflection

Analytical Present approach a (ANSYS11) % error0.00551mm 0.00552mm 0.181

Table 4Details of lay-up sequenceType of element

Lay up sequence

Layer number(starting from outside layer)

Thickness(mm) Fiber orientation (θ, degree)

Plate 1 1,5 1 02,6 1 03,7 1 04,8 1 0

2 1,5 1 452,6 1 -453,7 1 454,8 1 -45

3 1,5 1 902,6 1 903,7 1 904,8 1 90

Stiffener 1 1,5 0.5 02,6 0.5 0


3,7 0.5 04,8 0.5 0

2 1,5 0.5 452,6 0.5 -453,7 0.5 454,8 0.5 -45

3 1,5 0.5 902,6 0.5 903,7 0.5 904,8 0.5 90

Table 5Weight of stiffened panels

Pitch Length 187.5mm 150mm 125mm

Multicellular (Wt. in kg) 142.272 149.568 156.864

Hat(Wt. in kg) 97.584 104.88 112.176

Figure 1. Finite element model of composite multicellular and hat stiffened panel.

Figure 2. Simply supported square plate.

Plate 1 and Stiffener 1 Plate 2 and Stiffener 2


Plate 3 and Stiffener 3Figure 3. Lay-up sequence simulated in ANSYS.

(a) Plate1-Stiffener1 combination

(b) Plate2-Stiffener2 combination

(c) Plate3-Stiffener3 combination

Figure 4. Variation of deflection with pitch length for multicellular and hat stiffened panels.


(a)

(b)

(c)


(d)

(e)

(f)


(g)

(h)

(i)Figure 5. Variation of buckling load per kg with diffrent mode of buckling for multicellular

and hat stiffened panels.


Deform shape Contour plot

Figure 6. Buckling mode shape for hat stiffened panel.

Deform shape Contour plot

Figure 7. Buckling mode shape for multicellular stiffened panel.

laminated composite stiffened panels for bridge deck

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