D. ACI 318 - 99 Beam Design Principles
The ACI 318-99 Beam Design Brief is for single or multi span prismatic, rectangular solid or tee shaped members. The member sections must be defined as PRISMATIC sections in the STAAD.Pro data file.
Refer to D. Suitable Member Properties for more details.
Beams are designed for flexure, shear and torsion. Each member is divided into equally spaced sections along its length and the design is performed at each of these locations. The user can specify the number of segments to be considered (between 4 and 25) for each span. Sections are also taken at face of supports and at locations of the maximum positive and negative bending moments.
Design for Flexure
The main (longitudinal) reinforcement is calculated for both sagging and hogging moments on the basis of the section profile and parameters defined in the Design Brief. Compression reinforcement is provided where required.
The design of a beam is based on an envelope of the design forces and thus at each of the defined sections, the program determines the required area of steel for both the maximum hogging moment and maximum sagging moment values from the analysis.
The beam is then divided into sub-beams, those that can use the same reinforcement cage and having
- Same overall beam size
- Same cover requirements
For each sub-beam, the sections that have the largest sagging and hogging moments are identified and the most efficient reinforcement pattern is calculated for the range of bars specified in the Design Brief. The program does not have a limit on the number of bars in any one layer as long as the spacing requirements specified in the code are satisfied. The program can handle a maximum of 4 layers of reinforcement, two each for the top and bottom layers.
The program then performs a check at each of the defined sections to determine the number of bars, if any, that can be curtailed. The reinforcement bars will not be curtailed at these sections in the following cases:
- If the bars are required for compression or
- If curtailing these bars would result in a failure of crack-width checks or
- If curtailing these bars would result in a failure of the minimum reinforcement checks.
Design for Shear and Torsion
Stirrups are designed to resist the major axis shear force envelope, Fz, and optionally the torsional moments induced in the beam. The minor axis shear forces and torsional moments are not considered in the design.
The bar size for shear stirrups and the minimum number of shear legs to be provided are specified in the Design Brief. Therefore the required spacing for minimum stirrups can be defined. The program then checks each section to determine the shear stress v and concrete shear capacity vc. From this, the section is classified as either ‘minimum stirrup’ or a ‘high shear’ section. Adjacent sections of the same type are grouped into zones. For non minimum stirrup zones, the shear stirrups are designed for the maximum shear force within that zone.
If torsion is to be considered, the program checks whether the threshold torsion has been exceeded at each section and if so determines the corresponding reinforcement required and then adds this to the requirements for shear.
The number of shear legs and the shear link size is specified in the Design Brief. Therefore the required spacing for minimum links can be defined. The program then checks each section to determine the shear stress, v, and concrete shear capacity, vc . From this, the section is classified as either minimum link or a high shear section. Adjacent sections of the same type are grouped into zones. For non minimum link zones, the shear links are designed for the maximum shear force within that zone.
If necessary, additional legs may be added to the shear links in order to restrain tension or compression reinforcement.
Minimum shear links required,V, for shear forces between these values
Anchorage and Bond Lengths
Anchorage and bond lengths are calculated in accordance with the requirements of chapter 12. They can be displayed graphically on the main reinforcement diagram and are used for the schedule table.
Code Clauses Implemented
The following lists the code clauses used from ACI 318-99.
Chapter 3 - Materials
Chapter 7 - Details of Reinforcement
Chapter 8 - Analysis and Design - General Considerations
Chapter 9 - Strength and Serviceability Requirements
Chapter 10 - Flexure and Axial Loads
- 10.2.2 but not deep flexural members
- 10.2.6 parabolic stress block assumed except for T and L-sections
- 10.2.7 for T and L-sections only
- 10.11.2 - r always computed
- 10.12.1 - k controlled by brief
Chapter 11 Shear and Torsion
- 18.104.22.168 - minimum shear rft always provided
Chapter 12 - Development and Splices for Reinforcement.
- beta d factor.
- whether or not there is transverse loading.
- stability index Q (must be > 0.05 and ≤ 0.6).
- load case number that causes appreciable sidesway. If columns require different loadcases for sidesway, then they should be designed in separate design groups.
- beta factor for biaxial bending (default 0.65).