Beams - Design

Beam design and optimization can be performed for rectangular concrete shapes based on the following codes:

• The 2022, 2019, 2014, 2011, 2008, 2005, 2002 and 1999 Editions of ACI 318
• The 1992 EuroCode (EC2) and the British publication of the 2014 and 2004 Eurocode (BSEN)
• The 2004 and 2014 Editions of the Canadian code (CSA A23.3)
• The 2004 Edition of the Mexican code (NTC-DF)

The program designs the longitudinal and shear reinforcement for rectangular beams. 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 in the beam detail reports.

Apply a Concrete Design Code

To apply a Concrete Design Code:

1. On the Design tab of the ‘Model Settings’ window, choose the Concrete code from the drop-down list.
2. Click Apply.

Beam Spans

RISAFoundation automatically breaks 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 is supported. Note that for a physical member to see a support, there must be a point at that support point.

Beam members are supported by the following: Supports (Reaction, Spring, etc.) and other Beam Members that are supporting that member. 

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.

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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.

Limitations - General 

Torsion – Beams ignore torsion with respect to the design of shear reinforcement. A message is shown in the detail report to remind you of this.

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

Cracking – The effects of concrete cracking are not considered in the Foundation analysis.

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. 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.

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 can 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 - 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 beam. 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.

All of the load combinations that were run had the Service box checked on 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 can be missed and an unconservative 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: The shear tie spacing does not meet the code Minimum Requirement

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

Rebar spacing exceeds maximum allowable (ACI 318-14 Section 13.3.4.4, ACI 318-11 Section 15.10.4)

The 2008 and newer editions of the ACI 318 forbid the placement of rebar in foundation slabs exceeding 18 inches. The program will still optimize rebar up to the largest spacing specified in the Design Rules Spreadsheet, and may violate this ACI provision. Reduce maximum spacing in the Design Rules - Beam Rebar.