Concrete Beam Design
Requires: Design Level
- Concrete Beam Capabilities
- Concrete Beam Assumptions and Limitations
- Concrete Beam Parameters Overview
- Concrete Beam Parameters
- Concrete Design Parameters
- Concrete Beam Options Parameters
- Concrete Beam Stirrups Parameters
- Concrete Beam Design Process
- Concrete Beam Reports
Concrete Beam Capabilities
The concrete beam module allows VisualAnalysis to design and check concrete beams based on the ACI 318-11 Code Provisions. The beam may have one of three shapes:
- Rectangular (isolated beam)
- Spandrel Beam (slab extension one side only)
- Tee (slab extension both sides)
When modeling Tee and Spandrel shapes the slab thickness is defined in the modeled shape and is not changed by the design software. A design search for a shape that work will modify only the stem thickness and the overall depth. For Tees, the slab width for the modeled shape is not used during the checks, but the effective width calculated according to ACI and your input value for beam spacing is used. For Spandrels, the slab width (overhang) is assumed to be fixed and is used along with beam spacing in determining the effective width.
Bending behavior is assumed to be uniaxial only. (For biaxial checks or significant axial forces, use the column module.) The design module may be used to take into account the effect of torsion in addition to bending and shear. The module also suggests a reinforcing pattern and stirrup spacing. A standard beam detail, as shown below, with three sets of top bars and two sets of bottom bars is assumed. Once a reinforcing system is chosen, the module checks the design details for strength and spacing criteria.
Results tabulated include strength provided and strength required (both flexure and shear) as well as reinforcement spacing information. The design and subsequent checks are determined for those load cases that have been created with the strength Design Category (). You should only select load cases that have appropriate ACI load factors defined to use with the ACI Beam Design Module. The unity check values for flexure and shear for the worst load case are shown graphically in Design Windows.
TL Bars: Main reinforcing steel for the top left (node 1 end) third of the beam.
TC Bars: Main reinforcing steel for the top of the beam, and continuous across the supports..
TR Bars: Main reinforcing steel for the top right (node 2 end) third of the beam.
B Bars: Main reinforcing steel for the bottom center third of the beam.
BC Bars: Main reinforcing steel for the bottom of the beam, and continuous across the supports.
S Bars: Reinforcing steel for the sides of the beam for deep beams or torsional strength. The number of bars shown must be provided on EACH side of the beam.
Concrete Beam Assumptions and Limitations
This section summarizes the assumptions made in the concrete beam design module. It also includes several discussions that are very critical to the proper use of the design module. You are encouraged to read the attached sections before using the software.
- Beams are assumed to be modeled from column-to-column. You should not split your beam into multiple member elements unless you combine those elements using the Combined Member feature. Combining members so one Combined Member spans multiple supports may also cause inaccurate results.
- Left/Right sides of beam correspond to the member local axes, so the 'start node' is on the left and the 'end node' is on the right.
- Rebar is calculated for each 1/3 span. Rebar is assumed to be terminated at the 1/3rd points of the beam. Development length issues are not considered in the software.
- Normal weight concrete is assumed.
- No axial forces or biaxial bending is considered. Use the column design module if these forces are present.
- When a member is subjected to high torsion, you will be prompted to provide information regarding whether or not this member is part of an indeterminate torsional system. The ACI code allows higher strength provisions for indeterminate torsion. See ACI section 126.96.36.199 for more information.
- Special moment resisting frame is not used for seismic. This module assumes that the conditions of ACI 9.3.4 do not apply. In other words, the member is not part of a structure that relies on special moment resisting frames or special reinforced concrete structure to resist earthquake effects. Only the shear f (phi) factor modifications of ACI 9.3.4 will be used if you so indicate in the Beam Parameters area of the inspector.
- The Design shear demand is calculated at distance d from the face of the support column,
where 'd' is the depth of the reinforcement steel measured from the compressive face to the steel centroid.
This is consistent with ACI 11.1.3 assuming the conditions specified in that section are met.
It is left to the user to ensure the requirements of 11.1.3 are satisfied. If they are
not satisfied, it is left to the user to perform the design outside of the software.
Shear steel (stirrups) are assumed to begin at 3" from the face of the support, as shown in the standard beam detail.
- Top flexure steel checked at the support. For flexural calculations, the critical section for top steel is taken at the face of the support since the member has nearly an infinite depth once it reaches the support.
- Splitting members is not recommended. You should be careful about trying to design members that have been split into multiple member elements. The reinforcement details assume that the endpoints of the members are at supports. For the best design results, you should use the feature to combine elements into a single member.
- Cutoff lengths and locations are not determined by the software
- No "diagram" is produced to graphically display the bar locations or stirrup arrangements. There is a 'schedule' in the report with a list of bar requirements.
- Bracing is not a parameter for concrete design. All beams are assumed to have adequate lateral support to prevent rotation.
- Deep beams, as defined in ACI 11.7.1, cannot be designed in VisualAnalysis. VisualAnalysis will warn you if a beam has a clear-span length less than 4h. However, if you use a combined member to span across multiple supports the program's check will not be accurate. VisualAnalysis does not check for concentrated loads within 2h of the support face. It is left to the user to detect this situation and perform an approapriate design outside of the software.
Concrete Beam Parameters Overview
In selecting member sizes, reinforcing details, and subsequently checking a concrete beam, several parameters must be defined. The design parameters are controlled completely through the Modify tab of the Project Manager in the Design View. Prior to executing design checks, you should select one of the members that belongs to the design group in the Design View and go to the Modify Tab of the Project Manager and set up the design parameters.
Keep in mind that the parameter information you enter applies to all members of the design group, so it may be wise to choose the most conservative condition that applies to any member in the group.
Concrete Beam Parameters
Member Type: Indicates the design shape (Beam or Column).
Overstrength?: Causes the member to be designed for overstrength forces per ASCE 7 and ACI 318
Disable Checks?: Causes selected design group to be omitted from design checks.
High Seismic Risk - (Use Reduced Phi Factors for Members Resisting Earthquake Effects) This tells the design module to apply the lower PHI factors as indicated by ACI Code section 9.3.4 for members which are designed to resist earthquake effects and are part of a structure that relies on special moment resisting frames or special reinforced concrete structural walls to resist earthquake effects. Note that the program relies solely on this parameter in determining whether or not to use the reduced f (phi) it does not attempt to calculate whether the shear capacity is greater than the shear corresponding to the development of the nominal flexural strength of the member. Only the Phi factor for the shear is affected by this entry.
Slab Thickness: (Not shown) For Tee or Spandrels, the slab thickness is defined by the modeled shape, and is not entered directly in the design parameters.
Minimum Depth: The first value for depth that the program tries as it iterates over sizes in search of a satisfactory section. It will start from this size and go up. If the width needs to be fixed at a certain size, that size should be specified as the Minimum and Maximum Depth and the Increment Depth parameter should be set to zero.
Maximum Depth: The maximum value for depth that the program tries as it iterates over sizes in search of a satisfactory section.
Increment Depth: The amount by which the program increases its "trial" depth as it iterates over sizes in search of a satisfactory section. If the depth needs to be a specified value, enter zero for this increment (the depth value will be that specified by the Starting Depth parameter).
Minimum Width: The first value for width that the program tries as it iterates over sizes in search of a satisfactory section. It will start from this size and go up. If the width needs to be fixed at a certain size, that size should be specified for Minimum and Maximum Width and the Width Increment parameter should be set to zero. For Tees and Spandrels, this is the Stem Width.
Maximum Width: The maximum value for width that the program tries as it iterates over sizes in search of a satisfactory section.
Increment Width: The amount by which the program increases its "trial" width as it iterates over sizes in search of a satisfactory section. If the width needs to be a specified value, enter zero for this increment (the width value will be that specified by the Starting Width parameter).
Beam Options Parameters
Main Fy:: Specified yield strength of the longitudinal (flexural) reinforcement in the beam.
F'c: The specified compressive strength of the concrete.
Beam Spacing: The perpendicular spacing between beams (centerline to centerline). This is used for Spandrel and Tees to determine the effective flange width. This parameter is not necessary for beams with rectangular cross sections and will not be available.
Top Cover: Concrete clear cover at the top of the section. Calculated as the distance from the top of the stirrup to the top of the section.
Bottom Cover: Concrete clear cover at the bottom of the section. Calculated as the distance from the bottom of the stirrup to the bottom of the section.
Start/End Column Widths: Widths of supporting columns at start and end of beam. These widths are used for determining where critical shear and moment sections are at the ends of the beam. The critical moment is located at the face of the column and the critical shear is located at "d" from the face of the column. These ends correspond to the member's local axes, where local x goes from the start-node to the end-node.
Concrete Beam Stirrups Parameters
Stirrup Fy: Specified yield strength of the stirrups in the beam.
Size: Size of the stirrup bars.
Number of Legs: Number of vertical legs per stirrup. A value of 2 indicates normal U-shaped stirrups.
Symmetric? Option specifying that the beam stirrups are to be laid out symmetrically about the beam centerline.
Concrete Beam Design Process
1. Specify preliminary member size and Material (Model View) , and Design Parameters (Design View)
Concrete members are created using Standard Parametric cross sections. You will also need to apply loads and set up strength design load combinations. For more details on these 'basic' steps, please see: The Design Process)
2. Analyze your model, and select the Design View Tab to check the Unity Ratio.
If the cross section created in the Model View meets strength requirements, reinforcement will be sized to meet demands.
3. Manually adjust reinforcement (optional)
The sized longitudinal reinforcement and stirrup spacing can then be adjusted to your preference while in the Design View (Modify Tab, Details Heading, Adjust Rebar [...] button). Unity checks will then be updated in the Design View.
You can iterate steps 1-3 to obtain a final design, or you can use the Design feature in VisualAnalysis for optimizing design by continuing with the following steps:
4. Design The Group
Select a group from the Design View window. Then use the menu item.
5. Select a Beam Size
From the dialog you a may select an appropriate beam size. Unity Ratios are shown in the table to indicate just how close you are to code allowed maximums. Once you chose a size, a suggested set of reinforcement will be shown if possible. The ~ symbol indicates unity ratios must be validated with another analysis (because member stiffness may have changed).
6. Adjust Suggested Reinforcing Details
Initially the suggested reinforcing patterns that should meet code requirements are presented. If you are not satisfied with the suggestions, change the values as you see fit (Similar to step 3).
7. Synchronize Design Changes
Analysis results exist for the modeled shape, and are affected by member stiffness. In order to provide final verification of designed shapes, the menu item Design | Synchronize Design Changes must be selected. This will update the modeled shape to that selected in the design process.
8. Verify Unity Ratios in the Design View
The final step in the design process is to verify that Unity Ratios (Design View) are less than unity. If member sizes were drastically changed during the design process, final unity ratios can differ from predicted unity ratios because the analysis results may vary significantly.
Concrete Beam Reports
The design reports produced by the design module are available by simply double clicking on the member while in the Design View. This will report section material, reinforcement and code checks. The right click context menu also allows quick generation of design reports.