Analysis of Trough Shaped Ferrocement Roofing Elements Using ANSYS

L.Andal, Assistant Professor, Civil Engg. Department, Velammal College of Engineering and Technology, Madurai, M.S.Palanichamy, Vice Chancellor, Tamil Nadu Open University, M.Sekar, Dean, College of Engineering Guindy, Chennai, V.Vanitha, Lecturer, Civil Engg. Department, Mepco Schlenk Engineering College, Sivakasi

Ferrocement is a composite material made up of cement matrix and reinforcement in the form of multiple layers of mesh. Ferrocement structures are flexible and strong, due to the fact that they are thin and the steel reinforcement is distributed widely throughout the mortar. Though the raw materials required for Ferrocement construction is easily available, it is not widely used in our country due to the non-availability of proper design guidelines/code books. This paper aims at the study on the behavior of Ferrocement beams using ANSYS, finite element software. Finite Element Models have been developed for the trough shaped ferrocement roofing unit. The results obtained using the finite element software is compared with the analytical results computed from the formulae available in the literature and also with the experimental results. Design charts are developed for ferrocement trough unit to serve as an aid for a prospective designer to arrive at a section to suit their requirements.

Introduction

Housing shortage is a well recognized problem in this world. As result of uncontrolled population growth, migration to urban centers and the decay of existing low cost housing units, the cumulative needs of housing of many countries are already beyond easy corrective measures. Development of new materials of construction, low cost housing and accelerated construction methods are some of the different measures that research workers and engineers have been attempting towards meeting this challenge. Studies on building costs show that roofs/floors account for as high as 20-25% of the total building cost and saving in these items will go a long way in reducing the overall cost of a building. Traditionally, either ferrocement or corrugated galvanized iron sheets or asbestos cement sheets are being used for low cost roofing. Ferrocement as a roofing component was conceived in this context. As a composite material made of wire mesh, cement and sand, ferrocement posses unique qualities of strength and serviceability. If a roof/floor system in ferrocement is evolved and adopted on a large scale, it will prove a fitting contribution to housing the millions of homeless in the country.

The following are the analytical and experimental works related to ferrocement. Andal et.al (studied the flexural strength of modified ferrocement with polymer mortar. 45 specimens were tested and the load deflection curves, first crack loads and ultimate loads were analysed to obtain the optimum percentage of polymer to be used in modified ferrocement. Expressions were derived for the plastic moment and First Cracking Moment. Ramesht, M.H.(2) compared various analytical procedures which have been developed to predict the ultimate moment of Ferrocement under flexure. Balaji Rao,(3)proposed two methods to predict the first crack and ultimate moments of Ferrocement elements and concluded that both the methods are found to give satisfactory agreement with test data. Mathews,et.al.(4) studied the analytical and experimental investigations of cracking load, ultimate load, deflection, crack spacing and crack width of hollow ferrocement roofing system. Ramachandra Murthy, D.S(6)studied the applications of Ferrocement as housing units, water tanks, grain silos, roofing components, irrigation channels, boats and marine structures. Damodar Maity, et.al (9)studied the deflection and stress behavior of nine different types of ferrocement roofing elements. From the parametric study, it was found that deflection is less in double T-type of element and more in case of trough shaped element. Principal stresses are less in the segmental shell element and more in case of inverted V–shaped element. On the basis of cost analysis it was found that the shell element is the most economical shape as a roofing element.

Moreover, the theoretical analysis of ferrocement trough shaped elements are very tedious and time consuming. So this paper deals with the analysis of ferrocement elements using ANSYS software.

Objective

The main objective of this work is to develop design charts for the Ferrocement trough units. Design aids are essential and will be of immense help to the majority of the users of ferrocement. Moreover, they will increase consideration of ferrocement as an alternative construction material in various applications. In view of this, design charts are developed using the results obtained from ANSYS for ferrocement trough shaped roof/floor system.

ANSYS (Version 8.0)

ANSYS is a general purpose Finite Element analysis software package. The software implements equations that governs the behavior of these elements and solves them all creating a comprehensive explanation of how the system acts as a whole. This type of analysis is used for solving problems which are too complex to analyze manually.

Details of Trough Element

Trough shaped section with lips as shown in Figure 1 is selected for the roofing element. The cross sectional details are given below.

• Span =3.0m
• Thickness of section, t =20mm
• Depth of the section, H = 100mm
• Angle of web with vertical = 59Ú
• Angle of web with horizontal = 31Ú
• Characteristic compressive
• Strength of mortar = 40 N/mm²
• Moment of inertia, Ig (mm⁴) = 5956199.0
• Neutral axis depth Y_b =47.5mm
• Section modulus Z (mm³) = 125393.66

Analysis Using ANSYS

The following steps are involved in analyzing the trough element using the finite element software package ANSYS

• Element selection
• Defining material properties
• Model creation
• Meshing
• Applying boundary conditions and loading
• Analysis
• Viewing results

The accuracy of the results depends upon the type of element used and the way in which the model is meshed. Among these steps the element selection and meshing are very important.

Elements Used

In modeling the trough unit the elements chosen were:

• SOLID 65 for modeling the mortar layers and
• SHELL 181 for modeling the chicken mesh and weld mesh layers.

The trough element modeled using ANSYS is shown in Figure 2

Calculation of Cracking Moment

• Method I: (Ref 3)
  where,
  f_r - modulus of rupture of cement mortar
• I_g – Gross moment of Inertia
• Yb – Neutral axis depth
  
  
• Method II: (Ref 3)
  
  
  
  
  where,
  f_rm - modulus of rupture ofmesh-mortar combination
  
  I_g – Gross moment of Inertia
  
  Yb – Neutral axis depth
  
  
• Method III :( Ref 4)
  
  
  
  
  where,
  f_cr - modulus of rupture of cement mortar
  
  I_g – Gross moment of Inertia
  
  Yb – Neutral axis depth
  
  
• Method IV :( Ref 1)
  
  
  
  
  M_cr = a x f^cu xZ
  
  where,
  
  α - Cracking Moment constant Characteristic compressive strength of mortar
  
  Z - Section modulus
  
  S = Percentage of reinforcement
  
  F = Percentage of flyash

Cracking Moment from ANSYS

The stress value from ANSYS is given below from which the Cracking Moment is calculated.

Stress at first visible crack = 14.740 N/mm²

Cracking Moment M_cr= 14.740 x (5956199.0 / 47.5)

= 1.857 KNm

Experimentally the Cracking Moment obtained at the first visible crack (Ref 8) was 1.075 KNm.

The comparison of Cracking Moment for 20 mm thick Ferrocement trough element by various methods with ANSYS results are given in Tables 1 to 4.

Development of Design Charts

Design charts are developed for ferrocement trough unit by varying the following parameters

1. Thickness of the section,t: 20, 25, 30 &35 mm
2. Characteristic compressive
   Strength of mortar, f_cu : 35, 40, 45&50 N/mm²
3. Mesh mortar parameter, x : 0.2 (4 layers of chicken mesh &1 layer of weld mesh), 0.25 (6 layers of chicken mesh &1 layer of weld mesh) & 0.30 (8 layers of chicken mesh &1 layer of weld mesh).

The calculation of mesh-mortar parameter "x" for the Ferrocement Trough Element with 4 Layers of Chicken Mesh and One Layer Weld Mesh is given below.


= 0.059 + 0.116
= 0.175
H" 0.200.

Design charts developed for ferrocement trough for various thicknesses of the specimen and the mortar strengths are given in Figures 3 to 8.

Conclusion

Based on the detailed analysis of Ferrocement trough units using ANSYS the following conclusions have been drawn:

• The cracking moment by method 4 is found to be in good agreement with both the ANSYS and Experimental results.
• The methods 1, 2, and 3 underestimated the cracking moment for the Ferrocement trough units.
• From the results obtained from ANSYS design charts are developed for the ferrocement trough unit. The design charts developed in this paper will serve as an aid for the designer to arrive at a suitable section.

References

• Andal, L. Palanichamy, M.s. Ponraj Shankar And Vaidyanathan, R.(2005) 'Behaviour of Ferrocement Flexural Member with Polymer Modified Mortar,' Proceedings of International Conference on Advances in Concrete Composite and Structures (ICACS-2005) Jan 6-8.
• Ramesht, M.H. And Vickridge, I.g.(1996) 'Faoferrs- A Computer Program for the Analysis of Ferrocement in Flexure,' Journal of Ferrocement Vol.26, No.1, pp.21 -31.
• Balaji Rao, K.(2002)'Estimation of Cracking and Ultimate moments and load-deflection behavior of Ferrocement Elements', National seminar of Ferrocement 18-20 Feb' 2002 @ Anna University, Chennai. pp.G1– G10.
• Mathews, M.S. Sheela, S. Seetharaman, P.R. And Sudhakumar, J.(1991) 'Analytical and Experimental Investigations of Hollow Ferrocement Roofing Units'.Journal of ferrocement. , Vol.27; NO.1, pp.1 -14.
• Perumal, P. And Vidhyanathan, R.(1987) 'Strength and Corrosion Resistance of Ferrocement slabs', Proceedings of National Seminar on Modern trends in building materials, Design & Construction, Institution of Engineers (India) Allahabad, Nov.13-15.
• Dr. Ramachandra Murthy, D.S. (2002) 'Low cost Housing in Ferrocement Technology,' National seminar of Ferrocement 18-20 Feb' 2002 @ Anna University, Chennai, pp. j1 – j8.
• Rao, P.K. (1992) 'Stress-strain Behavior of Ferrocement Elements under Compression,' Journal of Ferrocement Vol.22, No. 4, pp.343 – 352.
• Sujatha, T. (2005) 'Flyash based Ferrocement trough shaped roofing/ flooring element for low cost housing,' M.E. Thesis.
• Damodar Maity, Kalita, U.C. And Nazrul Imam (2002) 'An Investigation On the Shape of Ferrocement Roofing Elements,' Journal of Ferrocement Vol.32, No. 4, pp. 271-284.
• Naaman, A.E.'Ferrocement and Laminated Cementitious Composites,' Techno Press 3000, USA, pp.1471, 143 &145.
• ACI Committee Report No. 549 'State–of–the–Art Report on Ferrocement,' ACI-549 -97, American Concrete Institute, Detroit, Part 5, pp. 549.R-2.
• ANSYS Help Manual (Version 8.0).

From the Editor

Structural Engineering Research centre at Roorkee/Ghaziabad carried out R&D work on ferrocement under the able guidance of its director Prof. G.S. Ramaswamy from 1968 onwards and a very large number of applications were developed and technology released for commercial use through NRDC. The Centre supported the activities of international ferrocement information centre A.I.T. Bangkok by deputing its experts for conducting Try Programme and preparation of do-it-yourselfManuals on tanks, Bins, Roofs, Boats, Bio Gas Plants etc. Large number of papers and manual were produced and the pre–patents were file. The technologies developed are in use by leading engineering department, pre–cast component Producers.