Masonry Wall - Seismic Design

Masonry walls can now be designed for seismic design provisions. Here the process is described.

Seismic Design Rules

For seismic design of masonry walls to be performed, the masonry wall panel must be assigned a Seismic Design Rule. Three default Seismic Design Rules are provided in the Seismic Design Rules spreadsheet - Masonry Walls tab as a starting point. Users are responsible to edit/create seismic design rules that fit specific design scenarios.

Masonry Wall Seismic Design Rule

Column	Description
Label	The label is a user defined text string that is used as a unique identifier for each of the seismic design rules defined for the structure. The program comes pre-loaded with a number of generic seismic design rules that you can edit to fit their design projects.
Wall Type	These are the three masonry wall types supported for masonry wall seismic design in RISA-3D.
Special Boundary Elements	This setting indicates that the wall will be designed with Special Boundary Elements per Section 9.3.3.6. The program will not check for the maximum area of vertical reinforcement requirements for walls with this checkbox turned on in the seismic design rules. Refer to the Design Considerations section for additional information on checks provided for walls designed with Special Boundary Elements. This setting is turned on by default for Special wall types. But you can edit the seismic design rules to meet your design needs.
1.5x Shear ASD	This setting applies a 1.5x factor to the shear demand of the walls with the seismic design rule applied in models solved with either TMS 402-16: ASD or ACI 530-13: ASD codes. This setting is turned on by default for Special wall types columns.

The following are the three possible states for the Label and Wall Type columns.

• Ordinary: This wall type will perform design checks based on the minimum reinforcement requirements of Ordinary Reinforced Masonry Shear Walls per section 7.3.2.4.

• Intermediate: This wall type will perform design checks for Minimum Reinforcement Requirements of Chapter 7, in addition to additional requirements of Intermediate Reinforced Masonry Shear Walls per section 7.3.2.5.

• Special: This wall type will perform design checks for Minimum Reinforcement Requirements of Chapter 7, in addition to additional requirements of Special Reinforced Masonry Shear Walls per section 7.3.2.6. These requirements include:

  • Additional requirements are checked for Special Reinforced Masonry Walls per section 8.3 (ASD Solution) and 9.3 (Strength Solution) in RISA-3D.

Design Considerations

All Wall Panels with Seismic Design Rules applied will be checked for the following Minimum Reinforcement Requirements.

Minimum Reinforcement Requirements

All masonry design walls assigned a seismic design rule will be checked for minimum reinforcement requirements per ACI 530/TMS 402 section 7.3.2. including:

• Vertical Reinforcement

  • At Wall Ends (for wall regions at the ends of the wall panel)

  • Minimum Bar Size

  • Maximum Bar Spacing Around Openings (for applicable Wall Panel Regions)

• Horizontal Reinforcement

  • At Top of Wall Region

  • Minimum Bar Size

  • Maximum Bar Spacing Around Openings (for applicable Wall Panel Regions)

Additionally, all wall types are checked for Mortar type requirements per ACI 530/TMS 402 section 7.4.4.2.2.

Special Reinforced Masonry Walls - Additional Requirements

Additional checks are provided for wall types assigned a “Special” seismic design rule per section 7.3.2.6. These additional checks include:

• Maximum vertical spacing of vertical reinforcement

  • Shall be the smallest of one-third of the length of the wall region, one-third the height of the wall region and 48 inches.

• Maximum spacing of horizontal reinforcement

  • Shall be the smallest of one-third of the length of the wall region, one-third the height of the wall region and 48 inches.

• Minimum cross-sectional area of vertical & horizontal reinforcement

  • The sum of vertical and horizontal cross-sectional area shall be at least 0.002A_g(vert), where A_g(vert) is the cross-sectional area of the wall panel region (on a horizontal plane)

  • Minimum vertical & horizontal cross-sectional area shall be 0.0007A_g, where A_g is the gross cross-sectional area perpendicular to reinforcement being considered

ASD - Additional Requirements

The ACI 530/TMS 402 code has additional seismic detailing requirements for masonry shear wall panels, some of which are considered in RISA-3D. The seismic detailing requirements in this section are only considered if the model is solved using one of the supported Service (ASD) masonry codes.

Max. Flexural Rho Check

Masonry Wall Panels with a Seismic Design Rule of “Special”, a shear span ratio (M/Vd_v) ≥ 1, and an axial load ≥ 0.05f’_mA_n, shall be checked for a maximum reinforcement ratio equal to ρ_max, calculated based on ACI 530/TMS 402 Equation 8-20:

Where:

• n is the modular ratio of steel reinforcement

• f’_m is the compressive strength of masonry

• f_y is the yield strength of steel reinforcement

ASD Wall Shear Stress Increase Check

Masonry Wall Panels with a Seismic Design Rule of “Special” will be checked if a ASD shear stress increase of 150% (1.5x factor) is applied to the in-plane shear demand as required in Section 7.3.2.6.1.2. The program automatically handles this requirement if the “1.5x Shear ASD” setting is turned on in the Seismic Design Rules - Masonry Walls tab, and will apply a 1.5 factor to the shear demand in the wall region.

LRFD - Additional Requirements

The ACI 530/TMS 402 code has additional seismic detailing requirements for masonry shear wall panels, some of which are considered in RISA-3D. The seismic detailing requirements in this section are only considered if the model is solved using one of the supported Strength (LRFD) masonry codes.

Special Boundary Elements Requirements

For Masonry Wall Panels with a Seismic Design Rule of “Special” and walls “Special Boundary Elements” setting turned on in the Seismic Design Rule, the program will determine if Special Boundary Elements are required per Section 9.3.6.6.1. Two conditions are considered to determine if Special Boundary Elements are required:

If any of the conditions above hold true, then Special Boundary Elements are not required per Section 9.3.6.6.1. If both of the conditions above do NOT hold true, then Special Boundary Elements are required. The program does not design the walls with the Special Boundary Elements. Users shall proceed to the requirements of Section 9.3.6.6 for further design requirements if special boundary elements are required.

Design Seismic Shear Capacity Limits

For Masonry Wall Panels with a Seismic Design Rule of “Special”, the program checks if the Design Shear Strength, phi_Vn, exceeds a shear value (Vu) that corresponds to 1.25 times the nominal flexural strength of the wall, calculated as follows:

Where:

• M_n is the in-plane nominal flexural strength of the wall panel region

• M_u is the in-plane moment (flexure) demand in the wall panel region

• V is the actual shear demand in the wall panel region

Additionally, the Design Shear Strength must not exceeds 250% of the shear demand in the wall panel region:

If any of the minimum and maximum limits to the Design Shear Strength is not met, a “Fail” will be indicated in this check in the Seismic Detailing section of the Wall Panel Detail Report for the wall panel region.

Maximum Area of Flexural Tensile Reinforcement (max. Flexural Rho Check - Strength)

For walls not designed with Special Boundary Elements (without the “Special Boundary Elements” and designed using a Strength code, all wall panel regions will be checked against maximum flexural reinforcement requirements per Section 9.3.3.2. Section 9.3.3.2 stipulates minimum levels of strain at the extreme tension fiber, which the program handles individually for each wall panel region, with a minimum tension reinforcement strain factor, “α”, for each masonry seismic wall type.

The tension reinforcement strain factor is dependent on the wall type, seismic response modification R factor, and the shear span ratio (M/Vd_v) ratio, as shown in the table below:

The strain ratio for the extreme rebar layer, calculated as e_s/e_y, where e_s is the strain in the steel at the extreme tension rebar layer and e_y is the yield strain of reinforcement.

The procedure for determining compliance is as follows:

• Wall Panel Region in-plane reinforcement design is done based on the flexure (moment) demand in the wall panel region.

• Shear span ratio (M/Vd_v), depth to neutral axis c and seismic response modification R factor are reported.

• Required minimum tension reinforcement strain factor is obtained per the table above.

• Strain ratio, calculated as e_s/e_y, where e_s is the strain in the steel at the extreme tension rebar layer and e_y is the yield strain of reinforcement, is compared to α.

• If e_s/e_y ≥ α, then reinforcement in the wall panel region is in compliance with Section 9.3.3.2.

• If e_s/e_y < α, then the reinforcement in the wall panel region is NOT in compliance with Section 9.3.3.2.

• The strain in the steel at the extreme tension rebar layer is calculated based on the depth to neutral axis c, and the effective depth of the wall region, d, as shown in the diagram below:

  

  Click on image to enlarge it

  The calculation of the strain at the extreme tension reinforcement layer is as follows:

  

Assumptions and Limitations

• All walls are assumed to be laid in a running bond.

• No checks are provided for seismic detailing requirements at roof and floor diaphragms, at wall corners, for dowels or through control joints. Additional requirements may apply for seismic design categories D, E and F. Refer to TMS 402-16 or ACI 530-13 Chapter 7 for additional requirements.