ETABS 2016 Analysis of Mat Foundation ... Examples utilized: Examples on Page 143 and Page 642;...

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Transcript of ETABS 2016 Analysis of Mat Foundation ... Examples utilized: Examples on Page 143 and Page 642;...

  • ETABS 2016 – Analysis of Mat Foundation Assoc.Prof.Dr. Emre AKIN

  • Introduction

    In finite element analysis, the mat foundation should be meshed into parts which have approximately 1 m2 area. The multiplication of spring coefficient (ko; unit: kN/m

    3) and max. vertical displacement (δz,max., found by analysis) will then give the max. stress under the foundation (σz,max.= ko × δz,max.).

    ko→ unit: kN/m 3

    δz,max. → unit: m

    σz,max.→ unit: kN/m 2

    Check:

    σz,max. ≤ σz,allow.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Introduction

    The following mat foundation (for a 4-story building) will be modeled and analyzed by using ETABS. The thickness of the mat is selected as 0.80 m. The soil type is hard clay and corresponding modulus of subgrade reaction (ko) is assumed as 20000 kN/m3. The effective depth may be taken as 0.75 m. Materials: C25/S420. Note that cantilever portion of the mat is assumed to have a length of 0.60 m. (away from the column face at all sides). The allowable stress of the soil will be taken as 200 kN/m2.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

    The analysis was performed considering

    column loads corresponding to

    «1.4G+1.6Q» combination for the analysis

    of building, which yields highest axial

    column loads. The axial column loads was

    taken from the analysis of 4-story building.

    Note: You should take the axial loads of the

    columns and shear walls at the ground story

    of your own model. Hint: In your own

    project, you may distribute the axial loads

    of shear walls at the nodes where the the

    finite elements of shear wall are joining. Examples utilized: Examples on Page 143 and Page 642; Betonarme

    Yapıların Hesap ve Tasarımı; Doğangün, A.

  • Introduction

    The column axial loads at the base which were taken from «1.4G+1.6Q» load combination of the analysis for 4-story building are shown below. The mat foundation will be analyzed for these point loads at the column joints (Load Case: Dead).

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

    Note that each portion between the columns will be

    divided into 4×4 parts (meshing). This will provide

    approximately 1 m. dimension for each meshed part.

  • Modeling

    Click «File-New Model». In «Model Initialization» window, choose «Display Units» as «Metric SI» under «Use Built-in Settings with:» and click OK! «New Model Quick Templates» window will pop-up. In our example we will have 5 grid lines along both horizontal directions. Let us give a constant spacing of 0.6 m between the grids (we will correct this in the following stage). We have no stories, however we cannot write «0» for the number of stories; so we write «1» here. Fill the appropriate parts as shown below and click OK!

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    The model will be formed where only grids exist. Right click and choose «Add/Modify Grids..». In the following «Edit Story and Grid System Data», click «Modify/Show Grid System». You should correct the «X» and «Y Grid Data» in the «Grid System Data» window as shown below in order to reflect the real distances given three slides before.

    Click OK twice when you finish!

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    The grid system should now look as shown below. Go to the «Base –Z=0 (m)» level in the «Plan View» by using «Move Up/Down in List» arrows.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Let us define the material in the next stage, by choosing «Define» and «Material Properties». In the «Define Materials» window, click «Add New Material». The «Add New Material Property» window will appear. Here, choose the region as «User», material type as «Concrete» and click OK! You may fill in the values as shown below in the «Material Property Data» and click OK twice!

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Now we should define the section by choosing «Define», «Section Properties» and «Slab Sections». «Slab Properties» window will appear. Here, click «Add New Property». In the «Slab Property Data» window, choose slab material as «C25» which you defined in the previous stage. You may define the property data as «Mat». Choose «Shell-Thick» members for modeling type. The property data type and thickness should be «Slab» and «800 mm», respectively. Click OK twice!

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Then choose «Draw», «Draw Floor/Wall Objects», «Quick Draw Floor/Wall» or simply click .Draw the area members starting from the left bottom corner moving to the right each time. Note that section property should be the one that you have defined for the mat (in our case we defined this as «Mat»).

    At the end your model should look like as below. You may check, when you right click on any area.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Click «Set Display Options» button and activate «Labels» for either joint and shell assignments on «Set View Options» window. After you click OK, the labels for joints and shells will be shown. Here, the order of labels may not be proper.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Shell areas F2, F3, F9, F10, F11, F12, F13, F14, F15, F16, F18, F19 will be meshed. The number of meshing along the x and y directions are shown below for each shell label.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Activate the shell areas F2, F3, F18 and F19 first by simply clicking on these areas. Then click «Edit» and «Edit Shells» and «Divide Shells». In «Divide Selected Shells» window, choose «Divide Quadrilaterals/Triangles into» as 4 by 1 corresponding to the division operation along x and y directions.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Repeat the same operation for all similar shell areas by using proper numbers of division along the x and y directions as given in the table (two slides before). At the end your model should look like this. In order to arrange order of labeling properly, choose «Edit» and «Auto Relabel All». It will ask you that this operation cannot be undone; click «Yes» for this. Now all labels should be in an order starting from the bottom left corner as shown below.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Our shell members are only expected to bend about the horizontal x and y directions (in plan) and translate along the vertical z-direction when subjected to out-of-plane loads (loads along the z-direction). Therefore, we will define support conditions at the bottom of our model to eliminate the translational degrees of freedom in plan (along x and y-directions) and rotation about z-axis (twist). In order to do this, select all your model first (Ctrl+A). Then choose «Assign», «Joint» and «Restrains». In the «Joint Assignment» window, activate restraints for «Translation X», «Translation Y» and «Rotation about Z». Click OK!

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    We should also define springs under the shell areas to reflect the resistance of soil under the mat foundation when shell members are subjected to out-of-plane loads and displace along the z-axis. The spring coefficient should be 20000 kN/m/m2 (modulus of subgrade reaction, ko). In order or to define the springs under shell members, choose all your model (Ctrl+A). Then, choose «Assign» «Shell» and «Area Springs».

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Then, click «Modify/Show Definitions» on «Shell Assignment-Area Springs» window. The «Area Spring Properties» window will pop-up. Here, select «Add New Property». In the following «Area Spring Property Data» window, it is important to write 20000 kN/m/m2 for spring constant corresponding to local 3 direction (others should be zero). You may choose not to change property name. Click OK three times!

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

  • Modeling

    Next we should apply the vertical point loads transferred by the columns (axial loads corresponding to «1.4G+1.6Q» load combination). These loads and the joint label for each load are shown in the table below.

    Assoc.Prof.Dr. Emre AKIN

    ADU Civil Eng. Dept.

    First select the joint where the load is to be applied (start selecting joint «13»). Then choose «Assign», «Joint Loads» and «Force». In the following «Joint Load Assignment- Force» window, write «-403.60» kN for the «Force Global Z». This will apply 403.60 kN vertical downward load at joint 13. «Load Pattern Name» may remain as «Dead». Repeat the same procedure to apply loads to the other joints as given in the table.

    Joint 13 should be selected first!

  • Modeling

    At the end, if you choose «Display», «Load Assigns» and «Joint» when the 3-D screen is selected, you should see the applied loads as shown below.

    Assoc.Prof.Dr. Emre AK