UNIT IV 2 BEHAVIOUR OF STRUCTURES AND SOIL.pptx

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1. UNIT IV BEHAVIOUR OF STRUCTURES AND SOIL Performance of structures during past earthquakes – Lessons learnt from past earthquakes – Liquefaction – Resistance against liquefaction – Soil types and strength – Soil-structure Interaction (SSI) effects

2. Loads Static Dynamic Dynamic Periodic SHM Non-periodic / Random

3. Before that … What is Cyclic load? Cyclic loading is defined as the continuous and repeated application of a load (fluctuating stresses, strains, forces, tensions, etc.) on a material or on a structural component that causes degradation of the material and ultimately leads to fatigue. Cyclic loading causes materials to deteriorate due to fatigue, often at lower loads and after a shorter time than normally expected.

4. Cycle loading is a type of dynamic loading. This study of cyclic loading is useful. why? because capacity estimation studies and cyclic loads offer a way to estimate the capacity of structure from that we can estimate a number that will how much load the structure can resist in elastic and inelastic Ranges.

5. Contd., Fatigue is appropriately defined as the progressive and localized damage of a material’s molecular structure, which occurs when the material is subject to cyclic loading, eventually weakening the material. The repeated stress, strain, force and tensile load acting in such a situation is one of the reasons why the load needed for fatigue failure is far less than the ultimate tensile strength of the material .

6. Different types of fatigue/cyclic loading zero-to-max-to-zero Example: chain used to haul lugs behind a tractor. Varying loads superimposed on a constant load suspension wires in a railroad bridge Fully-reversing load A rotating shaft with a bending load applied to it

7. Masonry structures-is it important? predominant building material for 43.7% of the walls - “Burnt Brick ” 32.2 % “Mud and Unburnt Brick ” Designed for vertical load acting on them in the form of dead and live loads effect of lateral thrusts that is produced during earthquakes are normally ignored . gap in availability of experimental data for evaluation of degrading nature and hysteretic behaviour of the masonry walls under reversible loading

8. Performance of structures under dynamic loads: Masonry They are actually good in durability, fire resistance, heat resistance and formative effects . Causes of failure Very heavy mass of masonry structures attracts large inertia Unreinforced masonry so weak in tension that is due to horizontal forces and shear. Therefore, performs poor during earthquakes. Usually of low time periods (launch in plane rigidity) and results in large seismic forces Lack of integrity this is due to lack of through stones Absence of bond between cross walls Absence of diaphragm action

9. Contd., They are actually good in durability, fire resistance and heat resistance Causes of failure Very heavy mass of masonry structures attracts large inertia Unreinforced masonry so weak in tension that is due to horizontal forces and shear. Therefore, performs poor during earthquakes. Usually of low time periods ( large in plane rigidity) and results in large seismic forces Lack of integrity this is due to lack of through stones Absence of bond between cross walls Absence of diaphragm action

10. Contd., Masonry structures or masonry they are actually good in durability fire resistance heat resistance and formative effects Causes of failure Very heavy mass of masonry structures attracts large inertia Unreinforced masonry so weak in tension that is due to horizontal forces and shear. Therefore, perform poor during earthquakes. Usually of low time periods (launch in plane rigidity) and results in large seismic forces Lack of integrity this is due to lack of through stones Absence of bond between cross walls Absence of diaphragm action

11. Provisions to improve the strength of masonry structures Follow the Indian codal provisions IS 4326: 1993

12. Performance of structural materials under cyclic loads : Steel Example: A thin rod bent back and forth beyond yielding fails after a few cycles of such repeated bending. This is termed as the ‘fatigue failure’. The fatigue failure is due to progressive propagation of flaws in steel under cyclic loading. This is partially enhanced by the stress concentration at the tip of such flaw or crack. The stress at these points could be three or more times the average applied stress. These stress concentrations may occur in the material due to some discontinuities in the material itself. These stress concentrations are not serious when a ductile material like steel is subjected to a static load, as the stresses redistribute themselves to other adjacent elements within the structure.

13. Stress concentrations in the presence of notches and holes

14. Contd., At the time of static failure, the average stress across the entire cross section would be the yield stress. However when the load is repeatedly applied or the load fluctuates between tension and compression, the points m, n experience a higher range of stress reversal than the applied average stress. These fluctuations involving higher stress ranges, cause minute cracks at these points, which open up progressively and spread with each application of the cyclic load and ultimately lead to rupture.

15. Stress pattern at the point of static failure

16. Fatigue failure The fatigue failure occurs after four different stages, namely : 1. Crack initiation at points of stress concentration 2. Crack growth 3. Crack propagation 4. Final rupture Crack growth and fatigue failure under cyclic load

17. S-N curves and fatigue resistant design S-N curve, where the total cyclic stress (S) is plotted against the number of cycles to failure (N) in logarithmic scale.

18. Performance of structural materials under cyclic loads : Concrete Repeated compressive loading The envelope curve for cyclic loading could be represented by the response of concrete to monotonic loading .(Approximately) The residual strains are a function of the strain at unloading; an increase in unloading strain causes approximately the same increase in the accumulated residual strain . The unloading and reloading curves do not coincide and are not parallel to the initial loading curve. The average slope of the unloading and reloading curves is inversely proportional to the plastic strain. This result is based on the overall observations of several experimental results compared with available models. This suggests that there is stiffness degradation for the entire stress–strain beyond elastic .

19. Contd., Continuous degradation of the concrete is reflected in the decrease of the slopes of the reloading curves . Reloading curves are nearly linear up to the intersection with the unloading curve, after which there is a softening in the response. The shape of the unloading curve is strongly dependent on the location of unloading plastic strain rather than the envelope unloading strain . There is no additional strain accumulation in the partial reloading curve until the stress level exceeds a certain limit (stability limit ). Concrete exhibits typical hysteretic behaviour where the area within the hysteresis loops, representing the energy dissipated during a cycle, becomes larger as the unloading strain increases. Based on previous test results for full unloading and full reloading , and random cyclic loading, the envelope reloading strain is always greater than the envelope unloading strain regardless of partial or full unloading .

20. Performance of concrete- Repeated tensile loading In cyclic loading tests, one may define the envelope curves as the line on which both the starting points of unloading and the end points of reloading lie. Comparison of the monotonic loading curve in uniaxial compression with that in tension shows that the descending branch in tension immediately beyond the peak is considerably steeper than in compression and the ratio between the ultimate strain corresponding to the peak stress in tension is considerably larger . The unloading curve softens gradually while stress is decreasing and the stiffness of the unloading curve at a given stress level is smaller for larger strains . The unloading curve in tension becomes a loading curve in compression, which becomes stiffer with increasing compressive stresses .

21. Performance of structural materials - Soil influence of the geotechnical nature of the soil profile on the recorded ground surface motion . In general soft alluvial deposits - amplify the incident ground motion, especially in the low frequency range Example: Loma Prieta earthquake

22. Abstract of lessons learnt from Past Earthquakes

23. Liquefaction Introduction Liquefaction-Related Phenomenon Effects of Liquefaction Remedial Measures Conclusion Reference

24. During an Earthquake, large scale devastation occurs due to failure of buildings, dams and other structures. There are various factors resulting to this failure. In this paper we study one such Geotechnical factor causing large scale damages. We study a particular phenomenon called Liquefaction. Liquefaction is caused due to Earthquake.

25. This phenomenon was very little known until it drew the attention of Geotechnical Engineers in 1964 when a devastating earthquake occurred in Alaska followed by Niigata earthquake in Japan which caused huge scale damages due to Liquefaction including slope failures, bridge and building foundation failures. Liquefaction is a phenomenon by which loose saturated sand becomes liquid when rapid loading occurs under undrained conditions.

26. Solids Pore Water During Earthquake, the saturated sand is vibrated as a result of this, it tends to densify. As the particles tend to come close to each other, the excess pore water pressure increases and hence effective stress decreases.

27. The Mohr-Coulomb strength equation is given by, τ = c + σ ’tan Φ Where, τ is the shear strength, c is cohesion, σ ’ is effective normal stress, Φ is the angle of internal friction As the sand is cohesionless, c = 0, But, σ ’ = σ – u , where σ is total normal stress that depends on unit weight and u is pore water pressure. In case of loose sand, σ itself is very small and since there is a possibility of the excess pore water pressure developed during Earthquake being equal the σ, the effective stress σ’becomes zero. As a result of this the shear strength, τ = 0 and the sand becomes a liquid.

28. The effects of Liquefaction is divided into three-parts: Alteration of Ground Motion Development of Sand Boils Settlement

29. Non liquified Liquefied Pile Potential Effects of Subsurface Liquefaction on Pile. The development of positive excess pore water pressures causes soil stiffness to decrease during an Earthquake resulting into large displacement. These displacement may affect the buried structures, utilities and structures supported on pile foundations that extend through liquefied soils. Non liquified

30. Before Earthquake After Earthquake G.L Sand Boils The surface soils are often broken into blocks separated by fissures that can open and close during the Earthquake.

31. Liquefaction is often accompanied by development of sand boils. During and following Earthquake shaking, seismically induced excess pressure are dissipated predominantly by the upward flow of pore water. The upward pore water flow carries the solid particles and ejects at the ground surface to form sand boils.

32. The tendency of sand to densify when subjected to earthquake shaking is well documented. Subsurface densification is manifested at ground surface in the form of settlement. This type of settlement causes distress to structures supported on shallow foundations and damage to utilities that support the pile supported structures.

33. There are basically three possibilities to reduce Liquefaction hazards when designing and constructing new buildings or other structures as bridges, tunnels and roads. These are as follows: Avoid liquefaction susceptible soil Build liquefaction resistant structures Improvement of Soil

34. The criteria by which liquefaction susceptibility of the soil is judged include: Historic Criteria Geologic Criteria Compositional Criteria State Criteria

35. There are basically two aspects to construct liquefaction resistant structure. These are: Shallow foundations aspects Deep foundation aspects

36. A stiff foundation mat (below) is a good type of shallow foundation, which can locally transfer loads from locally liquefied zones to adjacent stronger ground.

37. Liquefaction can cause large lateral loads on pile foundations. Piles driven through a weak, potentially liquefiable, soil layer to a stronger layer not only have to carry vertical loads from the superstructure, but must also be able to resist horizontal loads and bending moments induced by lateral movements if the weak layer liquefies. Sufficient resistance can be achieved by using piles of larger dimensions and/or more reinforcement.

38. The main goal of most soil improvement techniques used for reducing liquefaction hazards is to avoid large increases in pore water pressure during earthquakes. This can be achieved in the following ways: Vibro floatation Dynamic Compaction Compaction Piles Compaction Grouting

39. Vibrofloatation involves the use of a vibrating probe that can penetrate granular soil to depths of over 100 feet. The vibrations of the probe cause the grain structure to collapse thereby densifying the soil surrounding the probe. To treat an area of potentially liquefiable soil, the vibrofloat is raised and lowered in a grid pattern.

40. Densification by dynamic compaction is performed by dropping heavy weight of steel or concrete in a grid pattern from heights of 30 to 100 ft. it provides an economical way of improving soil for mitigation of liquefaction hazards.

41. Installing compaction piles is a very effective way of improving soil. Compaction piles are usually made of prestressed concrete or timber. Installation of compaction piles both densifies and reinforces the soil. The piles are generally installed in a grid pattern and are generally driven to depth of up to 60ft.

42. Compaction grouting is a technique whereby a slow flowing water/sand/cement mix is injected under pressure into granular soil. The grout forms a bulb that displaces and hence densifies the surrounding soil. It is a good option if the foundation of an existing building requires improvement since it is possible to inject the grout from the side or at an inclined angle to reach beneath the building.

43. Thus, alteration of ground motion, development of sand boils and settlement are the effects of liquefaction which can be reduced by avoiding liquefaction susceptible soils, building liquefaction resistant structures and by densification of soil. Liquefaction resistant structure include shallow and deep foundation aspects while soil improvement techniques include vibroflotation, dynamic compaction if soil, installation of compaction piles, compaction grouting.Thus in these ways the liquefaction related hazards can be reduced to a great extent.