ram concept tutorial

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44 Mat Foundation Tutorial

This chapter will walk you through the steps for modeling a mat foundation, also known as a raft. Alternative metric values and units are identified in square brackets [] next to the US units. The metric values are not exact conversions. The code used is ACI 318-05.

For information on creating a new file, see Creating and opening files on page 5. You should ensure that you select mat foundation in the new file dialog box.

Most mats support columns and walls. You may choose to model the columns and walls but you should be aware that this could affect the mat behavior. In particular, if there are lateral loads then you should be very careful in defining the supports above as having no horizontal restraint. Otherwise, the supports above rather than the soil (springs) below could resist some lateral moment and shear.

A mat need not have columns and walls modeled above. The reasons to model columns and walls above include improving the appearance of the model, and providing snap points for point and line loads. Additionally, a wall above will stiffen the mat in a beneficial way.

44.1 Import the CAD drawing

The CAD file you import is located in your RAM Concept program directory.

Import the CAD file:

1 Choose File > Import Drawing.2 Select the CAD drawing file mat_tutorial.dwg [mat_tutorial_metric.dwg].

The File Units dialog box appears.

3 Select Inches [Millimeters] (the units used in the CAD file) and click OK.

44.2 Define the structure

To use the CAD file you need to make it visible on the Mesh Input layer.

Show the drawing on the mesh input layer:

1 Choose Layers > Mesh Input > Standard Plan.2 Choose View > Visible Objects ( ).

Note: You can also right click to see a popup menu that includes the Visible Objects command.

3 Click the Drawing Import tab.4 Click Show All, and then click OK.

Draw the slab area:

1 Turn on Snap to Intersection ( ) and Snap to Point ( ).

2 Double click the Slab Area tool ( ) to edit the default properties.3 In the Default Slab Area Properties dialog box:

Choose a Concrete Strength of 4000 psi [25 MPa for AS3600; C25/30 for BS8110].

Set Thickness to 30 inches [750 mm].

Leave Surface Elevation as 0 and Priority as 1.

Click OK.

4 With the Slab Area tool ( ) selected, define the four corners of the slab by snapping to the imported drawings slab corners.

Note: You can type c to close the polygon instead of entering the last point.

Define the column locations and properties:

1 Turn on Snap to Center ( ).2 Double click on the Column tool ( ).3 In the Default Column Properties dialog box:


Set Height to 10 feet [3 m].

Set Support Set to Above.

Set Width to 30 inches [750 mm].

Set Diameter to 30 inches [750 mm].

Check Roller at Far End.

Uncheck Fixed Near and Fixed Far.

4 Click OK.5 Click at the center of all 11 column locations shown on the imported drawing.

Define the wall location and properties:

1 Turn on Snap Orthogonal ( ).2 Double click on the Wall tool ( ).3 In the Default Wall Properties dialog box:


Set Height to 10 feet [3 m].

Set Support Set to Above.

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Set Thickness to 12 inches [300 mm].

Uncheck Shear Wall.

Uncheck Fixed Near and Fixed Far.

4 Click OK.5 Define each wall by snapping to the start and end points of the wall centerlines shown on the CAD drawing.

Define the area spring location and properties:

1 Double click on the Quad-Area Spring tool ( ).2 In the Default Area Spring Properties dialog box:

Set an r-force constant of 0.1 pci [0.00001 N/mm3].

Set an s-force constant of 0.1 pci [0.00001 N/mm3].

Set a z-force constant of 250 pci [0.07 N/mm3], and click OK.

Note: You need horizontal springs (r and s) with very small stiffnesses since there are lateral loads.

3 Define an area spring over the entire slab by clicking four corners of a quadrilateral. This shape need not match the slabs exact dimensions, but should cover the entire slab.

You have now defined the structure but the element mesh does not yet exist.

Generate the mesh:

1 Click Generate Mesh ( ).

2 In the Generate Mesh dialog box set the Element Size to 2 feet [0.7 m].3 Click Generate.

View the mesh:

1 Choose Layers > Element > Standard Plan.

You will now see a somewhat random mesh. This will still produce reasonable results, but will significantly improve when you regenerate it later on.

View the structure:

1 Choose Layers > Element > Structure Summary Perspective.2 Use the Rotate about x- and y-axes tool ( ) to rotate the floor.

Figure 44-1 Mesh Input: Standard Plan

Figure 44-2 Element: Standard Plan

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44.3 Define the loads

Mat loads could consist of point, line and area loads for a number of loadings (such as live, other dead, north seismic, east seismic, north wind and east wind). For simplicity, this tutorial will not use area loads (except for the automatic calculation of self-weight) and will adopt loads belonging only to other dead, live, and ultimate seismic east loadings.

Define the other dead loading:

1 Choose Layers > Loadings > Other Dead Loading > All Loads Plan.2 Choose View > Visible Objects ( ).

3 Click the Drawing Import tab.4 Click Show All, and then click OK.

Showing the CAD file makes the following instructions easier to follow.

5 Turn on Snap to Intersection ( ).6 Double click the Point Load tool ( ).

7 In the Default Point Load Properties dialog box: Change Fz to 40 Kips [180 kN], and click OK.

8 Define 40 Kip [180 kN] point loads by snapping to column centers at the following locations:

A-1

A-3

D-1

D-3

9 Define the rest of the point loads as shown in Figures 44-3 and 44-4.10Double click the Line Load tool ( ).11 In the Default Line Load Properties dialog box:

Set Fz to 8 kip/ft [120 kN/m], and click OK.

12With the Line Load tool ( ) selected, draw a Line Load along the centerline of the wall on grid 2.13Repeat for the wall at grid 2.5 with a load of 5.5 kip/ft [80 kN/m].

Note: Draw these loads to the outside face of the inter-secting walls.

Copy to the live (reducible) loading layer:

For simplicity, use the same loads for other dead and live (reducible) loads

1 With the Selection tool ( ), select all of the other dead loads by fencing the entire slab.2 Choose Edit > Copy.3 Choose Layers > Loadings > Live (Reducible) Loading > All Loads Plan.

4 Choose Edit > Paste.

This pastes the other dead loads onto the Live (Reducible) Loading: All Loads Plan.

Figure 44-3 Other Dead Loading: All Loads Plan

Figure 44-4 Other Dead Loading: All Loads Plan [METRIC]

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Define the ultimate seismic east loading:

1 Choose Layers > Loadings > Ultimate Seismic East Loading > All Loads Plan.2 Choose View > Visible Objects ( ).3 Click the Drawing Import tab.4 Click Show All, and then click OK.5 Turn on Snap to Intersection ( ).6 Double click the Line Load tool ( ) and in the Default Line Load Properties dialog box:

Set the elevation above the slab surface to 360 inches [9000 mm].

Set Fx to 4.1 kip/ft [60 kN/m].

Set all other items in the dialog box to 0.

Click OK.

7 Draw a line load by snapping to the wall intersection points, as shown in Figure 44-5 and Figure 44-6.

Figure 44-5 East Seismic: All Loads Plan

Figure 44-6 East Seismic: All Loads Plan [METRIC]

8 Double click the Line Load tool ( ) and in the Default Line Load Properties dialog box:

Set Fy to -12.8 kip/ft [-174 kN/m].

Click OK.

9 Draw a line load by snapping to the wall intersection points, as shown in Figure 44-7 and Figure 44-8.10Double click the Line Load tool ( ) and in the Default Line Load Properties dialog box:

Set Fy to (+)12.8 kip/ft [(+)174 kN/m].

Click OK.

11Draw a line load by snapping to the wall intersection points, as shown in Figure 44-7 and Figure 44-8.

Figure 44-7 East Seismic: All Loads Plan (second set)

Figure 44-8 East Seismic: All Loads Plan (second set) [METRIC]

Note: The seismic loads are approximations for a five-storey building. The load elevation is the average floor height (third storey).

Note: The loads in the y-direction cancel the couple about the mat centroid.


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44.4 Create the design strips

Design strips are an essential part of RAM Concept because they link finite element analysis with concrete design. Their properties include reinforcement bar sizes, cover, and parameters that Concept uses to determine which code rules are applicable for section design. There are two directions called Latitude and Longitude.

Draw latitude design strips:

1 Choose Layers > Design Strip > Latitude Design Spans Plan.2 Double click the Span Segment tool ( ).

3 The Default Span Properties dialog box opens to the Strip Generation properties.

Set Column Strip Width Calc to Code Slab (this is the default for the AS3600 template).

Click the General tab.

Uncheck the Consider as Post-Tensioned box.

Click the Column Strip tab.

Change CS Top Bar and CS Bottom Bar to #8 [N25 for AS3600; T25 for BS8110].

Change CS Top Cover and CS Bottom Cover to 2 inches [50 mm].

Set the Min. Reinforcement Location to Tension Face.

Click the Middle Strip tab.

Check the Middle Strip uses Column Strip Properties box.

Click OK.

4 Click the Generate Spans tool ( ), or choose Process > Generate Spans.5 The Generate Spans dialog box opens with Spans to Generate set to Latitude (as shown in Figure 44-9).6 Click OK.

Figure 44-9 Generate spans dialog box

The span segments appear in the latitude direction.

Figure 44-10 Design Strip: Latitude Design Spans Plan.

Choosing span segments in a mat is a subjective matter. Concept uses imperfect algorithms that do not always produce acceptable span segments and span segment strips. It is recommended that some span segments in this tutorial are deleted.

7 With the Selection tool ( ), select the seven span segments highlighted in red in Figure 44-10 and press Delete.

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Generate the latitude strips:

1 Click the Generate Strips tool ( ), or choose Process > Generate Strips.

The design strips appear in the latitude direction.

Hatch the strips:

1 Choose View > Visible Objects ( ).

The Visible Objects dialog box will appear.

2 Check Hatching under Latitude Span Segment Strips, and click OK.

Note: You can also right click to see a popup menu that includes the Visible Objects command.

Figure 44-11 Latitude design strips (with hatching turned on). Some edit-ing is now required.

Two span segments are slightly skewed. How you treat skewed strips is also a subjective matter, but in this tutorial we suggest the span segment strips cross sections are manually reoriented.

Edit the cross section orientation:

1 With the Selection tool ( ), select span segments 5-2 and 6-2 as shown in Figure 44-11.

2 Click the Orient Span Cross Section tool ( ).3 Turn on Snap Orthogonal ( ).4 Click near one of the span segments, and then again above or below the first click.

The orientation line half way along the span strip is now vertical.

Regenerate the latitude span strips:


Figure 44-12 Latitude design strips after editing and regeneration.

Draw longitude design strips:

1 Choose Layers > Design Strips > Longitude Design Spans Plan.

2 Choose View > Visible Objects ( ).3 Click the Drawing Import tab.4 Click Show All, and then click OK.5 Double click the Span Segment tool ( ).6 Click the Column Strip tab.7 Change CS Top Cover and CS Bottom Cover to 3 inches [75 mm], and click OK.8 Click the Generate Spans tool ( ), or choose Process > Generate Spans.9 In the Generate Spans dialog box:

Set Spans to Generate to Longitude.

Click the up-down orientation button tool ( ).

Click OK.

10The spans appear in the longitude direction, as shown in Figure 44-13.

Similar to the latitude direction, some editing of the span segments is required.


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Figure 44-13 Design Strip: Longitude Design Spans Plan.

11With the Selection tool ( ), select the span segments over the walls (highlighted in red in Figure 44-13) and press Delete.12Turn on Snap to Intersection ( ).13With the Span Segment tool ( ), draw a span segment by clicking at the wall intersections at point A and B in Figure 44-14.14Choose Edit > Selection Properties, or right-click and choose Selection Properties.15In the dialog box, change:

Min Number of Divisions to 0.

Max Division Spacing to 30 feet [10 m], and click OK.

This span segment has been drawn to assist with Concepts span segment strip width calculation.

16Turn on Snap Orthogonal ( ) and Snap Nearest Snapable Point ( ).

17With the Span Segment tool ( ), draw a span segment by clicking at the wall intersection at point B and then at point C in Figure 44-14 (it should snap to the visible grid line).

18Choose Edit > Selection Properties, or right-click and choose Selection Properties.19In the dialog box:

Uncheck Detect Supports Automatically.

Uncheck Consider End 2 as Support.

Change Support Width at End 1 to 12 inches [300 mm], and click OK.

Figure 44-14 Manually drawn span segments

Generate the longitude strips:


The design strips appear in the longitude direction.

Two span segments are slightly skewed. We suggest the span segment strips cross sections are manually reoriented.

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Figure 44-15 Longitude design strips (with hatching turned on). Some ed-iting is now required.

Edit the cross section orientation:

1 With the Selection tool ( ), select span segments 9-3 and 12-1 as shown in Figure 44-15.2 Click the Orient Span Cross Section tool ( ).3 Click near one of the span segments, and then again to the left or right of first click.

The orientation line half way along the span strip is now horizontal.

Regenerate the longitude span strips:


Figure 44-16 Longitude design strips after editing and regeneration

Note: Many of the latitude and longitude design strips (span segment strips) have different widths either side of a column. You could rationalize these strips such that they have similar widths at the column, especially the canti-levers. See the discussion in Defining strip boundaries manually on page 88 of Chapter 21, Defining Design Strips. In particular, Example 21-2 on page 88 and Example 21-4 on page 90.

Check for punching shear:

1 Choose Layers > Design Strip > Punching Checks Plan.2 Double click the Punching Shear Check tool ( ).3 In the Default Punching Shear Check Properties dialog box:

Change Cover to CGS to 3 inches [60 mm] (cover to centroid of top reinforcement).

Click OK.

4 Fence the slab with the Punching Shear Check tool.


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Figure 44-17 Design Strip: Punching Checks Plan.

44.5 Regenerate the mesh

The presence of design strips can significantly improve the regularity of the finite element mesh. We recommend that once you have completed the design strips, you regenerate the mesh.

Regenerate the mesh:

1 Click Generate Mesh ( ).2 Enter Element Size of 2 feet [0.7m] and click Generate.

There is now a better mesh. View the mesh on the Element: Standard Plan.

Figure 44-18 Element: Standard Plan after regeneration.

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44.6 Calculate and view the results

After you run the model, you can view the results of the analysis and design calculations.

Review Calc Options:

1 Choose > Criteria > Calc Options2 Review the options.

3 Uncheck Auto-stabilize structure in x- and y-direc-tions, and click OK.

Note: See General options on page 126 of Chapter 27 for more information.

Calculate:

Click Calc All ( ), or choose Process > Calc All.

Look at reinforcement and design status:

1 Choose Layers > Design Status > Total Status Plan.

This shows OK for all design strips and punching checks. This means that there are no violations of code limits for ductility, one-way shear, and punching shear. Note that status does not flag excessive deflections.

Figure 44-19 Design Status: Status Plan.

There are punching shear status results at each column. You can see these more easily on the dedicated punching plan.

2 Choose Layers > Design Status > Punching Shear Status Plan.

Concept has noted Non-standard section at the corner column locations.

Non-standard Section is a warning, not an error. What it means is that at least one of the critical sections that Concept is investigating for that column does not perfectly fit one of the three ACI 318-02 cases: interior, edge and corner. Concept still calculates a stress ratio for non-standard sections. Refer to Non-Standard Sections: ACI 318 on page 135 of Chapter 28 for more information.

Figure 44-20 Design Status: Punching Shear Status Plan.

3 Choose Layers > Design Status > Total Reinforcement Plan.

This shows all the code-determined reinforcement for each of the design strips. The results are, however, too congested to be useful. You can access plans in the Design Status layer that separate reinforcement according to: face (top or bottom), direction (latitude or longitude), and type (flexural or shear). You should decide which plans best convey the results without too much clutter.

View Specific Reinforcement:

1 Choose Layers > Design Status > Latitude Bottom Reinforcement Plan.2 Choose View > Visible Objects ( ).

The Visible Objects dialog box will appear.

3 Check Bar Spacings under Latitude Span Designs, and click OK.


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Figure 44-21 Design Status: Latitude Bottom Reinforcement Plan.

44.6.1 Bearing stresses

Maximum bearing stress is a critical consideration when designing a mat. Contour plots of the bearing stresses are available in RAM Concept. These will vary according to the load combination. Note that the minimum and maximum bearing values often occur for different load combinations.

The Soil Bearing Design rule set envelopes the maximum and minimum bearing pressures for all load combinations. The maximum bearing pressure plan is probably the most useful for your design.

View bearing stress plans:

1 Choose Layers > Load Combinations > Service LC > Soil Bearing Pressure Plan.

Figure 44-22 Service LC: Soil Bearing Pressure Plan.

2 Choose Layers > Rule Set Designs > Soil Bearing Design > Max Soil Bearing Pressure Plan.

Figure 44-23 Soil Bearing Design: Max Soil Bearing Pressure Plan


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