Foundations provide the support and resistance of the loads of the structures above. They serve as structural systems that transfer loads to the soil below and that provide stability, including resistance to overturning, sliding, and uplift, for the overall structure. Due to the importance of their structural system to the overall structure, it is imperative that their structural integrity is maintained for the overall structure to function. However, in some cases, foundations can fail. We are now going to explore the different types of causes that can affect the failure of foundations, in order to prevent and remediate the failures.
 
Causes of failure

Uneven loading

The uneven distribution of loading from the superstructure can induce uneven stresses at different locations of the foundation. This can cause differential settlement at locations where vertical structural elements, such as columns and walls, directly transfer the superstructure loads to the foundation.  Differential settlement can eventually lead to cracks at the foundation.
 
Overloading

Overloading from the superstructure can also create foundation failure. Foundations can fail by cracking when the design moment and/or shear is above its moment and/or shear capacity. Failure can also occur when there are large concentrated or point loads, which can induce large punching shear onto the foundation, and when there is over designing of bearing pressure.
 
Different properties of soil at the foundation interface 
 
Different parts of the foundation can rest on different properties of soil. For example, one part of the foundation can sit on clay, while another part of the foundation can sit on rock. When all design checks are adequate for one part of the foundation due to that part resting on good soil and when checks fail for another part of the foundation due to bad soil properties at the other part of the foundation, the whole foundation can fail.
 
Ground investigation will need to be used to determine these different soil properties. The foundation structure will need to be designed in consideration of the different soil stiffness and soil properties.
 

Insufficient soil compaction 
 
The soil fill underneath the foundation might not be compacted properly and to its required degree of compaction. Since the soil is not compacted properly, air voids can be created within the soil, in which soil and water can displace in and out of. There will then be movement within the soil, which causes swelling and contracting. The swelling and contraction of the soil can cause pressure to the foundation that the soil supports.
 
Air voids within the soil can cause loose soil or soil with low density, which lacks adequate strength to support the foundation. Poor compaction equipment can also lead to foundation failure.
 
Therefore, it is best to compact the soil beneath the foundation to its required compaction degree before concrete placement of the foundation, in order to reduce soil displacement, to increase subgrade reaction and soil density, and to reduce differential and overall settlement of the foundation.

Uneven moisture levels of soil beneath the foundation

Similar to the above scenario, uneven moisture levels of the soil can cause soil swelling and contraction at specific parts of the foundation. This can lead to stress at intersecting locations where the soil is swelling and contracting and where the soil is not. 
 
Changes in moisture levels of soil beneath the foundation
 
Moisture levels of the soil can change due to varying humidity levels, rainy weather, or poor drainage conditions, which can cause soil to swell (or heave) and contract, therefore leading to cracks. Similar to insufficient soil compaction, the voids within the soil can be filled up by water or other fluids, which can create pressure onto the soil particles from the fluid.
 
However, when there are dry periods, the water evaporates from the soil and leaves from the voids within the soil. This can cause soil shrinkage.
 
Moreover, when there are cracks within the foundation, water seepage can also occur.
  
Vibration from adjacent construction
 
Vibration from nearby construction can displace soil particles underneath the foundation. This can then create air voids within the soil, which can loosen up the soil and lower the soil density. The lower the soil density, the lower the soil strength for the support of the foundation. This will then cause foundation failure.
 
Transpiration
 
If there are trees adjacent to the foundation, the trees can evaporate the water from the soil into its roots and into the atmosphere. This can cause changes in the moisture level of the soil.
 
Next, we will explore the different remediation and prevention techniques.

But before we do that, are there any other causes of foundation failures you have encountered? Any examples that you experienced? Be sure to let us know in the comment section below!

Bolts, welds, end plates, base plates, stiffeners, ribs, cleats, bolts with welded fin plates, beam to column connection, beam web to beam flange connection, gusset plates for bracings... These are only the few types of steel connections in the midst of many different types of connection elements. On top of these different connection elements, we also have different combinations of connection elements. This can result in complex checks, including nonlinear analysis checks. Yes, you can use the steel connections formulas in national codes to check basic steel connections, such as bolts or welds. However, for more complex steel connections, these checks are idealized and have limitations. How can different steel connection checks be made both accurately and effectively, and at the same time, comply with national codes?
 
IDEA StatiCa has this solution, the IDEA StatiCa Connection software. The team at IDEA StatiCa with the Faculty of Civil Engineering in Prague and the Faculty of Civil Engineering of Brno University of Technology developed a new method for design and analysis for steel connections, CBFEM (Component Based Finite Element Model). CBFEM makes up the basis of IDEA StatiCa Connection software. It still keeps the essence of the Component Method, which designs connections as systems of interconnected components. It still uses the Component Method for certain connections like bolts or welds since national codes comply with the Component Method. However, CBFEM improves the Component Method by including finite element analysis and nonlinear analysis to certain connection elements that require special attention to their nonlinear behavior. You can read more about their CBFEM theory here. 

IDEA StatiCa Connection can:
1. Check for different customized steel connections
2. Display the steel connection’s FE model
3. Automatically generate FEM results
4. Calculate for stresses and strains of steel connections based on non-linear behaviour
5. Adhere to international codes, EN code and the AISC
 
In this post, we will go through an example of a steel connection, a simply supported beam to column connection, to test out how IDEA StatiCa Connection works.
 
Connection Geometry Input Function
 
You can input all your basic information that you use to calculate for steel connections, such as steel grade, your type of steel connection, which steel member is bearing, the geometry of the connection, the length of the connecting structural members, and end conditions. You can also choose or edit a predefined steel section in a diversified collection of standardized sections OR you can create your own. Predefined sections include:
1. Rolled sections (I sections, Channels, Angles, Rectangular and circular sections)
2. Welded sections
3. Cold-formed 

Load Effects Input Function
 
You can input any load reactions onto the connection.
Then, you can run the Equilibrium check, a handy tool in this load section. The Equilibrium check is based on equilibrium equations at the joints and checks to see if you have any unbalanced forces.
 
Joint design and manufacturing feature
​
You can choose additional connection elements to add to your connection type, such as stiffeners, end plates, bolts, gusset plates, member cuts, anchors, wideners to increase flanges or webs of members, ribs, openings and notches, splices, cleats, etc. After selecting your connection elements, you can input their structural properties. ​

For example, if you selected the stiffener or rib option, you can choose which member to add the stiffener/rib on, the position and the inclination of the stiffener or rib, its thickness and material, etc. There are other options, such as the typical fin plate. These can be connected to the flanges or webs of the steel members. You can define the fin plate’s material, thickness, whether the connection is bolted or welded, bolt and weld properties, and even whether there are notches or not. For welds, you choose whether the steel cross section is welded to a plate or the other connecting steel member, the number of welded edges, the type of welds (butt weld or fillet weld) or the throat thickness. If throat thickness is not inputted, the throat thickness is automatically determined according to the thickness of the connecting element ​

Plate Editor Feature
 
Another unique feature is the Plate Editor. Based off of realistic manufacturing plate conditions, you can edit your connecting or end plate to any geometry you want. You can offset any sides or all sides of the plate, have smooth circular rounding of the plate’s corners, have the plate chamfer, bevel, or arc cut, have holes and notches at specific places of the plate, etc. You can also use this function to visualize your bolt positions and change your positions to suit. Once you are finished with the setting out and dimensioning of the plate, you can then produce a drawing of the plate and bolt holes with dimensions and setting out, for manufacturing, design, or coordination purposes. ​

Calculator based on CBFEM
 
IDEA StatiCa Connection uses CBFEM to calculate all its connections. It provides comprehensive and diversified FEM checks for different connection types and for different failure modes. This Check function also helps you to cater for different efficiency levels of your analysis. Under the Code setup tab, you can do so by changing the number of analysis iterations.
 

After you set up and ran your analysis, IDEA StatiCa Connection shows whether all loads applied are used (okay state) or whether analysis has stopped (when applied load is more than the connection ultimate limit state capacity). ​

For each individual connection component, you can find your summary tables of design/capacity percentage based on specific load cases. ​​​

For plates, IDEA StatiCa Connection generates a summary table for individual connecting plates, including structural members’ flanges and webs. The table provides:

• Maximum equivalence stress in plate
• Maximum plastic strain in plate
• Whether analysis of connecting plate is okay.

For bolts, the summary table shows:

• Values of tension and shear forces applied
• Max tension and shear
• Max combined tension and shear
• Whether the detailing is okay (edge distances, spacing, etc)
• Whether analysis of bolts is okay

 

For welds, the summary table shows:

• Stress in weld and stress perpendicular to weld throat
• Shear stresses parallel and perpendicular to weld axis
• Max stress capacity
• Whether analysis of weld is okay

 
 
The program also shows a stress diagram for the connection so that you can see where the critical points of stress are.
There are various tools in the CBFEM ribbon that can aid analysis. The Strain Check feature shows a diagram of the relationship between the plastic strain and the plastic strain limit. The Buckling shape function shows the stress contour of the connection under the compressive stress of a selected load case.
 
Reporting Function
 
The Bill of Materials feature can generate summary tables of information and a section of the connection. The Bill of Materials can be used for shop drawings to clearly indicate to the supplier what the quantity and properties of bolts, plates, anchor, and other connection elements are needed for construction.
​
With the Report function, the software can generate a structural report with design equations used for the connection analysis. Depending on your needs, you can select whether you want a report with a single line of design results, a more detailed full page report, or an expansive detailed report with selected items from an all-inclusive list, such as geometry elements, cross-sectional pictures, strain check drawings, buckling analysis, etc. Idea StatiCa Connection also prepared commentaries on the theoretical background of the software, the advantages of CBFEM over the Components Method, its method of joint analysis check, and how the different connection elements are modelled (eg. Plates are modelled as plate/walls).

​All in all, IDEA StatiCa Connection is a ground breaking technology that can accurately and efficiently calculate for nearly all types of steel connections imaginable. Give it a try here!
 
This post is sponsored by IDEA StatiCa.