Introducing LEDGE-STM™: Visual and Automatic Strut-and-Tie Design for Beam Ledges under ACI 318-25
- Adisorn O.
- Jun 11
- 5 min read
Updated: Jun 12
Adisorn Owatsiriwong
At ALPS Consultants, we continue to develop practical engineering tools that help engineers perform rigorous calculations efficiently while maintaining a clear understanding of structural behavior.
We are pleased to introduce LEDGE-STM™, a new design tool developed as part of the ALPS ACI 318-25 Concrete Calculation Tools suite.
LEDGE-STM™ is designed specifically for the analysis and design of:
Reinforced concrete beam ledges
Bearing ledges
Precast bearing seats
Inverted corbels or dapped end
Transfer nibs
Similar disturbed regions (D-regions)
using the provisions of ACI 318-25 16.5, and chapter 23: Strut-and-Tie Method (STM).
Why Beam Ledge Design Is Challenging
Many reinforced concrete elements can be designed using conventional beam theory.
Beam ledges are different.
The load path is highly concentrated and occurs over a short distance. As a result, assumptions of linear strain distribution and conventional sectional design become less reliable.
The engineer must consider:
Bearing stresses
Local compression zones
Horizontal tie forces
Vertical hanger reinforcement
Shear friction
Nodal zone capacities
Strut capacities
Reinforcement detailing requirements
These checks often require engineers to revisit STM provisions and manually sketch load paths for each project.
Although the calculations are not inherently difficult, they can be time-consuming and prone to oversight.
source: Nasir and Shakir (2019)
ACI 318-25 Chapter 16.5 Approach
For a beam ledge, dapped-end or corbel with av/d < 1.0, the ACI 318*25 Chapter 16.5 approach can be applied.
ACI 16.5.2.4
The maximum Vu is min(0.2fc'*b*d, (3.3+0.08fc')*b*d, 11bd).
ACI 16.5.5.1
The area of primary tension reinforcement, Asc shall be
Asc = max(Af+An, 2/3Avf + An, 0.04fc'/fy*bd)
ACI 16.5.5.2 The area of closed stirrups or ties parallel to primary tension reinforcement with in top 2/3d
Ah = 0.5*(Asc-An)
ACI 16.5.6.3
At the front face of a bracket or corbel, primary tension reinforcement shall be anchored by
(a) A weld to a transverse bar of at least equal size that is designed to develop fy of primary tension reinforcement, OR
(b) Bending the primary tension reinforcement back to from a horizontal loop, OR
(c) Other means of anchorage to develop fy
ACI 16.5.6.4, 16.5.6.5
Primary tension reinforcement shall be developed at the face of the support. The develop of tension reinforcement shall be full along the distance.
ACI 16.5.6.6
Closed stirrups or ties shall be spaced such that As is uniformly distributed within (2/3)d measured from the primary tension reinforcement
ACI 1.6.5.4.2
∅ = 0.75 is used for all failure mode of beam ledge and corbel according to ACI 21.2
A Visual STM-Based Approach
Rather than functioning as a simple calculator, LEDGE-STM™ provides a visual representation of the structural load path.
The software automatically generates a Strut-and-Tie Model showing:
Applied vertical load (Pu)
Applied horizontal load (Hu)
Top CCT node
Bottom CCT node
Diagonal compression strut
Horizontal tie reinforcement
Vertical hanger reinforcement
Support compression field
As design parameters are modified, the STM geometry updates automatically, helping engineers understand how forces travel through the concrete element.
This visualization provides valuable engineering insight and assists in verifying the reasonableness of the assumed load path.
Comprehensive ACI 318-25 Checks
LEDGE-STM™ performs a series of STM-based strength checks including:
Compression Strut Check
The diagonal concrete strut is evaluated using ACI STM provisions and appropriate strut effectiveness factors.
Top CCT Node Check
The loaded node is evaluated for:
Diagonal strut face
Horizontal tie face
Bearing face
allowing a more realistic representation of stress transfer at the bearing location.
The dimension of nodal zone is given by
height = 2*covering, base = width of bearing pad
The the strut width is ws = height*cos(theta) + base*sin(theta)
Bottom CCT Node Check
The support node is modeled as a CCT node and includes:
Diagonal strut face
Horizontal compression face
Vertical tie face
providing a detailed assessment of nodal capacity.
The dimension of nodal zone is given by
height = 0.15-0.20 h (user defined), base = 50*(n/2-1) mm
n = number of hanging tie
The the strut width is ws = height*cos(theta) + base*sin(theta)
Bearing Stress Check
Bearing stresses beneath the supported element are verified against allowable STM nodal capacities.
Reinforcement Design
The program determines:
Flexural reinforcement
Horizontal tie reinforcement
Shear friction reinforcement
Vertical hanger reinforcement
in accordance with ACI 318-25 requirements.
Color-Coded Demand-to-Capacity Visualization
To facilitate rapid engineering review, LEDGE-STM™ displays color-coded performance indicators:
Green
Satisfactory performance (DCR < 0.6)
Orange
Approaching capacity (0.6 < DCR < 0.9)
Red
Exceeds allowable capacity (DCR > 0.90)
Engineers can immediately identify the governing component of the design and determine whether the critical issue lies in:
A compression strut
A nodal zone
Bearing capacity
Horizontal tie reinforcement
Vertical hanger reinforcement
This allows faster design iterations and more informed engineering decisions.
*Automatic Design using Intelligent Optimization
An embedded AI optimization algorithm enables the design of the lowest-cost solution of the beam ledge. The design variables are ledge height and all rebar quantifies, including top rebar, hanging rebar, and tie bars (hoop). The design target is to bring all DCRs as close as possible to 0.90 to keep the construction cost as low as possible.
Designed for Practical Engineering
One of the primary goals of LEDGE-STM™ is to reduce repetitive manual calculations while preserving engineering judgment.
The software is intended to:
Improve productivity
Reduce calculation errors
Provide transparent STM verification
Assist in design review
Support engineering documentation
The engineer retains control over the assumed load path and final detailing decisions while benefiting from automated calculations and visual feedback.
Part of the ALPS ACI 318-25 Tool Suite
LEDGE-STM™ joins a growing family of specialized reinforced concrete design tools developed by ALPS Consultants, including:
BEAM-CALC
CORBEL-STM
PILECAP-CALC
FLATSLAB-CALC
Other specialized concrete design utilities
Each tool is developed with the same philosophy:
Provide practical engineering solutions that combine code compliance, transparency, and computational efficiency.
Looking Ahead
Future enhancements under consideration include:
Dapped-end design
Precast bearing seat design
Deep beam design
Generalized STM from FEA stress field
These developments will continue to strengthen the ALPS ACI 318-25 Concrete Calculation Tools as a comprehensive platform for reinforced concrete design.
Engineering Made Clear
Beam ledges and similar D-regions are often viewed as difficult design problems because they require engineers to move beyond traditional beam theory and think in terms of force flow.
LEDGE-STM™ bridges this gap by combining rigorous ACI 318-25 calculations with intuitive STM visualization.
The result is a design tool that not only calculates the answer but also helps engineers understand why the answer is correct.
References:
ACI318-25, Building Code Requirements for Structural Concrete: Chapter 23 Strut-and-Tie Method
A Muttoni, J Schwartz, B Thuerlimann, Design of Concrete Structures with Stress Fields, Birkhauuser, 1997
SE El-Metwally, WF Chen, Structural Concrete: Strut-and-Tie Models for Unified Design, CRC Press, 2018
Naser and Shakir, Experimental Study of the Behavior of Reinforced Concrete Beams with Composite Dapped End under Effect of Static and Repeated Loads, International Journal of Applied Science; Vol. 2, No. 1; 2019
A Owatsiriwong, Strut-and-Tie-Modeling in Reinforced Concrete Structures: Basics and Applications, 2013 (PDF) Strut and Tie Modeling in Reinforced Concrete Structures,
