RAFT-FEM™: A Simple and Efficient Tool for Pile-Raft Foundation Analysis and Design
- Adisorn O.
- Jun 3
- 6 min read
Updated: Jul 24
Adisorn Owatsiriwong
ALPS CONSULTANTS
Design Faster with Confidence
In foundation engineering, there is often a gap between simplified hand calculations and large-scale finite element software. Engineers need a tool that is fast, practical, and easy to use for everyday projects without spending hours building complex models.
RAFT-FEM™ Mat Foundation Analysis was developed to bridge that gap.
The software provides rapid finite element analysis of mat foundations, pile-supported mats, and pile raft foundations, allowing engineers to quickly evaluate load distribution, foundation behavior, and preliminary reinforcement requirements.
Why RAFT-FEM™?
Many foundation projects require quick answers during the conceptual and preliminary design stages:
- How much load is carried by each pile?
- Is the mat foundation flexible or rigid?
-What foundation thickness is required
- Where are the critical bending moments?
- What reinforcement will likely be required?
- Is the pile arrangement balanced?
Traditional 3D analysis software can answer these questions, but model preparation often takes longer than the actual engineering evaluation.
RAFT-FEM™ focuses on efficiency.
Engineers can generate a finite element model within minutes and obtain meaningful design information immediately.
The program is particularly useful for:
### Flexible Foundation and Pile Raft Analysis
### Mat Foundations
- Pile-Raft, pile and shallow Foundations
- Combined foundations
- Large equipment foundations
- Industrial foundations
### Pile-Supported Mat Foundations
- Pile caps with multiple columns
-Shear walls with arbitrary shape
- Large pile groups
- Transfer foundations
- High-rise building mat foundations
### Pile-Raft Foundations
- Settlement-sensitive structures
- Hybrid pile-soil load sharing systems (k_soil_bearing + k_pile_spring)
- Preliminary optimization studies
The software helps engineers understand how loads are distributed between piles and the foundation slab before moving to detailed design.
User-friendly GUI
The current version of RAFT-FEM equips a simple but efficient user interface. This allows full interaction with the mouse and shortcuts. The simple or moderate-sized model can be constructed easily. Also, it includes a standard CRSI pile cap library and an automatic pile cap boundary generator. For complicated models, DXF import is a more efficient workflow (in development).
Pre-Installed Problems
Some common problems in the structural design of foundations are provided as design wizards. The user can generate those models quickly. Some of the pre-installed problems are
Combined footing with two columns
Combined footing with four columns
Mat foundation with lift core
Cylindrical tank on pile cap
Reissner-Mindlin Plate Formulation: No More Simplification
At the heart of RAFT-FEM™ is a robust finite element formulation based on the:
Isoparametric 4-Node Reissner-Mindlin Plate Element
The formulation includes:
- Transverse displacement
- Plate bending
- Shear deformation effects
- Thick and moderately thick slab behavior
-Reduced integration for shear stiffness as an option
This makes the program suitable for:
- Thick pile caps
-Combined Footings
- Transfer mats
- Deep foundation slabs
- Pile raft systems
The Reissner-Mindlin formulation provides a practical balance between accuracy and computational efficiency.
Rapid Pile Load Distribution Assessment
One of the most valuable capabilities is automatic pile load recovery.
The software calculates:
- Individual pile reactions
- Maximum pile load
- Minimum pile load
Engineers can immediately identify:
- Overloaded piles
- Uneven load sharing
- Effects of column eccentricity
- Foundation flexibility behavior
This capability is extremely useful during preliminary design and optimization studies.
Structural Design Information
Beyond displacement analysis, RAFT-FEM™ provides engineering quantities required for design.
Bending Moments
- Mx
- My
- Mxy
Shear Forces
- Qx
- Qy
Foundation Deformation
- Settlement contours
- Deflected shapes
Minimum Thickness Design
The program uses binary search to bracket the minimum cap thickness governed by punching shear failure. After computing the minimum thickness, the finite element analsyis is conducted for one-way shear check and rebar design.
Wood-Armer Moment
At any small area cut, the moment can be written as 2nd order tensor like stress tensor, i.e.
m = [mxx mxy; mxy myy]
By eigenvalue analysis, we can find the principal directions and magnitude of the principal moments where mxy = 0, i.e.
m = [m11 0; 0 m22]; where m11 and m22 are bending moment in the principal direcitons.
For slab near the entrant corner, column and wall support where both mx and my act simultaneously. mxy is low value at most typical slab region.
For reinforcement design, it is common to design the rebar align to the local x and y direction of slab for ease of construction. Wood-Armer's moment wasorigated from that practical design aspect by considering that twisting moment must increase to normal bending moment in a conservative sense, i.e.
mx_design = mx + |mxy| ; mx > 0
= mx - |mxy| ; mx < o and the same to my_design
Reinforcement Assessment
The program can compute the flexural rebar and perform shear check following ACI318-25 requirements:
Required reinforcement areas (mm2/m)
High-demand regions
Punching shear check from column dimension and cap thickness
One-way shear stress check
Punching shear strength check around columns
Punching shear check around pile shall be manually checked by user
This allows engineers to move rapidly from analysis to design decisions.
Rebar can be rearranged according to the specified bounds of spacing [smin smax]. The program employs bracketing searches to find the best rebar arrangement to achieve the goal.
Rebar Region and Local Coordinate Design
The mat can be divided into regions with different assigned rebar inclination angles. Using local coordinate transformation, the rebar grid can be aligned with those local coordinate axes. This is practical for mats with irregular shapes. The post-processed local moment is a 2D transformation of the moment tensor in global coordinates, i.e. [ mxx mxy = [T] [mXX mXY. [T]' mxy myy ] mXY mYY]
where transformation matrix [T] = [c s ; -s c]. c = cos(alpha), s = sin(alpha)
Note: For isotropic material (assumed in the analysis), no element stiffness transformation is required since we formulate the element stiffness in reference to the global coordinate.
Punching Shear Design according to ACI318-25
Detailed punching shear analysis and design are automatically performed using the ACI 318-25 approach. The program uses advanced computational geometry to trace the critical shear perimeter for columns and piles. The detailed calculations are summarized in the table for quick review.
Powerful Visualization Tools
Engineering decisions become easier when results are visualized clearly.
RAFT-FEM includes contour plotting capabilities for:
- Vertical displacement
- Pile reactions
- Mx moments
- My moments
- Twisting moments Mxy
- Shear forces Qx
- Shear forces Qy
-As, top & As, bottom in x and y direction based on Wood-Armer's concept
-One-way shear stress in x and y direction
Engineers can quickly identify critical regions and understand structural behavior visually.
Typical Workflow
### Step 1
Define foundation boundary as polygonal (or from a template model)
### Step 2
Specify pile locations and spring stiffness. (or from a template model)
### Step 3
Enter column locations, dimensions, and transferred loads.
### Step 4
Generate finite element mesh automatically.
### Step 5
Run analysis.
### Step 6
Review:
- Pile reactions
- Settlement contours
- Moment diagrams
- Shear distributions
### Step 7
Perform preliminary concrete design.
The entire process typically requires only a fraction of the time needed for conventional finite element software.
-Flexural rebar
-Punching shear check
-One-way shear stress check DCR
Practical Applications
RAFT-FEM™ is ideal for:
- Preliminary foundation sizing
- Feasibility studies
- Alternative pile layout evaluation
- Pile raft behavior assessment
-Conventional pile and shallow mat foundations
- Load distribution verification
- Foundation optimization studies
- Construction engineering support
Engineering Efficiency Through Simplicity
RAFT-FEM™ Mat Foundation Analysis was created with one objective:
Provide engineers with rapid, practical finite element analysis for everyday foundation design problems.
By combining:
- Easy model generation
- Reissner-Mindlin plate elements
- Flexible foundation analysis
- Pile load distribution assessment
- Concrete design evaluation
- Professional visualization tools
The software allows engineers to obtain reliable answers quickly and focus on engineering judgment rather than model preparation.
References
M Bischoff, Advanced Finite Element Method, Lecture notes at Technische Universität München, 2002
KJ Bathe, Finite Element Procedures, Prentice-Hall, 1996
Mukhopadhyay and Sheikh, Matrix and Finite Element Analyses of Structures, Springer 2022