Modeling of Masonry Structures Using ETABS

MODELLING OF MASONRY STRUCTURES USING ETABS 1. Saving the Autocad layout with .dxf extension

2. Running ETABS – from the bottom right menu choose the measurement units according with the self-habitude. For the lengths the units used in AutoCAD drawing it is obligatory. During the modelling process these units can be changed but usually, after each save and run option the program will return to the initials. There is an option to export the model as an “.e2k” extension text file, with the preferred units, to close than the model without saving and to import the “.e2k” file. Then save the file. In this way, the chosen units system will remain permanent. So pay a great attention when you’ll collect the results.

3. From the menu bar you must choose: FILE/IMPORT/DXF File of Architectural grid

4. A new window will open

5. Select - No

6. A new window will open in which you may find and select the “.dxf” extension drawing file you need to import.

7. After this selection, a new window will open, from which select the desired layers, holding [Ctrl] key pressed. To simplify the introduction of the structural walls you can select layers of axes and walls:

8. At this point in the two default ETABS windows the imported model drawing appears. By default it will have four levels each one with a height equivalent of three meters. Check that the import was done correctly, first like appearance and the dimensions:

The Import is correct

The Import is wrong – the unit length is chosen with a higher degree then the AutoCAD one

The Import is wrong – the unit length is chosen with a lower degree than the AutoCAD one

9. As for simplifying the data entry were selected as axes traces in ETABS (including the walls), is necessarily to reduce to zero the bubble size which label the axes. For this purpose double click on one of the axes:

10.A new window will open. Please select one of the axes:

11.A new window will open:

12.At Bubble the size must be zero and please put a check mark in the right boxes from “a Primary Line” and “Apply to all” options:

13.In the ETABS model the axes notations will disappear and the model is much easier to read and manipulate:

14.Taking into consideration the real number of the building levels you must keep or modify the initial data. Whether you want to introduce a number of additional levels or to delete a number of levels from 4 predefined levels, this will be mandatory in the range of 1 to 4. It is not recommended to add the levels above the 4th one because the axes will disappear. For this purpose enter Menu / Edit / Edit Story Data

If you’ll keep the number of levels and just to modify the heights you must enter in Edit Story. Here you can also to change the names of the levels. If you want to introduce additional levels enter Insert Story. If you want to delete levels please enter in Delete Story. You will select the level or the levels that will erase by holding pressed the [Ctrl] key and then select OK.

15.For example, removing two levels the following model form occurs:

16.To simplify the model data acquisition you may close the right 3D window by clicking the “X” which appear in the upper right of it: 17.You’ll obtain a single window - a plan view, in which both axes of the structural elements and the dimensions in the plan will occur.

18.For various reasons such as lack of axes you need it to define the structural vertical and /or horizontal elements. To enter them in ETABS you can use a little trick. Plot a FRAME element in the area where you want to introduce the axis, select this FRAME element and divide it into two parts:

19.Select the middle joint of the frame and insert a new axis, parallel to X in this case.

20.This operation will be repeated whenever you need, both on the X or Y axis by selecting from GRID the desired direction. Finally the plan with all the axes is complete (excluding the possible positions of the partitions - if there are no other structural elements in the same positions - beams in their area). Then you may delete FRAME lines artificially introduced for defining these axes.

21.After completion of "axes" continues to define the other input data. Choose the plan view and define the structural elements of the levels (preferably the latest) by selecting the "Set plan view" then the level.

22.You can select a new group of units, depending of designer experience (for example tf-m):

23.The materials used in modelling you must define by their characteristics, depending of the state limits you need for the calculations:

For example - CONC – the default concrete - you can change the name as you desire. Also will change in order from top to bottom: density, specific weight, elastic Young modulus E, Poisson's ratio (giving 0.25 - the program calculates its transverse modulus G) and coefficient of thermal expansion (if there are no calculations in this regard can enter zero).

You can introduce other materials, in this case the masonry:

24.As an important observation, although the trend of everyone is going fast forward, it is recommended that after every step of the way to save what was defined. For various reasons it can happen that software or computer to block and can lose everything. Initially using SAVE AS from the Menu/File and defines a name. Then use SAVE Menu/File or may use [CTRL +S].

25.In the next step you can define different types of structural wall using planar finite elements (2D). There are some predefined type items as WALL or SLAB of 25 cm thick reinforced concrete. So for example if you have two types of walls (25 and 50 cm) of masonry, first you may change the type of wall - WALL 1 – which is default:

26.Then you can define the other wall, with 50 cm thickness as an example. You have also the possibility to introduce two others elements, without to use de default one. The material must be masonry, defined before.

27.Also you can use the default SLAB 1 element, changing the thickness and eventually the material, according with the model:

If you just intend to make a structural computation, without the slab one, in a simplified manner you may choose to put a check mark at MEMBRANE. If the intention is to compute the slab also you must put the check mark at SHELL 28.In this moment you can introduce the structural wall (masonry or RC). First you must select a level where to define them (usually the last one). From the left menu bar select Draw Walls (Plan) and in the new open window you must select the WALL section you desire to use:

You can move the open window inside the screen surface in a convenient position to not disturb you. This you may apply to each interior windows.

29.You must position the cursor to the first joint (of the two that will be inserted to define the wall) and click on it:

Move the cursor to the second joint (in between you’ll define the wall) and click... and so on. When you fully defined a wall at the last joint you may give a double click (or left click right click), in order to move to the next wall.

You may repeat this operation in order to define all the walls with the same thickness.

30.To define the other walls with a different thickness you must select before from the opened window another property (Property) – for instance 50. To define these walls you may use the same manner described above.

31.To close the opened window you may click on the arrow button from the left bar menu (Select Object):

32.You may choose a 3D view to correct any mistake.

Then you may turn back in a 2D view window, to continue with the new data definition.

33.It will further define the type sections of beams columns, column ties, beam ties, beams etc. Select the MENU / DEFINE FRAME SECTIONS and introduce step by step the desired sections:

34.You must define the column ties positions in accordance with the pre-design rules. From the left vertical menu bar you must select „Create columns in regions or at clicks (plan)”:

35.A new window will open in which you may select the section as STALPISORI, for example:

36.You must position with a simple click the column ties step by step.

37.From the left vertical menu bar you must select then „Draw lines (Plan, Elev.,3D)” to position the beam ties or the beams. In the new open window you must select the section, CENTURI, as an example:

38.Define element positions fixing the cursor on the first node by clicking and moving on to the next node with one click. At the last node a double click (or left click, right click) you must use.

And so on until all elements are inserted

39.You must repeat all these operations, for different sections and elements.

40.You can further define the floor slabs. Select from the left toolbar "Draw Rectangular Areas (Plan View)”:

41.A new window will open from which you may select the 2D elements for the slabs:

42.You must select the 2D element you wish to use, for instance PLACA. You must define the slab panels, fixing the cursor on the first left bottom joint by a click and keeping the left mouse button push you will move the cursor until the right top joint. You can repeat the same operation style for each rectangular slab panel.

43.After these operations you must push the arrow from the left vertical bar menu „Select Object”:

44.You may select all the slab panels entering in MENIU/SELECT/by WALL/SLAB/DECK SECTIONS in function of the desire section (for instance PLACA) or in MENIU/SELECT/by AREA OBJECT TYPE using FLOOR selection.

Or

45.For a correct calculation is recommended to use the auto mesh option for the slabs.

46.Select the Auto Mesh Object into Structural Elements/Further Subdivide Auto Mesh with Maximum Element Size of…. For instance 0.3 m or 30 cm.

47.It will divide the FRAME elements in the finite element model by the position grids and walls. Use the shortcut by pressing [Ctrl] and then the [A] (which means select all) and then enter the MENU/EDIT/DIVIDE LINES which opens a new window, where you mark a check "Break with Intersections with selected lines and points".

48.You must declare the slab as a horizontal diaphragm by selecting all the joints from that level, starting from left bottom outside of the layout until outside of the layout right top:

49.From the top horizontal bar you must choose the DIAPHRAGMS icon and then D1 horizontal diaphragm

50.You may define all the load cases in MENIU/DEFINE/STATIC LOAD CASES. For instance DEAD=PERMANENTE, LIVE=UTILE, X=seism on X and Y=seism on Y.

A new window will open:

You may observe that the DEAD Self Weight Multiplier is by default 1 (one), to take into consideration the self-weight loads.

To define the seismic base coefficient you must select each load case separately (X and Y) with a click on „Modify Lateral Load”. For instance if CB=0.2. For X:

And also for Y:

51.You must introduce the vertical gravitational loads on the slab panels, previously calculated. You must have into consideration that the self-weight of the elements are computed by the program itself. The loads must be divided in DEAD, LIVE, SNOW, etc. As an example let say that the dead loads onto slab panels are 0.15 tf/sqm (1.5 kN/sqm) and a live load of 0.3tf/sqm. You can have different loads on different slab panels so you must pay attention during the defining. If all the slab panels have the same loads you can select all of them and assign the same load. Preferably the loads are introduced with normal values following that the coefficients will appear in accordance with the combinations.

From the MENIU bar you may select „Assign Uniform Load”

From the MENIU bar you may select „Assign Uniform Load”

A new window will open where you’ll select the load type DEAD=PERMANENTE or LIVE/UTILA or other load types previously described.

You must introduce the load values (according to measurement units). Pay attention to the loads direction.

Repeat the operation for each load type.

52.Define the loads combinations using COMBINATIONS

Continue with Add New Load:

MENIU / DEFINE / LOAD

As an example let’s use GF for gravitational loads in Fundamental Combination:

Then GS for gravitational loads in Special Combination:

Let’s use XP as a combination of GS previous combination and the X seism on positive sense:

Let’s use XN as a combination of GS previous combination and the X seism on negative sense:

Let’s use YP as a combination of GS previous combination and the Y seism on positive sense:

Let’s use YN as a combination of GS previous combination and the Y seism on negative sense:

Since when masonry wall structures, active areas for tensile or compression can be different in both senses of action for each main direction, the final model it will be copied by 4 different names and the envelope efforts are given properly.

As an example for the model where the x direction seism act in the positive sense.

Choose the Type – ENVE.

Finally the combination cases being:

53.You must define the level masses looking in MENIU/DEFINE/MASS SOURCE.

Please choose „From Self and Specified Mass and Loads” using DEAD with a coefficient 1 and LIVE with 0.4. If there are also different other loads you must count them.

54.All the introduced data until now are for just one level. For all the other levels, thinking that are similar (in a different case you must locally change the loads) you may select the entire level in a 3D view (using [CTRL] + [A] complete selection). From MENIU/EDIT/REPLICATE please select STORY and in our case STORY1..

The program will copy (replicate) all the selected data from the previous declared level to all the others (STORY 1 – in our case).

55.Attention should be given to supports of the building base - which must be set to FIX SUPPORTS. The program does not automatically consider in this way, so you have to go to the plane, select all nodes at the base (as we did on other occasions) or icon to select "Assign Restraints" on the bottom menu bar. It will open a new window in which to mark a check all the possible restrictions.

56.The final model must be with fix supports at the base.

57. In the next step will be split in finite element all the structural walls. Number and size of the FE (finite elements) will be determined so that they can easily declare assets compressed or tensed areas. For buildings with low and medium height system can accept finite elements with dimensions of about 30x30 cm.

58.Because all we import from AutoCAD seems to be axes for ETABS you must follow only the valid axes. As an example you may start with the first longitudinal axe which in this case is 100.

59.You may select the 100 elevation (as an example).

60.This is the 100 elevation.

61.If the plan you can measure dimensions, in elevation this cannot be done directly. You can however select "Draw Line" from the left menu bar. Without choosing a particular type of section.

62.You can fix at the pier left edge with left click and you may move to the right pier edge pushing a click.

63.Note that on the element type FRAME length is written, in the unit located in the bottom right corner. In this case, it is of 70.50 cm. To remain not select the frame element type, press the Escape key. If we choose the division into finite elements about 30x30 cm, we find that the size of per not divide exactly. How finite elements must not be absolutely equal, we can accept that the horizontal division of the element to be in two finite elements (each 35.25 cm – what we choose) or in 3 finite elements (each 23.5 cm). If the height is 300 cm (3m) we’ll divide the pier into 10 finite elements of 30 cm. We'll keep this division of pier heights for all the levels.

64.You may select the piers with more or less the same dimensions

65.From EDIT select please Mesh Areas.

66.At Mesh Quads/Triangles you declare in the first box 2 finite elements (on horizontal layout) and in the second box 10 finite elements (on the vertical direction) then push OK. Take care that if you mesh an element and then you save, it is impossible to renounce at this (the FRAME elements are not in the same problem)

67.In this moment the first pier is divided in proposed finite elements.

68.You may repeat the operation for each pier and elevation.

69.In this moment all the piers are divided in convenient finite elements.

70.In this moment all the piers are divided in convenient finite elements.

71.At this moment, because all the piers were divided in finite elements, we must do the same operation for Column and beam ties, beams and so on, in the idea to work together with the other elements.

71. From EDIT please select Divide Lines

72. You may mark as check the second possibility.

73. In this moment all the existing FRAME elements were divided like the piers.

74. In the followings the parapets and spandrels must be described.

75. Please select Draw Rectangular Areas from the left instruments bar.

76. Auto ownership the first property is chosen initialy. Suppose further that we forget to select the thickness of the wall 50 and we leave it on the 25.

77. We’ll start to declare the parapets and spandrels, in an order we wish.

78. In this moment in the 100 elevation we almost declare the parapets and spandrels positions. We must divide these elements.

79. Select all the 100 elevation

80. Select please EDIT, Mesh Areas.

81. Please mark as check the last two options and then push OK.

82. In this moment the parapets, spandrels and piers are divided in convenient finite elements like the tie columns and beams.

83. You must continue this operation for all the elevations.

84. Returning to the fact that initially we were wrong and we have no correctly selected a wall thickness of 50 cm for spandrels and parapets in some elevations, we must go back to those elevations, choose the correct property and select only those that finite elements thickness are wrong.

85. You can observe that in this moment all the wall finite elements have 50 cm thickness.

86. You may continue for to declare the spandrels and parapets for all the windows regions and spandrels for the door regions.

87. In this moment the model is complete, having all the wall piers, parapets, spandrels, column and beam ties.

88. Since dividing the piers in finite element the base restraints you must go to the base of the building, to change the restraints to fix supports.

89. Save the "witness" model - you keep itself in this form.

90. Save the model with 5 different names: One model for SLS displacements; one model for special combination with the positive X seismic action; one model for special combination with the negative X seismic action; one model for special combination with the positive Y seismic action; one model for special combination with the negative Y seismic action.

91. Suppose that you start with the model (s) for displacements. So open the saved model appropriately named "displacements"

92. You must introduce another concrete material with the same density and selfweight like the initial one but with half of the Young elasticity modulus. This will be for column ties, beams, and beam ties. The slab will remain with the initial one.

93. Modify the Young longitudinal elasticity modulus for the masonry material, to 500fk, in accordance with CR6 (Eurocode 6).

94. Change the material type for the FRAME elements.

95. Save the model

96. Run the model

97. The program finished the analysis.

98. From DISPLAY select Show Story Response Plot. Take care about the measurement units from the left right corner.

99. Select the load case – seismic action (X or Y) then Maximum Story Drifts

100. Using the cursor you may look at the values at each level. For instance, at the first level 0.0003726 meaning 0.3726‰.

101.

For instance, at the second level 0.0004081 meaning 0.4081‰.

102. Do the same thing for the Y seismic action. For instance, the drift at the second level is 0.0003998 meaning 0.3998‰.

For instance, at the first level 0.0003486 meaning 0.3486‰.

103.

104. These are relative rotations for each level that need to be changed because they are calculated with the seismic forces initially defined. The values obtained at the two levels, in SLS, must be multiplied by behavior factor q and a coefficient equal to 0.50 for 3rd grade of importance and exposure. In our case, assuming q = 3.125 (behavior factor) it results that the initial drifts you must multiply by 3.125x0.50 = 1.5625:

Level

Rotational drift for X seism from ETABS (‰)

Rotational drift for Y seism from ETABS (‰)

Rotational drift SLS seism X (‰)

Rotational drift SLS seism Y (‰)

2

0.4081

0.3998

0.64

0.63

1

0.3726

0.3486

0.58

0.54

You may observe that the drifts are smaller than the admissible values (accepting an allowable rotation in SLS about1.5 ‰ and 2.5 ‰ in SLU). These are different than the P100 admissible values, because in our case is about a masonry structure and not about frames with partitions or wall panels.

105. In the same idea you may observe other structural responses like total shear forces at each level, on both directions.

106.

Or the total overturning moments at each level, on both directions.

107.

Or the absolute displacements, at each level, on both directions.

108. You may observe the proper modes of vibration to determine which kind of displacements appear (translation or torsion).

109. Note, for example, the mass participation factors for instance the 1st mode in ETABS is the fundamental mode of vibration in the X direction, and the 2nd from ETABS is the fundamental mode of vibration in the Y direction (this means that there is little difference in stiffness between the two main directions of vibration) and the 3rd mode of vibration from ETABS it is the fundamental torsion mode.

110. To collect the efforts you must open the models, according to the direction and sense.

111. To be more easy you may hide some of the elements (for instance Floor Area and Beams), presented in the view. You may keep only the Walls and Columns.

112. If the building is uniform onto vertical, in a plan view you can select in one of the views „All Stories” from the bottom right menu.

113. In function of previous computations concerning the active zones for compression or tension, you may establish the names of the piers and you’ll declare them.

114. From the 3rd raw instruments bar you may select Assign Pier Label. Select all the finite elements of the pier one (for instance)

115. From the 3rd raw instruments bar you may select Assign Pier Label. Select all the finite elements of the pier one (for instance).

116. Note that the components of the first pillar was already fixed. You must give the same pier name also for the column ties.

117. In this idea you must enter in Assign/Frame Line/Pier Label and you’ll select the desire PIER.

118. Note that the column ties has been awarded with the same PIER name as the wall shell elements. This is repeated for all the wall piers that works in the longitudinal direction in the sense of the seismic action left to right. Proceed similarly for the other directions/senses.

119.

In this moment you may observe all the wall piers …

120.

And also all the column ties …

121.

A general view with all the piers from this model.

122.

Save the model.

123.

Run ETABS.

124. After the analysis please look first at the measurement units, at the bottom right corner.

125.

Push Display/Show Tables …

126. Make a check at what you want. For instance the vibration modes and the wall piers efforts

127.

For the wall piers you must select the loads you wish.

128. At the proper modes of vibrations you may observe the previous presented distribution (at displacements section).

129.

Please select Pier Forces to obtain the efforts table.

130.

The wall pier efforts table

131.

You can copy the table ... and...

132.

You can import in Excel

133.

The copied efforts in Excel …

134. You may sort the efforts as you wish. As an example from the load case point of view.

135. In Excel you have the efforts table, selected in the desired order. Further strength checks can be made. 136.

Repeat the operation for each model.

137.

Bending moments diagrams

138.

Shear forces diagrams

139.

Axial forces diagrams

Associate Professor Daniel Stoica UTCB – Bucharest – Romania [email protected] [email protected]