ASDIP CONCRETE is a structural engineering software for design of concrete members. It includes the design of multi-span continuous beams based on the latest ACI 318 provisions.
The design of concrete beams involves the calculation of shears for the factored load combinations. These forces in turn must be smaller than the provided design shear strength at any point of the beam, and in addition the design must comply with a number of code limitations. This document shows how to design the stirrups along a typical concrete beam using ASDIP CONCRETE.
- Click here to download ASDIP CONCRETE free 15-day trial.
How do you generate the shear diagram?
The screen shot below shows a concrete continuous beam subject to some uniform and concentrated dead and live loads. Since the concrete members are designed per the Ultimate Strength Design method, the factored load combinations apply. As you enter any load, ASDIP CONCRETE generates immediately the corresponding shear and moment diagrams, which update with any change in geometry as well. The software allows to sort the results per load combination, as shown below.
In this article we will focus on the shear diagram only. It should be noted that the maximum shear Vu to be used for design purposes corresponds to the value at the effective section, located at a distance "d" from the support face. ASDIP CONCRETE calculates and shows graphically this shear value at the effective section.
How do you calculate the shear strength?
The design shear strength has two components: the strength provided by the concrete alone, and the strength provided by the stirrups. Mathematically this means φVn = φVc + φVs. In this expression the first term is constant, but the second term varies with the size and spacing of the stirrups.
ASDIP CONCRETE shows graphically the two components of design shear strength in the same chart of the shear diagram for visual comparison purposes. This way the designer can immediately identify any deficiency and correct it. In addition, the reports also show design ratios with color messages to identify any problem.
Depending upon the magnitude of Vu at a given section, the ACI 318 has established the rules to calculate the required design shear strength at that section. In general terms, if Vu > φVc, stirrups must be designed to provide the required strength. if φVc / 2 < Vu < φVc, the minimum shear reinforcement must be provided. Finally, if Vu < φVc / 2, no stirrups are required. The code limits the maximum stirrups spacing to d/2 for most practical cases, which frequently controls the beam stirrups design.
How do you design the stirrups?
ASDIP CONCRETE provides the required tools to design the stirrups along the beam. The user can specify the number, size, and spacing of stirrups at both ends and at the center of the span. This usually provides a good balance between simpler designs that comply with the code provisions.
To design the stirrups, select Design Manager from the Design menu bar. This will open the Design Manager, where you can specify the preferred stirrups size and the number of legs. With this information ASDIP CONCRETE will calculate the required spacing of the stirrups at both ends and at the center of each span. The point that delimits these areas is where Vu = φVc. At the center of the span the software will calculate the minimum shear reinforcement. Please note that this is conservative, since probably there will be an area where stirrups are not required, but in practice some stirrups will be placed anyway to hold the longitudinal rebars.
This first design from the Design Manager is usually good enough for most cases. However, the user can always go to the Reinforcement tab and modify the stirrups spacing as required. ASDIP CONCRETE creates two tables, one shows Vu at the ends of each span, and the other one shows the shear strength and the required maximum stirrups spacing. With this numerical information the user can modify the spacing, and at the same time check these two tables. When something is not correct, a red message "NG" will show up, otherwise you will see a green message "OK", as shown below.
ASDIP CONCRETE generates an image with the beam elevation showing all the reinforcement, including the stirrups, as shown below.
ASDIP CONCRETE includes the design of continuous beams, with multiple options to optimize the design easily. This example shows how to design the stirrups along the beam quickly, and how to check the results both graphically and numerically.
For engineering background, see the post Continuous Concrete Beam Design: A Structural Challenge. For our collection of blog posts about concrete design please visit Structural Concrete Design.
Javier Encinas, PE
ASDIP Structural Software