PARAMETRIC STUDY OF DIFFERENT GEOMETRIES OF STEEL TRUSS AND OPTIMIZATION.

BY: KARAN SHAH KATHA DESAI

GUIDE :Dr. A.S SANTHI

CONTENTS

INTRODUCTION GEOMETRIC DEFINATION MODELLING DESIGN RESULTS OBSERVATIONS CONCLUSIONS REFERENCES ACKNOWLEDGEMENTS

INTRODUCTION

  There has always been a hue and a cry that the world is running out of natural

resources. Analysts and researchers over the globe predict that steel which is an abiotic natural resource would get depleted by the year 2060 looking to the current scenario of production as shown in fig1.1).

Industrialization is the key to the growth of every nation and in turn distinguishes the human race. Steel is a key component material used for long span roof trusses which are specially constructed to give a column free space. Apart from roof trusses steel also forms an integral part of all RCC structures, frame structures, towers etc. Looking to the irreplaceable structural need of steel need arrises to optimize the structures.

We experimented with the geometry of roof truss taking standard shapes like fink fan and n-type and creating a geometry which would be combination of both.

All the plan dimensions were maintained for all the three geometries. These geometries were tested against same dead, live and predominant wind load .The analysis and design is carried out and weight of structural steel is proposed to be calculated. 

Our attempt for conserving this integral and invaluable natural resource.

•MODEL-1•MODEL-2•MODEL-3•DIMENSIONS OF STRUCTURAL ELEMENTS

GEOMETRIC DEFINATION

MODEL-1

•A flink fan type truss with plan area of 45m x 30m and the purlins are spaced at 1.38m as shown in the figure.

•The space truss have a slope of 1 in 5 and each truss spaced at 4.5 m centre to centre distance.

• The truss is supported on ends only with hinged support making effective span as 30m.

Plan

Cross section

MODEL-2

•A N-type truss geometry is shown in figure. It is having the same dimensions in plan as in the case of model 1 as shown in figure. Plan area of 45m x 30m and the purlins are spaced at 1.38m.

•The space truss have a slope of 1 in 5 and each truss spaced at 4.5 m centre to centre distance.Plan

Cross section

MODEL-3

• Combined truss geometry with plan area of 45m x 30m and the purlins are spaced at 1.38m as shown in the figure.

•The space truss have a slope of 1 in 5 and each truss spaced at 4.5 m centre to centre distance.

• The truss is supported on ends only with hinged support making effective span as 30m.

Plan

Cross section

DIMENSIONS OF STRUCTURAL ELEMENTS

Both the models have been analysed with the following dimensions of structural elements.

Outer frame: 2ISA 75x75x6 LD Tie runners: ISA 75x75x6  

Purlins: ISMC 100

Inclined members: ISA 50x50x6

1. Outer frame

4. Tie runners

2. Purlins

3. Inclined members

MODELLING

MODELLING

All the supports of the truss were considered as hinged. The three models were analyzed with dead load,live load and wind

load. Dead and live load were taken as 0.136 kN/m and 1.02 kN/m

respectively. Self weight factor was considered as 1. For wind load, the recommendations were taken into consideration

according to ASCE-7-2010. Wind load was applied in lateral direction on one side of the truss which was auto generated considering height of the truss. For that the wind speed and exposure factor were taken as 85 mph and 1 respectively. Exposure type was considered as B.

Further manual loads in the form of upward pressure at all nodes of principal rafters were applied which would govern the design in case of roof truss. Nodal load of 7.452kN was applied in local Y direction to all the principal rafters.

The directions of loads can be seen as in figs 3.1 upward thrust due to wind 3.2 dead loads and live loads 3.3 lateral wind load as per ASCE.

fig 3.1 fig3.2 fig 3.3

DESIGN

DESIGN The static analysis was carried out for the given loads. basic load cases are considered as :-

1) wind load

2) dead loads including self weight as 0.136 kn/m on purlins as a UDL in the downward (gravity) direction.

3) live load of intensity 1.02 kn/m on purlins as a UDL in the downward direction.

The basic load combinations considered are given below:

1) (1.5*dead load)+(1.5*live load) 5) 1.5*(wind load + dead load )

2) 1.2*(dead load+live load+wind load) 6) 1.5*(dead load-wind load)

3) 1.2*(dead load + live load –wind load) 7) 1.5*dead load

4) 1.2*(dead load + live load) 8) 0.9*dead load

1)

All the Indian standard angle sections were considered for the design.

The code checking was done using IS:800,2007 code. The method of design considered was limit state method. The permissible ratio of actual to allowable stress was

considered as <1. The allowable slenderness limit (L/R) for compression member

was taken as 180. The yield strength of the steel was considered as 250 N/mm2

and the beams were checked at every 1/12th section of their length.

RESULTS

TABLE-1

Truss Geometry Weights Optimized (KN) Weight (tonnes) Truss weight/area (Kg/sqm)

Fink Fan 256.29 25.62 18.98

N-Type 216.58 21.65 16.04

Combined geometry 209.21 20.92 15.49

TABLE-2

Truss Geometry Steel (tonnes)

%Reduction in steel by

combined geometry Cost (INR)

Cost savings by

combined

geometry (INR)

Fink Fan 25.63 18.37 1537740 282480

N-Type 21.66 3.40 1299498 44238

Combined geometry 20.92 1255260

OBSERVATIONS

OBSERVATIONS

From Table I, it was observed that combined optimized geometry was better than fink fan geometry in terms of steel consumption for steel roof truss.

Table-II shows the steel reduction of 18.37% with a cost reduction of INR 2,82,480 when compared to optimised geometry.

Table I showed that n-type truss was almost comparable to fink fan from the point of view of steel requirement and cost. However, compared to combined geometry it is slightly on the higher side in steel consumption and cost.

Table II indicated the relative decrease in costs and the amount of savings done by adopting the optimized combined geometry rather than standard n-type optimized steel. Reduction of 3.4% is obtained with cost savings of INR 44,238 is reflected on comparison of n-type with optimised geometry.

 

CONCLUSIONS

It can be concluded that the performance of steel roof truss structures which are under predominant wind loads, can be improved by providing a combined geometry of fink fan and n-type roof trusses rather than simply adopting them as per standard geometries.

REFERENCES

Auty, 1993 R.M. Auty,Sustaining Development in Mineral Economies (The Resource Curse Thesis) RTZ, London and New York (1993)

Journal Stahlbau (Steel Structures), published in German by Ernst & Sohn (1928).

Naeim, F. (2001) , The Seismic Design Handbook second edition , Kluwer Academic Publishers

Indian Standard Code of Practice IS-800:2007 SP-38(S&T)-1987 handbook of typified designs for

structures with steel roof trusses

ACKNOWLEDGEMENTS

OUR SINCERE THANKS TO A.S SANTHI MADAM FOR HER VALUABLE TIME AND HER PRECIOUS INPUTS IN THE PROJECT.

THANKYOU