Comparative Study Of Different Rock Anchor Systems.pptx

INTRODUCTION Objective and Scope The scope of this project is to have a comparative study of different rock anchors (i.e., active rock anchor system and passive rock anchor system) for

ensuring the stability of structures and limiting the lift-off (loss of contact) under earthquake load situation.

INTRODUCTION DOCUMENTS AND APPLICABLE CODES IS 456:2000

Indian Standard Code of Practice of Plain and Reinforced Concrete

IS 1893(Part 1): 2002 Indian Standard Criteria for Earthquake Resistant Design of Structures Part1-General Provisions and Buildings

IS 1786:2008

Indian Standard “High Strength Deformed Steel Bars and Wires for Concrete Reinforcement- Specification”

GENERAL DESCRIPTION STRUCTURE Building taken for analysis is a framed structure of 5 storeys consisting of 3x3 bays. Each storey is separated by 5m distance in height. While span of each bay is 6m in both mutually perpendicular horizontal directions. The structure is supported on raft foundation of 2m thickness. The centre core is kept open surrounded by shear wall. Also ground floor consists of peripheral basement wall. The thickness of slab at each floor is 0.2m, while thickness of

basement wall and shear wall was taken as 0.6m. Dimensions of beam and column were 1.2mx0.6m and 1.2mx1.2m respectively. Except ground floor other floors consists of brick wall of 0.23m thickness along the perimeter of

each floor.

GENERAL DESCRIPTION STRUCTURE The structure selected as above to approximately simulate the behavior of nuclear building raft of KAPP-3&4, in a manner wherein the centre core

represents the containment structures of nuclear building while outside bays represents other framed structures present on nuclear building raft. This simple assumption was taken up to do a comparative study of different rock anchors for ensuring stability of this structure and limiting the lift-off (loss of contact) under earthquake load situation.

GENERAL DESCRIPTION MATERIAL Following are different properties of concrete: 

Characteristic Strength:

Concrete Grade adopted for raft is having a characteristic 28 days compressive strength (cube) of 45.0 MPa (fck). 

Modulus of Elasticity of Concrete As per clause no.6.2.3.1 of IS 456:2000 the modulus of elasticity of concrete is, E= 5000fck i.e., 33540 MPa.



Poisson’s ratio for concrete is taken as 0.2.

GENERAL DESCRIPTION ROCK ANCHOR SYSTEM Active Rock Anchor Active rock anchors are prestressed rock anchor provided in raft to ensure the stability and

to limit the lift-off (loss of contact) under earthquake loading or loading arising due to seasonal buoyancy. These rock anchors remains in stressed condition even in absence of any event-induced loading. The initial prestressing force given to rock anchor pulls the

structure downward ensuring the stability and limiting the lift-off of structure in absence or presence of event-induced loading. For study of active rock anchors systems, prestressed rock anchors with a spacing of 3.0m (force mobilization of 175T per rock anchor) are provided along the perimeter of raft.

GENERAL DESCRIPTION ROCK ANCHOR SYSTEM Passive Rock Anchor Passive rock anchor start carrying load and controls the upward displacement of raft with

respect to the founding medium as soon there is separation of raft from the supporting medium under action of any event-induced loading. For study of passive rock anchors systems, passive rock anchors were distributed uniformly over the entire raft area in a rectangular grid pattern with a spacing of 3.0m c/c in both the directions. The passive rock anchors consist of Single ribbed reinforcement bar of 45mm dia. with grade of Fe500 (material conforms to IS 1786:2008). Analysis of the raft has been carried out with a free length of passive rock anchors of 1000

mm.

GENERAL DESCRIPTION ROCK ANCHOR SYSTEM Passive Rock Anchor Details of individual rock anchors are as follows:

Stiffness of rock anchor = (E * A) / Lf = 32434.61366 T/m Where, E = Modulus of elasticity

= 2.0e+05 MPa

A = Area of anchor bar= 1590.4mm2

Lf = Free Length

= 1000mm

Capacity of Rock anchor = A*fy = 81.09 T Where, fy = yield stress of steel = 500MPa

LOADS AND LOAD COMBINATION DESIGN LOADS Dead load (DL): Comprising of self-weight of the raft and the dead load of all the structural components of structure. Dead weight of floor finish at each floor is considered as 0.1 t/m2 and is applied as uniformly distributed load. Dead weight of roof finish at roof is considered as 0.25 t/m2 and is applied as uniformly distributed load. Dead weight of brick wall at each floor except ground floor is considered as 2.0 t/m3 and is applied along the

perimeter of each floor except ground floor.

Live Load (LL): Live load of 0.4 t/m2 is applied as uniformly distributed load on each floor, i.e. on slab at each floor and raft top

LOADS AND LOAD COMBINATION DESIGN LOADS Uplift Water Pressure (UL): Uplift water pressure of 6 t/m2 is applied as uniformly distributed load at the bottom of raft and Uplift water pressure of 7.5 t/m2 is applied as

linearly varying load at the basement wall.

Earthquake load (EL): According to IS 1893(Part 1): 2002 the Base Shear Force was calculated as show below:-

Zone Factor, Importance Factor,

Z=0.24 I=1.5

Response Reduction Factor, R=4.0

(for Zone IV from table 2) (from table 6) (from table 7, case no :-(ix))

LOADS AND LOAD COMBINATION DESIGN LOADS

Approximate fundamental natural period of vibration is given by Ta=0.09h/√d = 0.424264068 sec

(clause 7.6.2) (h=20m; d=18m)

Average Response Acceleration Coefficient is given by Sa/g=2.357022604

(clause 6.4.5)

Design Horizontal Seismic Coefficient is given by Ah=Z*I*Sa/(2*R*g) Ah=0.106066017

(clause 6.4.2)

LOADS AND LOAD COMBINATION DESIGN LOADS Base shear forces were multiplied by a factor of 2 to get the sufficient lift-off for comparative study of different rock anchors. The seismic forces thus obtained are distributed across 4

columns at each floor. Hence, Seismic Forces (in tonnes) at each column in both horizontal orthogonal directions is: @ Raft = @ 1st floor = @ 2nd floor = @ 3rd floor = @ 4th floor = @ 5th floor =

1.0 ELx 0.0 7.626122 26.426770 59.460233 105.707081 111.500190

0.4 ELy 0.0 3.0504491 10.5707081 23.7840932 42.2828324 44.6000759

LOADS AND LOAD COMBINATION DESIGN LOADS The seismic forces thus obtained have been applied to the building as equivalent static load at appropriate locations along the height of building.

Active rock anchor load (RAL): Due to prestressing of rock anchor a downward concentrated force of 175 ton is applied at a spacing of 3.0m c/c along the perimeter of raft.

LOADS AND LOAD COMBINATION LOAD COMBINATIONS For raft without any rock anchor system and with passive rock anchor system a load combination for worst possible condition is given by: -

Load combination (LC-1) - DL+UL+1.0 ELx + 0.4 ELy For raft with active rock anchor system a load combination for worst possible condition is given by: -

Load combination (LC-2) - DL+UL+1.0 ELx + 0.4 ELy +RAL Thus analysis of raft has been carried for above load combination.

METHOD OF ANALYSIS FINITE ELEMENT IDEALIZATION Modeling of Building 3-D Finite element model of the building including the raft has been developed using

commercially available general-purpose software NISA. The finite element model of the building including the raft is developed using the following types of element: (i)

3-D General Shell Element is used to model raft, floors, shear wall and basement wall.

(ii) 3-D Beam Element is used to model the beams and columns. (iii) 3-D General Spring Element is used to model foundation stiffness.

METHOD OF ANALYSIS FINITE ELEMENT IDEALIZATION

F.E Model of Building

METHOD OF ANALYSIS FINITE ELEMENT IDEALIZATION

F.E Model of Raft

F.E Model of Building (Cross-Sectional View)

METHOD OF ANALYSIS FINITE ELEMENT IDEALIZATION Modeling of Foundation Medium The modulus of sub-grade reaction is estimated as per Vesic's equation, which is as follows:  Er  Er B 4 ks B  0.65     12 2 1   Ef I f

Where, ks =Modulus of subgrade reaction in T/m3 B =Least width of footing in meter =18 m Er=Average modulus of elasticity rock =1.42 X 106 T/m2 Ef=Elastic modulus of concrete as per IS: 456-2000=5000 fck = 3.354 x 106 T/m2 If=Moment of inertia of footing in cross-section=Bd3/12 =12 m4 for raft thickness of 2m =1.5 m4 for raft thickness of 1m =Poisson’s ratio for rock =0.26

METHOD OF ANALYSIS FINITE ELEMENT IDEALIZATION Modeling of Foundation Medium The modulus of subgrade reaction calculated using above expression comes out to be (for raft thickness of 2m) , ks = 109071.956 T/m3 (for raft thickness of 1m) , ks = 47641.565 T/m3 The raft is analyzed considering the following stiffness values of springs in the vertical

direction:@ Corner nodes kz = @ Edges nodes kz = @ Inner nodes kz =

for raft thickness of 2m 61352.9752 T/m 122705.9505 T/m 245411.9010 T/m

for raft thickness of 1m 26798.3804 T/m 53596.7608 T/m 107193.5216 T/m

METHOD OF ANALYSIS FINITE ELEMENT IDEALIZATION Modeling of Foundation Medium The stiffness value of soil springs in the two horizontal orthogonal directions shall be taken as fh times that of the soil spring in the vertical direction where fh is given by fh = 2x(1-2)/ z Where, x, z= geometric constants that are functions of the dimensional ration L/B. Here L/B=1, =>x=0.9 =>z=2.05 fh = 0.818692682 Thus stiffness values of soil springs in the two horizontal orthogonal directions are:for raft thickness of 2m for raft thickness of 1m @ Corner nodes kx = ky= 50229.2319 T/m 21939.6379 T/m @ Edges nodes kx = ky= 100458.4637 T/m 43879.2758 T/m @ Inner nodes kx = ky= 200916.9274 T/m 87758.5517T/m

METHOD OF ANALYSIS FINITE ELEMENT IDEALIZATION Finite Element Model Details of Raft The FE model details are given below Number of Nodes [only for Raft]

:

169

Number of shell elements (NKTP-20) [only for Raft]

:

144

Number of spring elements (NKTP-38) [only for Raft]

:

169

Boundary Condition used in the Analysis UX, UY, UZ = 0 at the bottom of the spring elements

METHOD OF ANALYSIS APPLICATION OF LOADS Dead load of the structure is computed by NISA software based on the unit weight of the concrete. Uplift water pressure is applied as upward pressure on the raft and also on the basement wall as lateral pressure. Dead weight of brick wall is applied as concentrated load of 3.45 t on each node along the perimeter of each floor except ground floor (i.e., raft). Dead weight of floor finish at each

floor is considered as 0.1 t/m2 and is applied as uniformly distributed load on raft and all slab excluding topmost slab. Dead weight of roof finish at roof is considered as 0.25 t/m2 and is applied as uniformly distributed load at topmost slab.

Seismic forces are applied at column on each floor (except raft).

METHOD OF ANALYSIS LOAD COMBINATION WISE ANALYSIS Iterative Analysis of Raft without Any Rock Anchor System The raft is analyzed for the load combination LC-1 in an iterative manner, starting with

normal soil springs. When tension occurs in any of the soil springs, then these spring elements are discarded (i.e., the stiffness of those springs is made negligible). Iterative analysis continues in this fashion till convergence is achieved. The convergence of the

analysis in load combination is considered to be reached when the equilibrium of forces is met without any spring being discarded in two successive iterations. The total influence area of discarded soil spring elements gives the lift-off area.

METHOD OF ANALYSIS LOAD COMBINATION WISE ANALYSIS Iterative Analysis of Raft with Active Rock Anchor System The raft is analyzed for the load combination LC-2 in an iterative manner as discussed in

above slide. The convergence of the analysis in load combination is considered to have reached when the equilibrium of forces is met without any spring being discarded in two successive iterations. The area of lift off is computed from the influence area of those discarded soil springs.

METHOD OF ANALYSIS LOAD COMBINATION WISE ANALYSIS Iterative Analysis of Raft with Passive Rock Anchor System The raft is analyzed for the load combination LC-1. Here only compression spring stiffness is considered in the analysis. The raft is analyzed for the load combination in an iterative

manner, starting with normal soil springs. When tension occurs in any of the soil springs, the stiffness of those springs is modified to axial stiffness of passive rock anchors. If the induced tensile stress in the rock anchor springs exceeds 0.9fy, the rock anchor is considered to be yielded, then these spring elements are discarded and equivalent downward force corresponding to 0.9fy is applied back on the raft. Iterative analysis continues in this fashion till convergence is achieved, i.e., when no spring is discarded in two consecutive iterations. The percentage yielding is computed from the ratio of influence area of those discarded

RESULTS AND DISCUSSION

RESULTS AND DISCUSSION From Table-1 we get that in case of active rock anchors lift-off (loss of contact) was

limited to great extent than in case of without any rock anchors. While for passive rock anchors lift-off is of no concern and there is no yielding of any passive rock anchor. Also from Table-1 it can be concluded that maximum design bearing pressure values are generally highest for active rock anchor system while lowest for passive rock anchor system due to presence of prestress forces in raft in case of active rock anchor system. While maximum upward displacement of raft element was least for active rock anchor system. Maximum upward displacement of raft element got increased significantly when

thickness of raft was reduced. Raft thickness of 1m with lesser values of modulus of subgrade reaction has lesser lift-off than the raft with same thickness with modulus of subgrade reaction same as that of 2m

thick raft as stiffness of soil springs is reduced.

RESULTS AND DISCUSSION

RESULTS AND DISCUSSION

From Table-2 it can be clearly seen that maximum design bending moments are always greatest for active rock anchor systems while for passive rock anchor system it is the least. By reducing stiffness of soil spring maximum design bending moments got increased.

Displacement contours showing the lift-off of raft in various cases are shown in slide 34 to 42.

The variations of MXX and MYY along the diagonal of raft are shown in slide 43 to 60.

SUMMARY AND CONCLUSION SUMMARY There are some major differences in active and passive rock anchor system. Active rock anchor are always in stressed condition due to mobilization of prestress force even in absence of any uplift of raft. While passive rock anchor takes up load as soon as any uplift in raft occurs due to earthquake load or seasonal buoyancy. There exists a possibility of leakage of ground water up to top of raft while using active rock anchor system as it needs to be anchored on the top of raft, because prestressing of the cable is done after the construction of the whole raft. While chances of leakage of ground water reduces when using passive rock anchor system as passive rock anchor can be embedded up to first pour itself. Passive rock anchor system is more effective than active rock anchor system in ensuring stability of structures and limiting the lift-off (loss of contact) under earthquake load situation, as passive rock anchors can be distributed uniformly over the entire raft area in a rectangular grid pattern while active rock anchor are best effective when distributed along the periphery of raft.

SUMMARY AND CONCLUSION CONCLUSION Passive rock anchors are best alternative of active rock anchors due to following reasons: the lift-off (loss of contact) is limited to great extent by passive rock compared to active rock anchors Chances of leakage of ground water reduced as passive rock anchors can be embedded in first pour itself. Bearing pressure below the raft is reduced

SUMMARY AND CONCLUSION FUTURE SCOPE The observation and conclusion drawn from this study is specific to the structural system adopted. For general conclusion in this aspect there is a need to extend this work to other

different types of structural system.

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