ETABS 2016 RC Frame Example (Part 3)

Transcription

1 ETABS 2016 RC Frame Example (Part 3) Reference: G. Özmen, E. Orakdöğen, K. Darılmaz, «Örneklerle ETABS 2013, Örnek 4: Perdeli ve Çerçeveli Yapı», Birsen Yayınevi, İstanbul, 2014

2 Part 3 Definition of Equivalent Seismic Loads In order to define the equivalent seismic forces along both x and y-directions (which we calculated in Part 2), you should open your previous Etabs model and choose «Define-Load Patterns». First let us define loading along x-direction which will be applied directly at the mass center (EX, no eccentric loading). In «Define Load Patterns» window, write «EX» for the «Load», choose «Seismic» type and lastly click «Add New Load». Then on the same window, click «Modify Lateral Load» when «EX» was selected. The «User Seismic Loads on Diapragms» window will pop up. Here, write the lateral story forces which were calculated for the x direction in «Fx» column for each corresponding story. «Apply Load at Diphragm Center of Mass» should be selected (if it not selected as default). «0» should be written «Additional Eccentricity Ratio (all Diaphragms)» since there will be no eccentricity for «EX» load pattern. Click OK! (This operation is illustrated on the next slide)

3 Part 3 Definition of Equivalent Seismic Loads

4 Part 3 Definition of Equivalent Seismic Loads You should do the same operation for «EY» loading as shown below. Here the lateral forces that we calculated for y-direction will be entered in «Fy» column.

5 Part 3 Definition of Equivalent Seismic Loads You should also define eccentric loads. «EXP» load pattern which is applied in the x-direction at +5% eccentric point (on the y-direction) will be defined here. Do the similar operations, but write «0.05» for «Additional Eccentricity Ratio (all Diaphragms)».

6 Part 3 Definition of Equivalent Seismic Loads Similarly, define load patterns of «EXN», «EYP» and «EYN». Note that you should write «-0.05» for «Additional Eccentricity Ratio (all Diaphragms)» in case of «EXN» and «EYN». Hint: While forming eccentric load patterns (EXP, EXN, EYP and EYN), you may right click and copy the seismic loads from the «User Seismic Loads on Diapragms» window of non-eccentric load patterns (EX and EY). The load cases should be automatically formed after load patterns are defined. Check this by choosing «Define-Load Cases». You should see the «Load Cases» window as shown below.

7 Part 3 Definition of Equivalent Seismic Loads Lastly, we should define the load combinations which include equivalent seismic loads. Here only the definition of «G+Q+EX-0.3EY» and «0.9G+0.3EX+EY» combinations will be explained so as to be an example. Choose «Define-Load Combinations» and click «Add New Combo» in «Load Combinations» window. In «Load Combination Data» window, write «G+Q+EX-0.3EY» for the «Load Combination Name». «G» exists with a scale factor of «1». Now, click «Add» and change «Load Name» as «Q» (scale factor is «1»). Do the same for both «EX» and «EY». Note that the scale factor for «EY» should be changed as «-0.3». Finally click OK!

8 Part 3 Definition of Equivalent Seismic Loads Repeat the same procedure for «0.9G+0.3EX+EY» load combination. Here, change the scale factor as «0.9» for «G» at the initial stage. The scale factors for «EX» and «EY» should be «0.3» and «1», respectively.

9 When you finish definition of all required load combinations, save your model and run analysis by clicking Note: You should first know the local axes of members (columns, shear walls, etc.) in order to understand which internal force you will choose in the results. Click to see the undeformed shape on 3-D view. Then click and click «Local Axes» in «Set View Options» window under «Object Assignments-Frame Assignments». Click OK!

10 At the end, you will see the 3-D window as shown here. In ETABS local as identification: Red axis: Local «1» axis Green axis: Local «2» axis 2 Blue axis: Local «3» axis 1 3 Therefore, the forces that we will be interested in: For beams: Shear 2-2: shear forces Moment 3-3: bending moments Axial Forces («N» along axis-1) For columns: Shear 3-3 and Shear 2-2: shear forces Moment 2-2 and Moment 3-3: bending moments Axial Forces («N» along axis-1) The highest shear force will be considered in design of lateral reinforcements. Generally, you may expect to see the highest shear force in the direction parallel to the larger seismic load direction. For example, here in the direction «axis-2» in case of «G+Q+EX-0.3EY» combination. As you should remember from «Reinforced Concrete» lectures, we consider the most critical combination for «N-M 2-2 -M 3-3» that gives us the highest longitudinal reinforcement ratio.

11 After the analysis is finished successfully, you may see the internal force diagrams by simply clicking. For example, if you want to observe the distribution of bending moment in beams, we should select «Moment 3-3» on «Member Force Diagrams for Frames..» window. Note that «Combo» should be selected at the top to see the moment distribution under any load combination. In this example, let us see the moment distribution of beams under «G+Q+EX- 0.3EY».

12 If you want to see the distribution of internal forces on the members more clearly, you may click. For example, here if we select «2», we will see the moment distribution of members on the frame that is along 2-2 axis as shown below. In order to investigate one beam, right click on that member and a window will pop-up where shear 2-2 (V2) and moment 3-3 (M3) distributions will be shown. You can change the load combination to see the most critical one. You can change the load type. If you are interested in the highest value only, click this! You can move your cursor on the diagram to see the changing values. Right click on the beam here and the window on the left will pop-up!

13 Another example is shown here for a column under load combination of «0.9G+0.3EX+EY» combination.

14 You may also obtain all results by choosing «Display-Show Tables». For example, choose «Tables-Analysis-Results-Frame Results-Beam Forces» to get the internal forces on beams. A table will appear where all internal forces are provided for beams. On this table, if you right click and choose «Export to Excel», all these results will be exported to an excel sheet that will open-up. If this does not work, change the position of the table, then first press «Ctrl+A» to select all data and «Ctrl+C» to copy all data. Then paste this data in excel. In excel, you can see all internal forces on any member that you are interested and for any load combination that you want. The results will be presented on certain points along the members (stations). On excel sheet, for example, the internal forces at differents points (stations at 0.25m, 0.75m,..., m, 5m) on «Beam 10» at the second story are given under load combination of «G+Q+EX-0.3EY».

15 Although there are shortcuts to obtain the results for story drifts in ETABS, at this stage it will be better for you to observe the story displacements visually and calculate the relative displacements and drifts of each story manually. In order to see the displacements, click, select «Story8» and click OK! This will open the «Plan View-Story8» instead of 3-d view. In order to check the displacements, click and «Deformed Shape» window will appear. For example, if you want to see the displacements under «EX» loading, choose this load case when «Case» is selected and click either «Apply» or «OK». Lateral displacements are in units of mm. Rotations are in units of rad. When you move your cursor on the deformed shape in plan view, the displacements along x, y, z axes (Ux, Uy, Uz) and also rotations about x, y, z axes (Rx, Ry, Rz) are shown at the joints. You may only consider the main joints (totally 24 here) where the beams and columns (or shear walls) joins. The main lateral displacement that you should consider is Ux (along x- axis) under EX loading ( mm). Write down the Ux displacements of each main joint. Note that this value does not change from one joint to another for such a symmetric structural system under a lateral load applied at mass center (and when rigid diaphragm is assigned). However, as demonstrated in the next slide, this value will change for each joint under an eccentric loading (EXP, EXN, EYP, EYN). In case of loading along y-axis (for example under EY load case), you should consider and write down the Uy displacements (along y-axis).

16 For example, if you select «EYP» load case on «Deformed Shape» window, the following displaced plan will be shown where the Uy displacements changes from one joint to another.

17 You should do the same operations to take the lateral displacement at different stories. In order to navigate between different stories, you may use «up-down arrows». When you do that, you will see «Plan View» changes from «Story8» to other stories. Now that you have all the displacements at the joints of different story levels (under different load cases). You may calculate lateral drift ratios and check these according to the limit values provided in the seismic code. [Also, the «Graduation Project Group» will check soft story and torsional irregularities on their model] Especially for the «Graduation Project Group» : When the lateral displacements does not change at different joints on the plan (as it happens in case of «Ux under EX loading» and «Uy under EY loading»), this means that there is no need to check torsional irregularity. However, if the lateral displacements change between joints on the plan (as it happens in case of «Ux under EXP or EXN loading» and «Uy under EYP or EYN loading»), there may be a torsional irregularity since maximum and average displacements will be different.