Mesh Input layer

Why is it necessary to have priorities?

Without the priority system the modeling of floors would require one of two methods:

•  Objects for slabs of different thicknesses, beams, openings etc. could not overlap - this would be very tiresome for all but very simple floors, or
•  Depths would have to additive. For example, you would have to deduct slab depth from beam depth. If you had to change the slab depth then a change would be required for the beam, unless its depth changed by the same amount.

Can I copy columns or walls below to the same above?

Yes.

1. Select all of the columns or walls you wish to copy.
2. Choose Edit > Copy (or right-click and choose Copy from the popup menu that appears). Then double click off the edge to select nothing.
3. Choose Edit > Paste (or right-click and choose Paste from the popup menu that appears). The pasted objects are the current selection.
4. Choose Edit > Selection Properties, or right-click and choose Selection Properties.
5. Change Support Set from Below to Above, and click OK.

Note: It is important that you do not abandon the process after pasting. Otherwise, you will have two supports below at various locations, which causes calculation errors.

Element layer

How can I view the slab without the mesh?

Choose Layers > Element > Slab Summary Plan or go to the visible objects dialog box and check the "Outline only" option under slab elements.

What is the difference between beam and slab elements?

There is no difference unless you modify their behavior. See discussion of behavior in “Slab area properties” on page 56 and “Beam properties” on page 57. The difference in only in the modeling, a beam always has two parallel edges offset from the two end points, while a slab area polygon can be any shape in plan.

How many nodes or elements are allowed?

There is no limit, other than the limitations of your computer. If you find the program performance too slow, consider any of the following to help:

1. increase the mesh size to reduce the number of elements 
2. delete an imported drawing file that is no longer needed 
3. reduce the number of load cases and/or load combinations
4. reduce the number of design strips or increase the spacing of the design strip cross sections.

How many elements should I use per span or panel?

This cannot be answered directly as it depends upon the structure and loads. The maximum is 32.8 feet (10 meters). To speed the analysis, it is useful to choose a coarse mesh for preliminary design and a fine mesh for final design.

• A coarse mesh might have an element size of span length /6.
• A fine mesh might have an element size of span length /12.

If in doubt, you should investigate the effects of different mesh element sizes.

Columns

Do columns restrain the slab?

Depending upon the defined fixity, columns can provide rotational and lateral restraint. If the far end of a column is defined as a “roller” support (or both ends of the column are pinned) then the column does not provide any lateral restraint to the slab. Columns above the slab do not support the slab vertically, they can only restrain the slab rotationally and laterally.

Why is the deflection at the face of a supporting column?

Columns in Ram Concept connect to the slab finite element mesh at a single node located at the column centroid. They are not solid objects nor do they provide vertical support to multiple nodes. This is apparent viewing the Elements - Standard Plan.

As such, deflections in the slab start from that node and increase towards the face of the column. For small columns this may not matter much, but for large area columns this is significant. 

To mimic the behavior of a stiff column support, we suggest modeling a thick and stiff slab object that overlays the column volume like a mini-drop cap. Be sure to assign a higher priority to this patch of concrete. It is recommended to model this patch with an elevated top of concrete elevation such that the slab centroid aligns with the mid depth of the patch in order to avoid eccentricity at this location.

The same approach could be taken for thick walls supporting the slab as well. A beam or slab object can be used there.

Beams

Where should you define the end points of a beam, at the face of the column, passing the column or at the center line? 

The general preference is to model the beams through the column, extended to the slab edge since it best matches the built condition and how things are formed. Stopping the beam at the column centerline results in a slightly more flexible system.

Walls

Do walls restrain the slab laterally?

Yes, if you select Shear Wall as a property. If the Shear Wall is unchecked then the slab is allowed to slip freely over the top of the wall. The walls rotational stiffness is independent of the Shear Wall setting; use the fixity settings to control the walls rotational stiffness about its longitudinal axis.

What is the effect of specifying walls above?

Wall elements can be used to model the stiffness and spanning ability of walls connected to the slab. Walls above behave similarly to beams in that they stiffen the floor. One could actually model the walls above the slab as beams instead, but it is not generally recommended.

Using beam or slab elements does have some advantages over using wall elements (“wall-beams”):

• Concept design strip cross sections automatically integrate the forces across slab-beam elements; wall-beam elements are ignored in these integrations, however.
• Also, Concept provides you many controls over how slab element results can be displayed; wall-beam elements (like wall elements) can only plot their reactions to the slab.
• However, Concept’s standard slab elements have a torsional stiffness that is proportional to their depth cubed. This can cause a large over-estimation of the torsional stiffness for a very thick slab element if it is adjacent to relatively thin elements. “Wall-beam” elements do not have this problem. As such, walls above that are modeled as upturned beams should use the “No-torsion” beam property.

When modeling wall-beams above the slab, Concept interprets some of the wall element parameters differently than for walls below.

• If the wall-beam is not rotationally fixed to the slab then the wall-beam will have zero torsional stiffness.
• If the wall-beam is not a shear wall then it will have zero axial stiffness. The vertically compressible and rotationally fixed at far end parameters are ignored.

Wall-beam elements have one advantage over slab elements.

• Slab elements of drastically differing thicknesses in the same structure can cause the automatic plotting controls to show (correctly) huge force variations in and adjacent to thick slab elements and almost no variation within the thin slab element areas. This does not generally happen if walls above are modeled as wall-beams.

Do walls above the slab provide rotational restraint?

There is no restraint at the far end of a wall above. (Even if “Rotationally Fixed at Far End” is checked, it is ignored).

Mats (rafts)

Does Concept ignore soil tension?

You can reduce the tension by iteration. The tension gets closer to zero with an increase in the number of iterations. See “Zero tension iteration options” on page 126 for more information.

What value should I use for the area springs Z force constant?

The geotechnical engineer commonly provides a value called the “subgrade modulus” or “modulus of subgrade reaction”. As a guide only: realistic values vary from 100 pci (approx. 25 MN/m3) for soft clay to 750 pci (approx. 200 MN/m3) for very dense gravel.

Note: Area springs are always assumed to be compression only springs in the Z direction, but they behave elastically in the R and S axes. Line and point springs are also linear elastic supports resisting tension or compression.

Does Concept design for soil heave?

Not directly. You could draw spring supports that approximate varying soil support.

Do I need to draw the columns above in a mat foundation model?

No, but it is a good idea. It ensures a node is placed at that location where there is likely to be a heavy point load. The columns also provide convenient snap points for tendons or design strips.

Can Concept design for pile supports?

Yes. Use either (flexible) columns under, or point springs. Skin friction is not considered.

Can Concept design for pile and mat (raft) action together?

Yes, but the results could be very susceptible to variations in geotechnical parameters. For example, if the soil’s stiffness is overestimated, the actual pile reactions could be significantly underestimated. Use caution.

Does the area spring support have to match the mesh?

No. An oversized area spring is fine. The program applies the individual nodal springs based on the mesh tributary area.

Can the soil stiffness vary?

Yes. You can vary the stiffness in two directions. See “Area spring properties” on page 55.

Where do I select the allowable soil bearing pressure?

This is not an input parameter. You need to look at soil bearing pressure plans (which have a maxima / minima legend) to assess the maximum pressures.