RAM Concept by Bentley is a popular modelling software for the design of suspended concrete slabs. It has both post-tension and conventionally reinforced design capability. However a lesser used function in RAM Concept is the ability to design mat foundations. This feature can also be used to design strip footings and isolated footings (or pads). For structures which are sensitive to settlements, RAM Concept is an efficient design tool to check your bearing pressure and structural design of your footing. So lets take a look at footing design in RAM Concept…
The steps in performing footing design in RAM Concept including isolated footings and mat foundations is as follows:
- Import a CAD file background for tracing (not mandatory)
- Model the slab or footing element
- Specify the soil stiffness parameters
- Model columns or walls over the footing
- Apply the design loads for each column and wall location
- Set up design strips and/or design cross-sections
- Model post-tension cables (for post-tension footings)
- Run analysis
- Check results including specification of reinforcement
Note that you don’t always need the assistance of an FEA software such as RAM Concept to design simple isolated footings. For a look at a step-by-step approach to isolated footing design, including a design spreadsheet you can use yourself, take a look at the article via THIS link.
(Note, if you like this article, don’t forget to check out another useful article located HERE where I walk through a step-by-step guide on how to perform vibration analysis on suspended slabs using RAM Concept)…
Example Footing Design In RAM Concept
Lets take a closer look at how to design footings in RAM Concept with a project example I’ve been working on. First lest define the specific example footing for consideration…
I have been working on an inland port facility here in Melbourne. The port is a storage facility for shipping containers and forms part of a newly constructed rail network. A critical Structural element of the project is the strip footings which support the shipping containers while they are stored at the facility. We will be focusing on this footing design in RAM concept in this article.
Here are some further dot points outlining the design considerations for this element:
- The loading assumes shipping containers stacked 5 high (determining the actual loads will not be covered in this article)
- Wind overturning loading on the shipping containers need to also be considered (determining the actual wind overturning load will not be covered in this article)
- Settlement of the strip footings is critical and needs to be controlled at 10 to 15mm maximum (0.4 – 0.6 inch)
- The strip footing arrangement needs to cater for support of either 20 ft or 40ft long shipping containers
Here is a CAD snapshot of the strip footing we will be looking at as an example in this arctiel…
Starting the Footing Design in RAM Concept
First thing you need to do is boot up RAM Concept, then go to…
File >> New...
The next thing you will see is the following dialog box…
This dialog box allows you to choose the Structure Type (Elevated Floor or Mat Foundation), we want to select “Mat Foundation”.
You can also select the design standard you wish to adopt. Considering this project is based in my hometown of Melbourne, Australia, I will be using the Australian Concrete code, AS3600.
Adding a CAD Background into RAM Concept
The next step is not mandatory however it is very helpful. That is to import a CAD file background for you to trace over in your model. You can do this by navigating to…
File >> Import Drawing…
You then navigate to where you have the CAD file saved then click “Open”.
While you can draw model elements “freehand” within RAM concept, the drawing tools are quite clunky and not that intuitive. It is much faster to use a CAD background for tracing purposes. If you don’t have an architectural or structural CAD file for your design yet, I have sometimes found it quicker to draw a quick sketch CAD file first from scratch then import that into RAM Concept.
The lack of intuitive freehand drawing tools within RAM Concept is a bit of a negative (However I still find this software very useful and efficient overall otherwise)…
Here is a snapshot of RAM concept for the strip footing we are looking at. As you can see it looks more or less exactly like the CAD file shown earlier in this article…
You can modify or move the CAD background by navigating to the following location within RAM Concept…
Layers >> Drawing Import…
This CAD file is good as is so I won’t be making any modifications to the CAD background.
Modelling the Footing in RAM Concept
Next we move onto the modelling stage. For this you need to navigate to…
Layers >> Mesh Input >> Standard Plan…
Now you will be looking at a blank screen. For some reason after importing a CAD file, RAM Concept doesn’t think it a good idea to automatically turn it on in-view on any of the layers. You will need to make the previously loaded CAD background visible by clicking on the all seeing eye of RAM Concept (its located in the top toolbar). This is a handy tool for you to manipulate how each of the layers look…
When the visible objects dialog box shows up, you need to click on the DWG tab, then select “show all” then click ok, as shown in this snippet below…
Now you can see your CAD background and you are ready to trace over the top.
The right-hand tool bar is where all the modelling tools are. Here is where you choose to model your slab/footing, columns, walls and soil supports.
I will just be modelling the strip footing only for this example as there are no columns but rather steel casters which the shipping containers sit upon. The slab/footing element tool looks like this…
Double clicking on the slab/footing tool brings up additional properties you can change prior to physically modelling the slab. If you single click on the button it will auto-use the previously used settings or the default settings. By double clicking you will see the following dialog box…
For this example strip footing I will be using a 500mm deep element. I have done a quick hand check to verify that this should be the ballpark thickness we need. I will use RAM Concept to verify this as a second check. I will also be specifying 32MPa Concrete. You then click OK and draw a polygon for the shape of footing you need. Here is what my freshly modelled strip footing now looks like with the CAD background still turned on…
To assist with snapping to your CAD background and to help you in modelling in general, it is highly recommended that you take advantage of the different snapping tools provided by RAM Concept. They are located in the top toolbar and look like this…
At the very least I’m always using the “Snap to Intersection” setting coupled with others (you can use more than one at the same time). This really helps you join elements effectively and accurately (don’t model without it!!).
Specifying Modulus of Subgrade Reaction (Area Spring) in RAM Concept
To support our footing we need to specify the soil spring supports. This is better known as the Modulus of Subgrade Reaction.
The Modulus of Subgrade Reaction of a soil is a measure of its stiffness. It is given by a unit displacement for a given applied pressure (usually resulting in a unit of kPa/mm or MPa/mm).
The Modulus of Subgrade Reaction is a factor that is specified by the project Geotechnical Engineer. Here are some indicative values you may expect from different soil types (but always get this verified by your friendly project Geotechnical Engineer!)…
| Soil Type | Modulus of Subgrade (kPa/m) | Modulus of Subgrade (MPa/mm) |
| Compacted Sand | 50,000 – 150,000 | 0.05 – 0.15 |
| Clay | 80,000 – 100,000 | 0.08 – 0.10 |
| Crushed Stone | 100,000 – 150,000 | 0.10 – 0.15 |
| Corse Crushed Stone | 200,000 – 250,000 | 0.20 – 0.25 |
For further explanation on the Modulus of Subgrade Reaction, its units and how its measured on-site by Geotechnical Engineers, take a look at THIS article.
The soil isn’t that great for my example site, the Geotechnical Engineer has specified a Modulus of Subgrade of 12,000 kPa/m (or 0.012 MPa/mm). To input this into RAM Concept, its the lower most button on the modelling toolbar to the right of screen which we looked at earlier…
Again double clicking on this option allows us to specify our spring stiffness value before drawing it in the model…
As you can see, you can specify area springs for lateral directions (R and S directions) as well as vertical (Z direction). I am only dealing with vertical loads and therefore vertical support springs in this example so I have used the Z Force constant only. You can see that RAM Concept is expecting to see a unit of “N/mm3” which is a little counter-intuitive. The unit of MPa/mm is the same as N/mm3 . Expressing the Modulus of Subgrade as newtons per millimetre cubed doesn’t truly represent what Modulus of Subgrade is in my opinion however its not uncommon for this unit representation to be used.
After clicking OK, you draw another polygon for the area where this spring support will be effective. I don’t have any varying soil conditions across the strip footing so I will be overlapping the area spring fully with the previously modelled strip footing I have just created. When that’s all done, it looks something like this…
Now you can see that the KFz of 0.0012 has been included over the same area as the strip footing.
Applying Loading to a Footing in RAM Concept
Now that our strip footing is modelled, I will input the applied loading. There are a number of load combinations I need to check for this footing and I won’t bore you with them all. In this example I will be applying the fully stackd 5 high load for each shipping container caster location. This equates to 405kN per caster as a point load. For this instance, I will be assuming the shipping container loading as a dead load.
To input loads you navigate to…
Layers >> Loadings >> (In this Case “Other Dead Loading”) >> All Loads Plan…
You can select different load types and also isolate your layer to point loads, line loads or area loads. I like to see everything on the one layer so I often choose to use the “All Loads Plan” layer for the given load I am working with.
Once your in this layer, you may see a blank screen again. This is because you need to mesh the modelled elements you just crated in order for them to appear on other layers (such as this loading layer). To do this, you need to click on the green mesh button which is located in the top toolbar and looks like this…
Now your footing should appear and you can input your loads. The right toolbar changes for this layer and becomes various tools for load input. You can choose from point loads, line loads and area loads. I will be using point loads for my shipping container casters, the loading toolbar looks like this…
Again you can double click and specify the load you want to use prior to drawing it on the model, her is what that dialog box looks like…
Now I simply click on all the load points until all the caster locations are covered. The finished product then looks like this…
Setting up Design Strips for Footing Design in RAM Concept
Next we need to specify our design strips in the longitudinal and lateral directions. First lets run longitudinal. You navigate here by clicking on…
Layers >> Design Strip >> Longitudinal Design Spans Plan…
My strip footing example is a little quirky. Instead of large columns applying load to the footing I have small shipping container feet (casters) performing this role. This can make the design strip set-out a little complex. Instead, I’m going to treat the centre point of each cluster of 4 casters a column location and model my design strips accordingly. This is what I’m talking about…
The toolbar on the right changes again to now provide tools for specifying your design strip, it should now look like this…
I will be using the third from the top to specify by longitudinal design strips. Double clicking on this again allows you to specify all the particulars of the design strip. I won’t be going through them all in detail in this article however just methodically navigate from tab to tab and update them to suit your specific application. They are reasonably straight forward and self-explanatory. Here is what that dialog box looks like…
I have then drawn a bunch of user-lines to give me something to snap my design strip to at the centre location of the caster “clusters”. The design strip arrangement now looks like this…
Simply repeat this process now for the lateral design strips and then we move on to the next phase of our modelling. Lateral design strips are specified by navigating this time to…
Design Strip >> Lateral Design Spans Plan…
Repeat the process which we just performed for the longitudinal design strips.
Running the Footing Analysis in RAM Concept
I don’t have any post-tensioning in my footing so won’t be going through this step, however after specifying design strips is when you would specify your post-tensioning cables. For a detailed article on what post-tensioning is, take a look over HERE.
Before running the analysis, we need to stabilise the footing in the horizontal direction. You will recall that I have only provided spring area support in the vertical direction (Z coordinate). This means that the system is not stable. Even though no horizontal loading has been applied to the footing itself, the analysis still requires stability in all directions for it to run correctly (or else you will receive an error).
You can achieve this through either providing a spring area support of a very small nominal value in the R and S directions. Or a better way to do this is to navigate to…
Criteria >> Calc Options…
The following dialog box appears (shown below). Within this you want to navigate to the general tab then check the “Auto-stabilize structure in the X and Y directions” (as highlighted below)…
Now we are ready to run the model. There are two options for this, both are located in the top tool bar…
- Calc All
- Calc Load History Deflections
The “Calc All” function gives short term deflection results as well as strength design. The “Calc Load History Deflections” performs the strength design as well as the deflections accounting for long term effects (creep and shrinkage of concrete). For the most comprehensive results I use the “Calc Load History Deflections” most of the time until I am happy with the structures deflection then focus on the “Calc All” option when I’m finalising the reinforcement design.
Checking Results After Analysis in RAM Concept
To check your footing design in RAM Concept for strength (bending and shear) you can navigate to…
Layers >> Design Status >> Status Plan…
On this layer, you are looking for everything to be green. Any red errors means that your structures is failing!! If you see failures you need to investigate and increase your structural sizing or change your design assumptions until everything passes. Here is what my example strip footing looks like, everything is green as a pass which means that the 500mm thick strip is working (so far)…
Next, lets check the footing design in RAM Concept for soil bearing. There is no ability for the user to input the design bearing capacity of the soil into RAM Concept. Rather you must read off the soil bearing plots to make sure that you are below the specified bearing capacity provided by the Geotechnical Engineer. For this example project, the Geotechnical Engineer has specified an allowable bearing capacity of 250kPa. Because this is allowable bearing, I will be using the un-factored D + L load case to check this (Dead + Live). You can navigate there by clicking on…
Layers >> Load Combinations >> Max Service LC: D + L >> Max Soil Bearing Pressure Plan…
This is what my bearing stress plot looks like…
From this contour plot, we can see that the bearing pressure is hovering around the 200kPa mark at its peak. To drill down and look in more detail, the “Selected Plot Distribution” tool (or slice tool) is great. Its located on the right hand side toolbar in this layer and looks like this…
Using this tool to cut a slice right up the middle of the strip footing in RAM Concept, I can see exactly what the bearing pressure is more accurately in the footing…
Our bearing pressure on the soil is 200kPa with is less than the soil bearing capacity of 250kPa so we are all good.
Now we need to check our footing settlements. To do this, I like to check the long-term deflection plot which can be found via…
Layers >> Load History Deflections >> Final Instantaneous Load >> Std Deflection Plan…
Here is what my settlement/deflections look like (below). I am using the slicing tool again for accuracy. As you can see, I’m little over the 15mm I was shooting for. So I will either need to make the strip footing wider or twist the Geotechnical Engineers arm to get a higher spring stiffness value (Modulus of Subgrade Reaction)…
Finally you simply need to read off your reinforcement plans and rationalise the outputs and you are finished… That is one strip footing in RAM Concept successfully designed!!
RAM Concept is also a great tool for checking vibration of suspended slabs (both reinforced and post-tensioned). Take a look at THIS article for another step-by-step walk through on how to complete this in RAM Concept.
