Errors in construction specifications lead to lost time and money for all parties involved. Errors can occur due to references to outdated standards, contradictory or conflicting requirements throughout a specification, general omissions of important information, or a misunderstanding of what is present in masonry standards.

As part of CMDC’s goal to provide designers and our members with the tools and resources to design masonry structures, this new initiative is meant to provide a resource to anyone involved in a project where masonry work is being specified. The main specification page is now available under Resources, linked here: www.canadamasonrydesigncentre.com/resources/specifications

The following are examples of some of the typical issues that CMDC engineering staff have encountered over years of experience in the industry. The purpose of this initiative is to provide a general resource to help inform mason contractors, specification writers, architects, engineers, and masonry designers in general on what types of specifications might lead to issues on a masonry job.

The specification examples available today are:

1. Tying Veneer to a Structural Backing
2. Specifying Mortar for Nonloadbearing Masonry
3. Specifying Mortar for Loadbearing Masonry
4. Bond Beams in Partition Walls
5. Workmanship and Tolerances for Walls
6. Workmanship and Unit Appearance
7. Workmanship and Mortar Joints
8. Vertical Movement Joints
9. Acoustic Block in Reinforced Masonry
10. Masonry Grout

CMDC is available to answer any design or construction questions for designers or our contractor members. Please do not hesitate to contact us if you are looking at a specification that is raising some questions or you are looking to specify something in your design and want to ensure that the way it is written in a way that is clear and reflects the codes and standards that are being used for your next project.

The known bugs page was formerly hosted here and has since been moved to the MASS software documentation site:

Known Bugs in MASS

For questions about specific bugs, or to report a bug, contact mass@canadamasonrycentre.com

The MASS software is a useful tool for designing the individual structural elements of a masonry building. While there is a shearline module for simple, single-storey elevations, there is no such equivalent when it comes to multi-storey shear wall design (yet). It is left to the designer to model each element of their shear wall within MASS in a way that accurately represents its behaviour. This article will touch on a few of the major aspects that come up when using MASS for multi-storey designs.

To jump straight to a specific aspect, click the heading links below:

• Load Distribution
• Total Height
• Top End Fixity

Click here to jump straight to the summary. 

Load Distribution

Before diving into the multi-storey specifics of shear wall design, it is useful to first recall the scope of the shear wall module in MASS.

Shear wall module scope:

The shear wall module in MASS designs an individual shear wall element for in-plane moment and shear based on the loads that are applied to that individual element.

The shear wall module can be used for multi-storey buildings so long as the designer has taken into account the various ways in which a shear wall element interacts with the structure around it.

This includes:

• The accumulation of axial loads applied above the storey being designed
• The accumulation of lateral loads applied above the storey being designed
• Any overturning moments resulting from lateral loads applied above the top of the storey being designed

Example Exercise

Consider the example below, where the second storey of a four storey shear wall is being designed using MASS. In order to design the second floor shear wall element, all loads must be distributed to the top of the wall from all of the walls above. Each storey is 4m tall and all dead loads include self-weight.

Before expanding the solution, it may be a worthwhile exercise to calculate the solution yourself to test your skills.

Note: All axial loads are assumed to be placed at the centre of the wall and evenly resisted by the full cross section (no load dispersion is considered within the section). In cases where axial loads are applied with some eccentricity, this can be accounted for in MASS using an applied moment with a moment arm equal to the eccentricity of the load.

While distributing loads makes up the majority of the work needed to design multi-storey shear walls in MASS, there are still three other important aspects to consider.

Total Height

It is possible that while the full shear wall is not considered “squat”, an individual storey may have a height to length aspect ratio less than 1. In this case, it is important to change the total height to match the height of the full shear wall so that MASS doesn’t treat the individual storeys as squat shear walls. By default, the total height is set to the same value as the shear wall height so if it is unchanged, there may be an unnecessary reduction in moment resistance.

A full article explaining the difference between “Height” and Total Height” can be found here, including examples showing between 15% to 24% of moment resisting performance losses for not taking the total height into account.

End Fixity

When looking at a shear wall element within a larger shear wall, the objective is to take all aspects of being part of a larger shear wall into account. While there is an option in MASS to fix the top of a shear wall from rotating, the effect on the design can be seen in the difference in bending moment profile below:

Applying a rotational fixity at the top of the wall effectively divides the moment between the top and the bottom of the wall’s supports. While the “Fixed (R)” end condition was added to MASS for the purpose of shear wall designs with significant masonry above which prevent rotation, the scenarios where it is appropriately used would more closely resemble what is pictured below, taken from section 5-7 of the MASS Help files:

As a result, there is no need to change the end fixity of the top of the shear wall, as long as the loads have been properly distributed. Using the cantilever configuration for a multi-storey shear wall , it can be designed element-by-element, accurately designing each storey for the same shear and moment profile as would be used if the full multi-storey shear wall were designed at once.

Only by using the default cantilever fixity selection can an individual storey be adequately modeled without having to apply additional loads to cancel out the effect from a fixed top rotational end condition.

Summary

To quickly summarize, there are three main things to consider when designing a multi-storey shear wall using MASS:

1. Load Distribution: all loads not applied directly to the storey being considered must have their effects included. In particular, the accumulation of axial loads, lateral loads, and overturning moments due to loads applied from storeys above.
2. Total Height: To avoid being penalized for squat shear wall moment arm reductions, be sure to change the total height in order to accurately reflect the full height of the wall.
3. End Fixity: While it may at first seem reasonable to factor in the rotational stiffness from storeys above by changing the top fixity to Fixed (R), it will not result in a moment profile that accurately reflects the moments experienced by the storey in question. The default cantilever selection with properly distributed overturning moments is a more appropriate selection.

The MASS software is a product of a joint partnership between CMDC and CCMPA. CMDC is the authorized provider for MASS Technical Support.

If you have ever designed a multi-storey shear wall and wondered why the moment resistance is less than expected, the reason is likely CSA S304-14: 10.2.8:

MASS automatically identifies shear walls that have an aspect ratio less than 1 and designates them as squat shear walls. Keeping all calculations and design results in accordance with the CSA Standards, it also correctly reduces the moment arm of all steel in tension when applicable which is why there is a reduction in moment resistance. While it is often the first reaction of many users to assume that this behaviour comes from a bug in the software, MASS is behaving as intended.

Multi-Storey Applications

What if you are designing just one element within a larger shear wall where the element has an aspect ratio less than one but the full shear wall does not? Is it correct to be applying the reductions from clause 10.2.8 to elements such as these? Consider the example below:

This example which was used in the Multi-Storey Shear Wall Design article demonstrates an instance where this clause comes into play. The entire shear wall itself is clearly not squat as it’s aspect ratio is 3.2. As it is loaded, it is behaving as a non-squat shear wall so it is not correct to be applying clause 10.2.8 to the design of an individual storey. In order to design this wall in MASS, only the individual elements can be modeled and designed separately. As you can see, the wall input into MASS on its own is designated as a squat shear wall which is where the Total Height input comes in handy: it allows the user to tell MASS that while an element may be “squat”, it should not be treated as such.

“Height” vs. “Total Height”

The scenario described above is the reason multiple height inputs are available in MASS.

Height refers to the vertical dimension of only the shear wall element being modeled while Total Height refers to the vertical dimension of the full shear wall assemblage, beyond just what is being modeled.

If Storey 2 is modeled in MASS without any consideration of the larger shear wall it is apart of, it is designated as being squat as it’s 4/5 aspect ratio is less than one. When the total height is changed to the full 16m, the aspect ratio used to apply squat reductions from clause 10.2.8 increases to 3.2 and the result is an improved moment resistance.

Impact on Design

How much of a change does this make to a shear wall design? Using the example from earlier, when designing using a 20cm, 15MPa concrete masonry unit, taking the total height into account means the difference between using No. 15 and No. 20 bars placed exactly the same. If using No. 15 bars for both designs, the squat version of the MASS file would need to go all the way from a 15 to 30MPa strength unit to compensate. Furthermore, if the masonry and reinforcement properties were both fixed to the same design, the difference in capacity can be seen on the interaction diagrams below:

Comparison of moment envelope curves for shear wall design both including and neglecting the total height

For the critical load combination (#15: 0.9D + 1.4W), this means that the moment resistance of the wall is reduced from 1333.5kN*m to 1111.5kN*m, or by 222kN*m, simply by not taking the aspect ratio of the full wall into account!

This effect is further demonstrated in the example below where 70% of the vertical reinforcement is concentrated on either end of the wall. This significant reduction in moment is a direct result of a reduced moment arm for the steel that is in tension and furthest away from the compression zone. Note that this design uses the exact same materials simply arranged differently.

Comparison of moment envelope curves for shear wall design both including and neglecting the total height

There is now a 330 – 430kN*m reduction in the moment resistance compared to the 200 – 275kN*m reduction observed when the reinforcement is evenly distributed. One thing to note for all of these comparisons is that the difference in moment resistance diminishes when the applied axial load approaches P_f,max.

For those curious, a comparison of the uniformly distributed reinforcement and concentrated end steel designs can be found by expanding the section below:

If you have any questions, please do not hesitate to call or email the Canada Masonry Design Centre.

The MASS software is a product of a joint partnership between CMDC and CCMPA. CMDC is the authorized provider for MASS Technical Support.