Tie beams in foundation systems

A key concept in structural engineering is understanding that geometry and stresses are closely related. It is precisely this relationship that makes controlling differential settlements necessary, as relative displacements generate increases in internal stresses. These displacements will not always be vertical; they could also occur within the horizontal plane. Tie beams are the key element for controlling these relative displacements.

What is the contribution of tie beams?

It is not unusual to see certain provisions in design codes that initially seem somewhat arbitrary, as their effect on the design is not explicitly quantified in the standard. A prime example is the requirement for tie beams; they are often mandatory without implying any modification to the design.

The truth is that including tie beams (also known as struts or foundation ties) transforms a group of individual foundations into a cohesive foundation system, producing effects that, while not explicitly quantified, certainly provide great assistance to the performance of the foundation system:

• Displacement Synchronization (Rigid Block Effect): During a seismic event, ground waves do not impact all footings at the same time or with the same intensity. Tie beams force the foundations to move uniformly, preventing the supports from "separating" or moving out of phase. This prevents the emergence of massive secondary stresses in beams and slabs of the superstructure that were not designed to absorb such angular distortion.

• Mitigation of Differential Settlement Effects: Although tie beams are primarily designed for axial forces, their presence provides extra stiffness that helps redistribute loads if one point of the ground yields slightly. By being connected, a footing tending to settle finds "support" in its neighbors, limiting relative sinking.

The Chilean seismic design code (NCh433) states:

7.2.2 Foundations on isolated footings that do not have adequate lateral movement restraint must be joined by tie beams designed to absorb a compression or tension of no less than 10% of the vertical demand on the footing.

Tie Beam Design

Most design codes indicate that tie beams should be designed to resist only axial forces. These axial forces must include both tension and compression, equivalent to 10% of the maximum vertical load between the foundations the beam connects.

Tensile Strength

In beam design, the contribution of concrete to tensile strength is neglected. Therefore, the total tension must be resisted by the longitudinal steel of the beams. Thus, the calculation of the required steel area is quite simple:

The distribution of steel within the beam section is not critical, as it is expected to work only in tension. It is common to specify this amount of steel equally distributed in the corners of the section, considering that these sections are generally not of large dimensions.

Note: In cases where the section dimensions are greater than 0.9m, it is recommended to provide additional skin reinforcement to the beam to prevent cracking. An adequate amount for this reinforcement is $\phi 8 @ 25$.

Compressive Strength

Compressive strength is calculated analogously to the pure compressive strength of a column. It is assumed that both the concrete and the steel are in an ultimate compression state. Additionally, the ACI 318 code considers that no load is perfectly centered, so the strength is reduced by 0.80 to account for accidental eccentricity. Thus, the nominal strength is given by:

Furthermore, since concrete crushing failure in compression is brittle, the strength reduction factor is lower than in the case of tensile failure. In this case, a factor of $\phi = 0.65$ is used. If a tie beam does not meet the minimum required compressive strength, it can be strengthened by specifying more reinforcement $A_s$, but the most effective way is to increase the cross-sectional dimensions (larger $A_g = b \cdot h$).

Tie Beams in Foundaxis

In Foundaxis, the modeling and design of tie beams are fully automated. We have an algorithm for the automatic creation of tie beams based on the specific layout of the foundation system.

The tie beam proposal generation algorithm scans all isolated foundations within the project. For each isolated foundation, it searches in the orthogonal directions ($\pm X, \pm Y$) for foundations to which it can be tied, with an angular deviation of up to $\theta = 15^\circ$. In this way, for a reasonably typical foundation arrangement, most isolated foundations are correctly tied using beams.

Subsequently, the design loads for all beams are determined (10% of the highest vertical load between the tied foundations), and the beams are designed for both tension and compression, using exactly the same approach explained above.

Finally, all tie beams in the project are automatically grouped to facilitate their detailing and construction. That is, all beams are adjusted to share the same cross-section, adapting to the one under the most unfavorable conditions.

 

Discover how tie beams design is integrated into the complete Foundaxis workflow:

Foundation Design Software: A Complete Guide for Structural Engineers 2026