Concrete Member - Design

Concrete design and optimization can be performed for standard concrete shapes based on the following codes:

• The 2022, 2019, 2014, 2011, 2008, 2005, 2002 and 1999 Editions of ACI 318
• The 1997 Edition of the British code (BS 8110)
• The 1992 EuroCode (EC2) and the British publication of the 2014 and 2004 Eurocode (BSEN)
• The 1994 and 2004 Editions of the Canadian code (CSA A23.3)
• The 2000 Edition of the Indian code (IS 456)
• The 2001 Edition of the Australian code (AS 3600)
• The 1995 Edition of the New Zealand code (NZS 3101)
• The 2004 Edition of the Mexican code (NTC-DF)
• The 2007 Edition of the Saudi Building Code (SBC 304)

Note:

• Unless otherwise specified, all code references below are to ACI 318-14.
• Beams and Columns designed in RISA meet all of the requirements for Ordinary Moment Frames except for the additional requirements indicated in ACI 318-14 Section 18.3. Those provisions should be checked by hand outside of RISA.

• ACI 318-19 (22) re-approves previous ACI 318-19 without any technical changes, therefore all references in the help documentation and in the program continue to use ACI 318-19.

The program will design the longitudinal and shear reinforcement for rectangular beams and rectangular or circular columns. These calculations encompass all the code requirements except those noted in the Limitations section of this document. The program also provides reinforcement detailing information for concrete beams and interaction diagrams for concrete columns in the member detail reports.

Concrete Parameters (General):

• Unbraced Lengths
• K Factors (Effective Length Factors)

ACI 318 Design Parameters:

• Cm Coefficients (Interactive Bending Coefficients)
• Sway Flags
• Icr Factors (Cracked Moment of Inertia Factors)

British Eurocode Design Parameters

Known Limitations:

• ACI
• Canadian
• British
• Indian
• EuroCode
• Australian / New Zealand

To Apply a Concrete Design Code

1. On the Code tab of Model Settings Dialog, select the concrete code from the drop down list.
2. Click Apply or OK.

Concrete Spans

RISAFloor will automatically break a concrete physical member into spans based on the number of internal supports. Each internal point is NOT automatically treated as a support. Instead, we go through the whole model geometry to determine where a beam or column is supported. Note that for a physical member to see a support, there must be a point at that support point. If a physical column and a physical beam cross each other without a joint at their intersection, then no support / span will be detected and they will not be connected.

Beam members are supported by the following: Column Members and other Beam Members that are supporting that member. 

Column members are supported by the following: Beam Members and Rigid Diaphragms or Decks.

Note

• The program's ability to recognize spans is important because it will give you more relevant span to span information without overwhelming you with independent design results for each finite element segment that comprises your physical member.
• For continuous beam members, the program will evaluate the framing to determine which beams elements are supporting other beam elements so that only supporting members are treated as supports and not vice versa.

Concrete Design Parameters - Columns 

The Concrete tab on the Columns Spreadsheet records the design parameters for the code checks of concrete columns. These parameters may also be assigned graphically. See Modifying Column Design Parameters to learn how to do this.

The following parameters can be defined for each concrete column. The pull down list at the top of the spreadsheet allows you to toggle between floors.

Label

The Label field for column members is dictated by the label entry on the Column Stacks Spreadsheet and may not be edited here. If you would like to edit this entry, you must do so on the Column Stacks Spreadsheet.

Unbraced Length

You may specify unbraced lengths or have RISAFloor calculate them for you. The unbraced lengths are Lu-yy and Lu-zz.

The Lu values, Lu-yy and Lu-zz, represent the unbraced length of column members with respect to column type buckling about the member's local y and z axes, respectively. They are listed on the Concrete tab of the Columns Spreadsheet. These Lu values are used to check the column for Euler buckling, and for the Moment Magnification Procedure in RISAFloor and older editions of the ACI code.

If the Lu values are not entered (left blank), the unbraced lengths for each segment of a physical column or lift will be automatically calculated by RISAFloor based on the distances between floor levels and/or splices. This means that each physical column can have multiple unbraced lengths if the entry is left blank. However, if a number is entered, RISAFloor will use that value for ALL segments of the physical column. Unbraced lengths that have been set or modified in RISA-3D will also be used in RISAFloor.

Note

• The calculated unbraced lengths are listed on the Member Detail report.
• The "Segment" code cannot be entered for column members in RISAFloor.

For additional advice on this topic, please see the RISA Tips & Tricks website: www.risa.com/post/support. Type in Search keywords: Unbraced Lengths.

K Factors (Effective Length Factors)

The K Factors are also referred to as effective length factors. Kyy is for column type buckling about the member's local y-y axis and Kzz is for buckling about the local z-z axis. 

If a value is entered for a K Factor, that value will be used for the segment of the physical column between the current floor level or splice above and/or the floor level or splice below. When in RISA-3D via RISAFloor, the largest K factor entered for any segment of a physical column will be used for the entire physical column. If an entry is not made (left blank), the value will internally default to '1' for that column segment. See ACI 318-14 Section R6.2.5 (ACI 318-11 Section R10.10.1) for an explanation of how to calculate K Factors.

Sway Flags

The Sway Flags indicate whether the member is to be considered subject to sidesway for bending about its local y and z axes. The y sway field is for y-y axis bending and the z sway field is for z-z axis bending. Click on the field to check the box and indicate that the member is subject to sway for that particular direction, or leave the entry blank if the member is braced against sway. These sway flags influence the calculation of the K Factors as well as the Cm.

Sway flags may be applied to any column segment at any floor level. However, when in RISA-3D via RISAFloor, if a sway flag is checked for any segment of a physical column, the entire physical column will be assumed subject to sway. Beam members in RISAFloor are assumed to be braced against sway.

Cm – Equivalent Moment Correction Factor

The Cm Coefficients are used to check the column for Euler buckling, and for the Moment Magnification Procedure in RISAFloor and older editions of the ACI code.  Cm-yy is for bending about the columns's local y-y axis and Cm-zz is for bending about the local z-z axis. If these entries are left blank they will be automatically calculated. Each segment of a physical column will receive its own calculated value. However, when in RISA-3D via RISAFloor, the highest Cm value calculated for a column segment in RISAFloor will be used for the entire physical column in RISA-3D.

In the ACI design code, the Cm values are only applicable for non-sway frames. Therefore, this value will be ignored if the corresponding sway flag is checked.

Icr Factors (Cracked Moment of Inertia Factors)

The Icr Factor is used to reduce the bending stiffness of concrete columns per ACI 318-14 Table 6.6.3.1.1(a) (ACI 318-11 Section 10.10.4.1). If this entry is left blank, default values of 0.35 for beams and 0.70 for columns will be used. 

Note

• The Icr Factor will be ignored if the “Use Cracked Sections” box is not checked on the Concrete tab of the Model Settings dialog. 
• The alternative calculations in ACI 318-14 Table 6.6.3.1.1(b) (ACI 318-11 Equations 10-8 and 10-9) are not considered.
• The sustained load reduction of ACI 318-14 Section 6.6.3.1.1 (ACI 318-11 Section 10.10.4.2) is not considered.

Note

Concrete Design Parameters - Beams

The Concrete tab on the Beams Spreadsheet records the design parameters for the code checks of concrete beams. These parameters may also be assigned graphically. See Modifying Beam Design Parameters to learn how to do this.

The following parameters can be defined for each concrete member.

Label 

You may assign a unique Label to all of the members. Each label must be unique, so if you try to enter the same label more than once you will get an error message.

Length

The beam Length is reported in the second column.  This value may not be edited as it is dependent on the beam start and end points listed on the Primary Data tab. It is listed here as a reference only.

Composite

The Composite field should be "checked" if the beam design is to consider composite behavior with the floor deck above. A T-beam or L-beam will be designed if this box is checked. If the Composite field is "un-checked", the beam will be designed as the shape indicated on the Beam Primary Data tab and any values entered for either B-eff Left or B-eff Right will be ignored. See T-beam & L-beam Sections below for more information.

Flexural and Shear Rebar Layout 

The user may choose to manually create the reinforcement layout for the beam. This must be done if the user wishes to take advantage of compression steel, or multiple layers of reinforcement. See Concrete Database - Rebar Layouts for more information. If Use Design Rule is specified, then the program will design for one layer of reinforcing and may vary that reinforcing based on ACI minimums, maximums, and the moment and shear demand at each section along the span using the Member Design Rules as the parameters of the reinforcement selection. If you define your own rebar layout, and compression reinforcement is defined, then the program will consider the compression reinforcement in the analysis.

Icr Factors (Cracked Moment of Inertia Factors)

The Icr Factor is used to reduce the bending stiffness of concrete beams.

For ACI and Canadian codes (ACI 318-14 Section 6.6.3.1 and A23.3-04 section 9.2.1.2), if this entry is left blank, the Icr factor will default to a value of 0.35 for beams and 0.70 for columns.

For Australian and New Zealand codes (per section 6.6.2 of AS3600-2001), this will default to a value of 0.4 for beams and 0.8 for columns.

For Indian and Saudi codes, this entry will default to a value of 1.0 for beams and columns.

Note

• The Icr Factor will be ignored if the “Use Cracked Stiffness” box is not checked on the Concrete tab of the Model Settings dialog. 

T-beam & L-beam Sections- RISAFloor ES Only

T-beams and L-beams may be specified by indicating a beam as composite when creating the beam or after the beam is created by checking the composite check box on the Concrete tab of the Beams Spreadsheet. This modification may also be made graphically via the Modify Properties tab of the Draw Beams tool.

Note:

• The T-beam & L-beam design is done only when a one-way slab is defined. See One-Way Slab Design for more information.
• Composite beam design is supported in both one-way and two-way slabs in RISAFloor ES. The composite action between the beam and slab is considered in the finite element analysis for both slab types, the only difference is that the beam rebar design uses rectangular shape in two-way slab and T or L shape in one-way slab (if Composite checkbox is on).

RISAFloor will automatically calculate the effective slab widths, B-eff Left and B-eff Right, based on ACI 318-14 Table 6.3.2.1) ACI 318-11 Sections 8.12.2 and 8.12.3) if no values are entered by the user in the Beams Spreadsheet. You may override the values calculated by RISAFloor by entering your own values for B-eff Left and B-eff Right in the spreadsheet. It should be noted that if the value entered is in excess of the values allowed by ACI 318-14 Table 6.3.2.1 (ACI 318-11 Sections 8.12.2 and 8.12.3), RISAFloor will reduce the value to meet the code requirements.

If the composite check box is un-checked, the beam will be designed as the shape indicated on the Beam Primary Data tab and any values entered for either B-eff Left or B-eff Right will be ignored.

RISAFloor automatically calculates the slab thicknesses for T-beams and L-beams based on the slab thickness defined in each slab area. See the Slab Definitions Spreadsheet for information on defining slab thicknesses.

Note:

• B-eff Right corresponds to the positive local z-axis of the beam. Subsequently, B-eff Left corresponds to the negative local z-axis.

Parabolic vs. Rectangular Stress Blocks 

You can specify whether you want your concrete design to be performed with a rectangular stress block, or with a more accurate parabolic stress block. While most hand calculations are performed using a rectangular stress block, the parabolic stress block is more accurate. In fact, most of the PCA design aids are based upon the parabolic stress distribution. A good reference on the parabolic stress block is the PCA Notes on ACI 318-99.

Biaxial Bending of Columns 

You can specify whether you want your column design to be performed by using Exact Integration, or by using the PCA Load Contour Method. While most hand calculations are performed using the Load Contour Method, this method is merely an approximation based on the uniaxial failure conditions and the Parme Beta factor. In contrast, the Exact Integration method uses the true biaxial strain state to design the member. A good reference on the Load Contour Method is chapter 12 of the PCA Notes on ACI 318-99.

British Eurocode Design Parameters (BS EN 1992-1-1: 2004)

General

• f’_ck – Can not be more than 50 MPa (7252 psi) for normal strength concrete
• α_cc is assumed to be 1 (recommended value) : See 3.1.6
• Effective length of T and L: L_o=.7*span length and b_eff,i =L_o/10
• φ_concrete = 1.5
• φ_rebar = 1.15
• Maximum bar spacing for beams = 300 mm

Tension Development Length

• α_ct= 1 (assumed in Eq 3.16)

• η₁= η₂=1 (assumed in Eq 8.2 to calculate bond stress)

• λ₃, λ₅, λ₄=1 (assumed in Eq 8.4)

• C_d in Table 8.2 assumed to be 1 bar diameter rebar spacing

• Development length when hooks are provided uses same assumptions as BS 8110-1: 1997

Shear Capacity of Concrete

To compute the shear capacity of concrete the following recommended values are being used:

• C_Rd,c =0.18/γ_c for Eq 6.2.a

• v_min =0.035 k^3/2 f_ck^1/2

• ν =.6*[1- f_ck /250]

• V_max is calculated from Eq. 6.5.

• θ is assumed 45 degrees in Eq 6.8.

Slender Column Design

• Biaxial column design done using Eq. 5.39

• Design based on nominal curvature

• λ_lim = 20 A B C/n^1/2 (A=.7, B= 1.1, C=0.7 for unbraced)

• K_ϕ =1 in Eq 5.34; the effect of creep is neglected.

Limitations - General 

Torsion – Beams and columns ignore torsion with respect to the design of shear reinforcement. A message is shown in the detail report to remind you of this. You can turn the warning messages off on the Concrete tab of the Model Settings Dialog.

Creep / Long Term Deflections – No considerations are taken in the analysis to account for the effects of creep or long term deflections.

Beam Design – Beams are not designed for weak axis y-y bending, weak axis shear, or axial forces. A message is shown in the detail report to remind you of this. You can turn the warning messages off on the Concrete tab of the Model Settings Dialog. Beams currently do not consider any compression steel in the calculation of the moment capacity. Beam "skin reinforcement" per the requirements of ACI 318-14 Section 9.7.2.3 (ACI 318-11 Section) 10.6.7 for beams with "d" greater than 36" is currently not specified by the program. The provisions in ACI 318-14 Section 9.9 (ACI 318-11 Section 10.7) for deep beams are not considered.

Column Design – Columns with biaxial moment and no axial load will currently be designed using the PCA Load Contour Method even if Exact Integration is selected on the Model Settings dialog. This is shown on the detail report.

Limitations - ACI 

Shear Design –When ACI 318-19 is selected, the shear strength of concrete (Vc) uses equations in Table 22.5.5.1. Note that for members meeting the minimum shear reinforcement requirement (Av≥Av,min), Vc is taken as the larger of the results calculated by the equations (a) and (b) in the table. ACI 318-19 code suggests ρw may be taken as the sum of the areas of longitudinal bars located more than two-thirds of the overall member depth away from the extreme compression fiber. Therefore, RISA calculates ρw as the sum of the areas of longitudinal bars on the tension face.

When other ACI 318 editions are selected, the shear strength of the concrete alone is limited to the standard 2*λ*sqrt (f'c) equation from ACI 318-14 Section 22.5.5.1 (ACI 318-11 Section 11.2.1.1) and does not use the more detailed calculations of ACI 318-14 Table 22.5.5.1 (ACI 318-11 Section 11.2.2). Also, note that for members with significant axial tension (greater than 0) the program designs the shear reinforcement to carry the total shear per ACI 318-14 Section R22.5.7.1 (ACI 318-11 Section 11.2.1.3).

Deep Beam Design – The program does not design deep beams as defined in ACI 318-14 Section 9.9.1.1(a) (ACI 318-11 Section 10.7).

Limitations - Canadian Code

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is assumed same as PCA method for the Canadian codes.

Bi-Axial Bending - The program uses the simplified uniaxial solution provided in the Canadian specification rather than performing a complete biaxial condition.

Mid-Depth Flexural Strain for Shear Design - The program uses the code equation (per the General Method) to calculate exwith the moment and shear at the section taken from the envelope diagrams. The maximum ex for each span is conservatively assumed for the entire span. Currently the program has no option for pre-stressing, so V_p and A_p are both taken as zero.

Shear Design - The shear strength of concrete is calculated using β and θ, which are both calculated per the General Method (Clause 11.3.6.4 from the 2004 CSA A23.3). S_ze is calculated per equation 11-10 and a_g is always assumed to equal 20 mm (maximum aggregate size).

Limitations - Australian and New Zealand Codes 

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is assumed same as ACI for the New Zealand and Australian codes.

Neutral Axis Parameter – Ku in AS code is always assumed to be less than 0.4.

Rebar Spacing –  NZS and AS codes: max spacing of rebar (beam) is 300 mm and minimum spacing is one bar diameter or 25mm whichever is bigger.

Shear Strength in Beams –  In AS code, when calculating the shear strength of a beam β2, β3 are always assumed to be unity.  This is always conservative for beams will little axial load, or beams in compression.  But, may be unconservative for members subjected to significant net tension.

Bi-Axial Bending – The New Zealand code does not appear to give a simplified method for solving biaxial column design.  Therefore, the PCA load contour method is being used instead.

Shear Tie Spacing –  Column/beam shear tie spacing is based on (a) and (c) of NZS 9.3.5.4 :1995.

Development Length – Development length in NZS is based on NZS 7.3.7.2 where αa is conservative assumed to be 1.3 (top bars) for all cases.  For the AS code, it is assumed that K1=1 and K2=2.4 in clause 13.1.2.1 of AS 3600:2001.

Slender Column Calculations –  EI is assumed to be equal to 0.25EcIg (with βd =0.6) in slender column calculations in AS and NZS codes (like in ACI).

Limitations - British

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is taken from the British specification.  

Cracked Sections –   Icracked defaults to 1.0 for the British code. But, a user entered value may be entered if desired. Service level stiffness is assumed to be 1.43 times the strength stiffness, but is not allowed to exceed Igross.

Bi-Axial Bending – The program uses the simplified uniaxial solution provided in the British specification rather than performing a complete biaxial condition.  

Limitations - Euro

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is taken from the EuroCode specification.  

Cracked Sections –  Icracked defaults to 1.0 for EuroCode. But, a user entered value may be entered if desired. Service level stiffness is assumed to be 1.43 times the strength stiffness, but is not allowed to exceed Igross.

Bi-Axial Bending – The program uses the simplified uniaxial solution provided in the EuroCode rather than performing a complete biaxial condition.

Limitations - Indian

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is taken from the Indian specification.  

Cracked Sections –   Icracked defaults to 1.0 for the Indian code. But, a user entered value may be entered if desired. Service level stiffness is assumed to be 1.43 times the strength stiffness, but is not allowed to exceed Igross.

Bi-Axial Bending – The program uses the simplified uniaxial solution provided in the Indian specification rather than performing a complete biaxial condition.  

Limitations - Saudi Code

Concrete Stress Profile –  Concrete stress strain curve (parabolic) is assumed to be the same as the ACI code.

Shear Strength–  The shear strength is based on 11.3.1.1 and does not include the more detailed provisions of section 11.3.1.2.

Yield Strength of Shear Ties - The yield strength of shear ties is not allowed to exceed 420MPa.

Shear Tie Spacing - Minimum spacing of shear ties is set to 50mm

Bi-Axial Bending – Both the Exact Integration and the PCA Load Contour methods for bi-axial bending are supported in the Saudi code.

Special Messages

In some instances code checks are not performed for a particular member. A message is usually shown in the Warning Log and Detail Report explaining why the code check was not done. There are also instances where a code check is performed, but the results may be suspect as a provision of the design code was violated. In these cases, results are provided so that they can be examined to find the cause of the problem. Following are the messages that may be seen.

No Load Combinations for Concrete Design have been run.

None of the load combinations that were run had the Concrete Design box checked on the Design tab of the Load Combinations Spreadsheet. Since there are no concrete design specific load combinations, there are no results or force diagrams to show.

Warning: No design for spans with less than 5 sections.

Certain very short spans in physical members can end up with less than 5 design sections. No design is attempted without at least 5 sections because maximum values may be missed and an un-conservative design may result.

Warning: No design for spans less than 1 ft.

Certain very short spans in physical members can end up with lengths less than 1 foot. No design is attempted for these sections.

Warning: Slender Compression Failure (Pu > .75Pc). No Slender calculations done.

Since RISAFloor allows you to specify a starting column size, it’s possible that for slender columns under substantial axial load you'll exceed the critical buckling load used in the slenderness equations in ACI 318-14 Section 6.6.4.5.2 (ACI 318-11 Section 10.10.6). Design results are still shown so the suggested shapes can be used to pick a new suggested column size that will not have this problem. Note that the design results shown are NOT valued because the slender moment effects have NOT been considered.

Warning: KL/r > 100 for this compression member.  See ACI 318-05 Section 10.11.5

Members that violate the KL/r limit still have design results calculated and shown. Note this is only checked for the 2005 code and older.  Newer codes require a P-Delta analysis and omit this consideration.

Warning: Exact Integration selected but PCA method used

This message is shown when you've requested the Exact Integration option on the Model Settings Dialog, but we weren't able to converge a solution for the column in question. When Exact Integration does not converge, the PCA Method is used instead to give an idea of the demand vs. the capacity.

Warning: PCA Method Failed. Axial Load > Axial Capacity.

One of the limitations of the PCA Method is that it requires the column being checked to have a greater axial capacity than the axial demand. Since RISAFloor allows you to set a starting size, it’s possible that the demand may be greater than the capacity. In this case a very rough estimate of the capacity is calculated by using the independent moment capacity about each axis considering the axial load. The resulting code check value is then based on the combined demand vector over the combined capacity vector and will always be greater than 1.0. The purpose of the results in this case is to show the column failed, not to give an accurate estimate of the over-demand. The redesign feature will suggest a larger shape to resolve this issue.

Warning: The shear tie spacing does not meet the code Minimum Requirement

This warning is stating that either minimum spacing or strength requirements are not being met for the shear reinforcement in the concrete member.