Design of Compression Members Based on IS 800-2007 and IS 800-1984 by 4mD4ar

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  DESIGN OF COMPRESSION MEMBERS BASED ON
     IS 800-2007 AND IS 800-1984- COMPARISON
                               M. KRISHNAMOORTHY, D.TENSING
                 M.Tech (Structures) Student, PRIST University, Thanjavur
          Principal, ASL Pauls College of Engineering and Technology, Coimbatore
                           dtensing@rediffmail.com, m.krishk@yahoo.com
ABSTRACT: The design methodologies for the steel structures namely, working stress design method and limit
state design methods are briefly explained. The importance of limit state design method is highlighted. Columns
form the main component of a structure which serves the basic purpose of supporting and transmitting the
entire loads both vertical and horizontal for which the overall structure is intended to the foundation system.
Beams are generally subjected only to flexure about the horizontal axis whereas columns are subjected to axial
load along with bending moment about the major axis. The minor axis moment in columns are generally nil or
very nominal since in standard structural system, the columns are so oriented that the frames along the major
axis of the columns are moment resistant frames, and column bracings are provided in the frames along the
other perpendicular direction. This paper focuses entirely to the procedure involved in design of compression
members. Typical problem have been worked out using allowable stress design methods and limit state method
and comparative studies is made.

ALLOWABLE STRESS DESIGN                                   all time during the life of the structure for which is to
With the development of linear elastic theories in the    be built. The various primary loads and other
19th century the stress-strain behavior of new            secondary effects required to be considered for Indian
materials like wrought iron & mild steel could be         condition m while computing maximum stresses in a
accurately represented. The first attainment of yield     structure are mainly as follows
stress of steel was generally taken to be the inset of    a) Dead load b) imposed load or live load c) wind
failure. The limitations due to non-linearity and         load d) seismic load e) erection load f) Secondary
buckling were neglected. The allowable stress is          effects due to contraction or expansion resulting from
defined in terms of a “factor of safety” which            temperature      changes,     shrinkage,      creep    in
represented a margin for overload and other unknown       compression members etc.
factors which could be tolerated by the structure.        As a general approach, a structure is analyzed for all
                                                          the probable primary load cases and their
                       Yield Stress                       combinations are mentioned above. Only for special
Allowable stress =
                     Factor of Safety                     structures or under stringent conditions, the
                                                          secondary effects are considered in the overall
LIMIT STATE DESIGN                                        analysis and in the design of connections of the
An improved design philosophy to make allowances
                                                          structural components. While designing a structure
for the shortcomings in the “allowable stress design”
                                                          using the popular “Allowable stress design method”,
was developed in the late 1970’s and has been
                                                          the above load combinations are considered with an
extensively corporated in design standards and codes
                                                          individual load factor of unity. As per IS: 800-1984,
formulated in all the developed countries. Although
                                                          the permissible stress can be increased upto 33%,
there are many variations between practices adopted
                                                          whenever wind or seismic load is taken in to
in different countries the basic concept is broadly
                                                          consideration.
similar. The probability of operating conditions not
                                                          In the proposed Limit state method of design also the
reaching failure conditions forms the basis of “Limit
                                                          above load combinations are considered, but with
States Design” adopted in all countries. Ultimate
                                                          variable load factors called the “partial safety factor
limit states are those catastrophic states, which
                                                          for load as described in Table4. This variable load
require a larger reliability in order to reduce the
                                                          factors basically account the loading and thus enable
probability of its occurrence to a very low level.”
                                                          to use steel efficiently and economically in different
Serviceability limit state” refers to the limits on
                                                          structural systems.
acceptable performance of the structure.
                                                          Similarly, to determine the strength of the member to
LOAD AND LOAD COMBINATIONS                                be designed against the factored loads as described
 To design a structure, it is analyzed first for its
                                                          above, a reduction factor for strength called “partial
intended structural configuration and assumed
                                                          safety factor for material” is taken into consideration,
sectional properties against various loads individually
                                                          which accounts for uncertainty in material strength
and in combination with each other in a way by
                                                          and quality as well as manufacture tolerance. Various
which the structure may be subjected any time or at


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material safety factors as have been adopted in IS:   The detail design procedure of compression member
800-2007 are given in the table –5                    using allowable stress design method as per IS: 800-
                                                      1984 and also limit state design method as per IS:
                                                      800-2007 have been discussed with the help of
                                                      example and comparatives study as been done
DESIGN PROCEDURES




DESIGN A MEMBER SUBJECTED HAVING A                    Step 1: Type of the Section
SPAN OF 3M WHICH IS FIXED @ BOTH                         b 100                   d 178
ENDS                                                              9.259                31 .22
LSM (As per IS: 800-2007)                               t f 10 .8               t w 5 .7
Let us take ISMB 200 @ 254 N/m
    Area = 3233mm2                                    The section is Compact
    Depth (d) = 200mm
    Width of flange (b) = 100mm                       Step 2: Determination of Effective Length
    Thickness of the flange (tf) = 10.8mm,            Leff = 0.65 x 3000 = 1950mm
    Thickness of the web (tw) = 5.7mm



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Step 3: Calculate the Slenderness Ratio
                                                                              Step3: Calculation of Compressive Stress
                       KL l x 1950                                                                             σac = 89.4 N/mm2
                                     23 .43
                       rx   rx 83 .2
                                                                              Step 4: Load Carrying Capacity
                                                                                                        σac x Area = 289.030 kN
                       KL l y 1950
                                    90.69
                       ry   ry 21.5                           COMPARATIVE STUDY
                                                              In this study we have compared Columns fixed at
Step 4: Determination of Non Dimensional                      both ends, column fixed at one end and hinged at
                                                              other, column pinned at both ends for a column
                            2                                 length of 2m, 2.25m, 2.5m, 2.75m, 3m, 3.25m, 3.5m,
                     KL 
                 fy                                         3.75m & 4m and also Graphical study has done for
        fy             r         250 x 23 .43 2             the Strength Vs Section and Strength Weight Ratio
x                                              0.2638
        f cc         E
                      2
                                     x 2 x10
                                      2        5              Vs Section. The Fig. 1, Fig. 2, Fig. 3 show the
                                                              comparative study of columns fixed at both ends of
                                                              2m, 3m, and 4m length. The Fig. 4, Fig. 5 and Fig. 6
                              2
                       KL                                   show the comparison between the S/w ratio Vs
                  fy                                        Section for a length of 2m, 3m and 4m. Similarly the
                       r 
                                                    2
          fy                         250 x 90 .69
y                                                  1.020 Fig.7 and Fig. 8 shows the section Vs the constants
         f cc         2E              2 x 2 x10 5           like stress reduction factor, Ф and effective
                                                              slenderness ratio.

                                                                              COMPARISON OF LOAD CARRYING

  0.51     0.2  2 
                                                                              CAPACITY VS DIFFERENT SECTIONS

  0.51  .34 1.02  0.2  1.02 2   1.16
Step 6: Calculation of Stress Reduction Factor
                       1                               1
 
              2
                          
                           2 0.5
                                      
                                           [1.16  (1.162  1.022 )]
                                                                      0.58

Step 7: Determination of design Compressive Stress
fcd

              f y /  mo                          fy fy                                            Fig.1
f cd                                                      132.64
            2  2 
                                             =
                             0.5
                                                  mo  mo

Step 8: Determination of Compressive stress P d

Pd  A  f cd  428 .82 kN

WSM (As per IS: 800-1984)

Let us take ISMB 200 @ 254 N/m
                                                                                                    Fig.2
                                       2
         Area = 3233mm
         Depth (d) = 200mm
         Width of flange (b) = 100mm
         Thickness of the flange (tf) = 10.8mm
         Thickness of the web (tw) = 5.7mm

Step1: Determination of Effective Length

          Leff = 3000 x 0.65= 1950

Step 2: maxleff/rmin = 83.33                                                                    Fig.3



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                                                          strength is two degree binomial. On comparison of
COMPARISON BETWEEN THE STRENGTH                           the strength of sections calculated using old and new
WEIGHT RATIO VS SECTION                                   code, it was found that the strength increases with
                                                          increase in size of the sections to the maximum of
                                                          15%
                                                          From Fig.4, 5 &6, it was found that for ISMB
                                                          100,125 and 150 the strength-weight ratio was
                                                          approximately the same. For ISMB 150,
                                                          175,200,225,250 & 300 strength-weight ratio was
                                                          found to increase with increase in size of the sections.
                                                          For ISMB 300,350 and 400 the strength-weight ratio
                                                          remains the same and for ISMB 400,450,500,550 and
                                                          600, it was found to increase with increase in size of
                                                          the sections.
                                                          Fig. 7 & 8 shows the curves drawn for the Stress
                                                          Reduction factor, inclination of tension field and
                                                          effective slenderness ratio with respect to different
                                                          Indian Standard Medium Beams.


                        Fig: 4




                                                                                  Fig: 7



                        Fig: 5




                                                                                  Fig: 8
                                                          CONCLUSION

                                                          1. The load carrying capacity of the compression
                                                          members as per IS 800-2007 is controlled by ‘stress
                                                          reduction factor, inclination of tension field stress in
                        Fig: 6                            web and effective slenderness ratio. The slenderness
From the chart it was found that the best fit curve for   ratio is inversely proportional to the stress reduction
describing the behavior of steel sections with respect    factor. The design compressive stress is directly
                                                          proportional to ‘stress reduction factor’.


ISSN: 0975 – 6744| NOV 11 TO OCT 12 | Volume 2, Issue 1                                                Page 76
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2. In IS 800-1984 for the design of compression
member is controlled by slenderness ratio which is
inversely proportional to the permissible stress in
axial compression.
3. The percentage increase in load carrying capacity
as per IS 800-1984 is marginally higher than IS 800-
2007. The maximum increase was found to be a
maximum of 5%.
4. The behavior of steel sections with respect to load
carrying capacity follows two degree binomial curve
for the design of sections as per both the codes.
5. The behavior of steel sections under strength-
weight ratio is controlled by the weight per unit
length.
6. The load carrying capacity of built-up columns
using ISA sections for various back to back widths as
well as for various lengths were found to vary for
smaller sections and for higher sections the values
become same irrespective of change in widths or
lengths.
REFERENCES
1.Arijit Guha and Dr.T.K. Bandyopandhya,
“Structural Member Design Based on Draft IS: 800
(Limit State Method), Insdag’s steel journal”,
Institute for steel development & Growth, Jan 2004,
Volume5.
2. N. Pandian, Arul Jayachandran, S. Seetharamal,
“Structural Efficiencies of New Indian Wide Flanged
Sections Compared With the Existing Rolled
Sections”, Insdag’s Steel Journal, Institute for steel
development & Growth, Jan 2004, Volume5.
3.Rangachar Narayanan, V.Kalayanarman, etal
“Teaching Resource on Structural Steel” Design
Volume 1 of 3, Institute For Steel Development &
Growth.
4.Indian Standard General Construction in Steel-
Code of Practice “IS: 800-2007”, December 2007.
5.Indian Standard General Construction in Steel-
Code of Practice “IS: 800-1984”.




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