Part 2 Architecture 1 st yr. Understanding stresses in structures compression stresses
- 1. Part 2:Architecture First Year UNDERSTANDING STRESSES IN STRUCTURES- Compression Prepared By: Ar. Harshada A.Bramhe For 1st yr B’Arch Students
- 2. ENGINEERING MECHANICS • PHYSICAL SCIENCES THAT DEALS WITH THE FORCES THAT ACT ON A BODY. • A BODY: ANY PHYSICAL OBJECT EG. A BOX ,SAY ITS 2KG, WHICH MEANS THAT THERE IS A FORCE OF 2KG ACTING ON THIS BOX IN THE DOWNWARD DIRECTION. FORCE 2 KG.
- 3. LOADS ( FORCE) LIVEDEAD REACTIONS FORCE DEVELOPED AT THE SUPPORTS IN RESPONSE TO THE APPLIED LOADS
- 4. CONCEPTS OF STRUCTURAL ANALYSIS LOADS REACTIONS( a force developed at the supports in response to the applies loads EXTERNAL FORCES TENSION COMP RESSI ON INTERNAL FORCES( CAUSED BY THE EXTERNAL FORCES) TENSION COMPRESSI ON STRESS( INTERNAL FORCE DISTRIBUTED OVER THE ENTIRE CROSS SECTIONAL AREA OF THE MEMBER) always expressed as force per unit area STRENGTH OF MATERIAL ( VALUE OF STRESS THAT THE MATERIAL CAN CARRY) EVALUATE THE PERFORMANCE OF MATERIAL STRESS< STRENGTH OF MATERIAL THE MATERIAL WILL NOT FAIL but IF STRESS > STRENGTH OF THE MATERIAL IT WILL FAIL 1. 2 4 5. 3
- 5. What does it mean for a structure to carry loads? • A structure carries loads successfully if: The stresses caused by external forces are less than the strength of the associated materials.
- 6. All vertical forces are balanced If a trains comes and slams its breaks from left we need a horizontal reaction at support to counter balance that force created . We need to also balance all horizontal forces so that the bridge is in equilibrium
- 7. COMPRESSION
- 8. What is Compression? • When the material gets pushed from both sides and tends to shorten , we say it is subjected to compression the molecules of the material are forced together. • Since atoms in solids always try to find an equilibrium position, and distance between other atoms, forces arise throughout the entire material which oppose this.It is the compressive stress in the materials.
- 9. • Concrete and Ceramics typically have much higher compressive strengths. • Compressive strength requirements can vary from 2,500 psi for residential concrete to 4,000 psi and higher in commercial structures. • Compressive strength is higher in ductile and brittle materials. • Exceptions to this include fiber-reinforced composites such as fiberglass, which are strong in tension but are easily crushed.
- 10. Forms that resist Compression ARCHES COLUMNS ANTI FUNICULAR FORMS VAULTS ANTI FUNICULAR FORMS DOMES
- 11. Trajan’s column in Rome early 2nd cent A.D Example for Analysis of Compression • The 98 feet shaft was made of 19 cylindrical marble drubs Each was 11 feet in dia. • Each stacked over one another, each weighed 32 Tons.
- 12. Trajan’s column analysis on basis of material failure
- 13. Strength of marble Stress in Trajan’s col. Hence FACTOR OF SAFETY IS 48 hence structure is 48 times stronger than its own weight without the stone failing in compression
- 14. Trajan’s column analysis on basis of material failure The column will have to go beyond this figure to fail under its own weight, but before it gets to that height there is another stability failure that will happens which is called buckling.
- 15. A purely axial load will cause a column to deflect laterally this is called BUCKLING Stiffness of material Moment of inertia measure of the cross sections resistance to bending and is affected by the distribution of the material from the axis. Hence is much of the material is far from the axis then the resistance to bending is greater hence moment of inertia is greater.
- 16. For all columns with same cross sectional areas DIFFERENT COLUMNS seen in PLAN Observation: Even if all columns have same cross sectional areas they will fail in buckling at different loading conditions. This is due to varying moment of Inertia.They will not fail in compression as all 3 have same failure loads.
- 17. For short columns IF SHORT COLUM IS LOADED TO FAILURE THE MATERIAL WILL CRUSH IN COMPRESSION BEFORE IT FAILS IN BUCKLING. HENCE LENGTH HAS THE GREATEST INFLUENCE ON BUCKLING STRENGTH SINCE ITS IN THE DENOMINATOR OF EQUATION AND IT IS SQUARED. SHORTER COLUMNS ARE STRONGER IN BUCKLING
- 18. THE COMPRESSION MEMBERS HAVE A BULDGE AT THE CENTRE SINCE IN BUCKLING THE COLUMN BENDS MORE IN THE CENTRE THAN AT THE ENDS. IT HAS GREATER NEED TO RESIST THE BENDING HENCE IT IS THICHER IN THE MIDDLE
- 19. EFFECTIVE LENGTH OF COLUMN BY ADDING A LATERAL SUPPORT THE BUCKLING STRENGTH OF THE COLUMN INCREASE 250% AS THE ORIGINAL HT. OF COLUMN IS SPLIT IN 2. THE STRENGTH OF COLUMN IS DETERMINED BY THE LENGTH BETWEEN THE LATERAL SUPPORTS. LATERAL SUPPORT GIVEN BY FLOOR
- 20. WORLD TRADE CENTRE FAILURE THE WORLD TRADE CENTRE SUSTAINED THE IMPACT OF AIR CRAFT THEN WHY DID IT COLLAPSE ? THE STRUCTURAL COLUMNS WERE ALL ALONG THE EXTERIOR WALL AND THE CORE. IN BETWEEN THE TRUSSES SUPPORTING THE CONCRETE FLOORS ACTED AS BRACING REDUCING THE EFFECTIVE LENGTH OF COLUMN AND STRENGTHENED IT IN BUCKLING.
- 21. WORLD TRADE CENTRE FAILURE THE COLUMNS SUCUMMED TO THE WEIGHT OF THE BUILDING AND IT COLLAPSED TO THE GROUND DUE TO THE HEAT OF INTENSE FIRE, THE STEEL TRUSSES STARTED BENDING AND GAVE WAY AND STARTED COLLAPSING . DUE TO THIS THE BRACING OF COLUMNS DISAPPEARED AND THE EFFECTIVE LENGTHS OF COLUMNS INCREASED AND CREATED MORE STRESS AND BUCKLING. TILL THEY COULD TAKE LOAD OF BUILDING NO MORE, AND FINALLY THEY COLLAPSED.
- 22. HOW DIFFERENT MATERIALS REACT IN COMPRESSION
- 23. I.S. CODE FOR CONCRTE
- 24. FAILURE DUE TO COMPRESSIVE STRESSES
- 25. FAILURE DUE TO BUCKLING
- 26. References & Acknowledgement Lectures and videos :Understanding the World’s Greatest Structures: Science and Innovation from Antiquity to Modernity Professor Stephen Ressler United States Military Academy at West Point Lectures and videos : Youtube DartmouthX –The Engineering of Structures around us. Images from Google.com
- 27. Thank You This Presentation has been developed by Ar. Harshada A Bramhe( Academician) .It is in 3 parts for First year B’Arch students to understand the Basics of Stresses- in Structures.