Length

A beam’s length represents the total length of the entire modeled assemblage including any overhanging length on the outside edges of the supports. This value is typically entered in first and must accommodate the length of the opening above which the beam is spanning as well as the bearing plates on either side. Any additional masonry outside of the primary span is not used when distributing loads and determining the factored moment or shear that must be resisted by the beam. Once specified for a new beam design, a clear span,explained below, is then assumed based on the length needed for bearing on either end.

Clear Span

The clear span refers to the length of the opening above which the beam is spanning. While a beam’s length is typically entered first into MASS, it is also possible to start a MASS design by specifying a clear span and let the software automatically fill in the total beam length based on the length required for bearing on either side, rounded to the nearest modular cell length.

Bearing Length

The bearing length is defined as the length along the beam under which a high concentration of stresses due to concentrated loads is transferred to the supporting structure below. It can be spread over a steel plate or an area of masonry under compression. The default bearing length of 300mm was chosen for MASS because it is the longest allowable bearing length (CSA S304-14: 7.14.1.2) that can use a triangular load distribution and not require additional detailing (ie. using a rocker plate) to ensure a rectangular load distribution. For triangular reaction distributions, the centre of reaction is at one third of the bearing length away from the edge of the clear span and for rectangular distributions, the distance to centre of reaction is at half of the bearing length.

Design Length

Design length is the distance between the centre of reactions between beam supports. It is less than the beam length and greater than the clear span, used to determine the factored moment and shear. For example, checking MASS results by hand and looking to replicate the maximum factored moment at mid-span for a simply supported beam, the design length is used in M_f = wL²/8.

Quick demonstration – From masonry elevation to design using MASS

Starting with an elevation containing an opening with masonry extending above and on either side, a portion must be designated as part of the modeled beam. This example where two courses are assumed for the beam’s height, the full beam length extends one full masonry unit to either side which allows room for the bearing area in addition to the clear span.

For the same elevation, it is also possible to design a single course beam or go all the way up to four courses which can all result in acceptable solutions. Smaller beams have reduced moment capacity mainly from a smaller moment arm between coupling tension and compression forces while taller beams can have intermediate steel requirements (S304-14: 11.2.6.3) and may also have to satisfy additional provisions for deep beams (S304-14: 11.2.7) and deep shear spans (S304-14: 11.3.6). Choosing how a beam is modeled is left to the discretion of the designer.

For all masonry beam designs, a load path for vertical loads must be assumed for transfer to the supporting structure below. The default bearing area with a bearing length of 300mm is shown below resulting in a triangular distribution of the reaction force. Had the bearing length been longer than 300mm, the reaction would have been spread over a rectangular distribution (S304-14: 7.14.1.2).

In order to determine the design length, the centre of each reaction force must be determined. For a triangular reaction distribution, this location is one third of the bearing length away from the edge of the clear span for each support. (Rectangular distributions have a centre of reaction point half way through the bearing area)

The locations of the reaction forces expressed as point loads is then used to determine the design span. They are also where MASS draws the support points underneath the bearing plates in beam drawings.

Note that the difference in unit arrangement between the figures above and MASS has no impact on the design as fully grouted masonry. MASS always starts an assemblage with a full unit however starting with a half unit as was done in the illustrations above is functionally the same design.

As always, feel free to contact us if you have any questions at all. CMDC is the authorized service provider for the MASS software which is a joint effort of between CCMPA and CMDC.

This is one of the most common question we get having to do with masonry beam design so if this is something you are stuck on, know that you are not the first! We’ll first break down the reasons for this issue and then quickly outline what you can do about it.

First of all, this is not a bug in the code. This issue is a product of the way the 2004 S304 standard limits stirrup spacing. It has since been addressed in the 2014 edition and MASS Versions 3.0 and newer will allow two course designs with stirrups.

When does this happen?

This issue is specific to beams designs where:

1. The beam is 2 courses high or less, and
2. The beam is failing in shear design for having a shear resistance that is too low

You can see in the screenshot below that only the “none” option is selected for stirrup placement and if you try and click the single or double leg selections, nothing happens because the whole input area is greyed out or “disabled”.

The fix seems obvious, right? Just add some stirrups to boost my shear capacity! This is where we run into problems.

Why MASS won’t let you just add stirrups to your beam

The error message you are seeing reads: “Design fails: There is insufficient steel stirrup area and/or bar spacing according to CSA S304.1-04: 11.3.4.7.1,2 and/or 11.3.4.8”.

For reference, 11.3.4.7.1 and 11.3.4.7.2 each refer to stirrup placement and minimum area requirements. The issue we are running into comes from 11.3.4.8 which covers “Spacing limits for shear reinforcement”.

Click to see full reference

The issue here and the reason MASS will not let you place stirrups in your beam is that clause 11.3.4.8 is specifying a maximum spacing of d/2 which for 2 course beams is less than 200mm. MASS recognizes this and greys out the option to even place stirrups because the modular nature of masonry restricts you to multiples of 200mm (or the space between the centre of adjacent cells in concrete block construction).

Since you cannot physically place stirrups closer than 200mm within a masonry beam, MASS disable this option.

As you can see, there is no way to place stirrups any closer than one per cell. For those curious, these beams were built for university research supported by CMDC. You can read more about that here.

What you can do to get a successful design

You have a couple of options to get your design to pass. You can always send your MASS project file over to us in an email and we can walk through these options together over the phone as well. To do this, visit our contact page to get in touch.

1. Re-examine your loads

Oftentimes when going through a design, there are assumptions that are made along the way that can have a big impact on the final design. In the case of load distribution, are all of the loads being carried straight down to the beam or can arching be assumed as outlined on page 305 of our textbook? Did you manually add the self-weight of the beam in your dead load? If so, you might be double counting if you have the self-weight option selected in the loads window. Are there other loads applied that might not actually be resisted by the beam? All of these can mean the difference between a beam design passing and failing in shear design based on the loads applied.

2. Model an additional course of masonry within your beam

Many masonry beams are supporting more masonry above them so why not take advantage of this by considering a third course to be part of your beam? The real world difference could mean as little as simply grouting an additional course but the key difference is that the increase in height and in turn, d (depth of tension steel from compression face), moves the d/2 spacing restriction to being greater than 200mm. This might not be possible if the clear span is very small since the additional course might bump you into deep beam territory but for most designed masonry beams, adding the third course to your design does the trick!

3. Think about using a high lintel unit

If you can’t get your d/2 value below 200mm, you can approach the problem from the other direction and remove the 200mm restriction by changing to a high lintel beam design. High lintel beams have a continuous grouted area that can fit any reinforcement configuration. They can even be modeled in MASS for those of you who are feeling more ambitious! Give us a call if you are interested and we’ll be happy to walk you through it.

4. Consider using a stronger masonry unit

No, this does not mean you also need to increase your grout strength (See note 4 of Table 4 within the S304 masonry standard for reassurance). Using a unit with a higher f’m value increases the shear strength of the masonry itself, possibly giving you enough resistance as to no longer require stirrups in your beam. This is not always practical and is only effective if the failing masonry shear resistance is close to the required factored shear force.

One more thing…

Seeing as this whole issue is born not from a software bug but the way the CSA S304 standard was written, MASS Versions 3.0 and newer will not have this issue. CMDC currently maintains an active role in shaping and developing masonry standards and as a result, the 2014 edition of the S304 standard includes an addition to the “problem” clause, 11.3.4.8, which now includes a minimum value of 200mm for the maximum spacing of stirrups.

It is for this reason that the issue you are dealing with now will no longer be a problem when you start designing with the 2014 S304 standard.

Still have questions? Feel free to call or send us an email! (Including your MASS file is very helpful)

Associated Professor, Section Structural Masonry, Department of architecture, building and planning, Eindhoven University of Technology, P.O. Box 513, 5600 MB, Eindhoven, The Netherlands, a.t.vermeltfoort@tue.nl

ABSTRACT

This paper describes research into the behaviour of so-called “composite lintels” i.e. load bearing masonry in combination with prefabricated concrete lintels. Eighteen identical walls were loaded in plane to rupture. Nine layers of stretcher bond masonry, 562.5 mm in height, were built on prefab concrete lintels (60×100 mm²) with a span of 2800 mm. The effects of two types of supports and two types of loading on the mechanical behaviour of in plane loaded composite lintels were studied. Roller supports were simulated by suspending steel blocks from the roof beam of the test frame. A support condition, often used in practice, was simulated by a layer of felt on a brick. Two series of six walls were symmetrically loaded at four points. A third series of six walls were asymmetrically loaded at one point. The mean failure shear load for the four point loading condition was V_fail = 31 kN. For the one point condition it was V_fail = 24.4 kN. On average, the ultimate load (F_ult) was 15% higher than the failure load (F_fail). Supported on rollers, three walls failed in the constant moment area (mid span). The fifteen other walls failed in the maximum shear load area near the supports. The height of the compression zone at mid span depended on the support condition and was largest for the felt support condition, where horizontal movement of the lintel was restrained. The support condition (rollers or felt) had a negligible effect on the load bearing capacity.

KEYWORDS: Composite lintel, support, shear, strength variation, load bearing masonry

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1. Associate Professor, School of Civil Engineering, Harbin Institute of Technology Harbin, Heilongjiang, 150090, China, xmzhai@hit.edu.cn
2. Senior lecturer, Centre for Infrastructure Performance and Reliability, School of Engineering, The University of Newcastle Newcastle, NSW, 2308, Australia, Yuri.Totoev@newcastle.edu.au
3. Professor, Centre for Infrastructure Performance and Reliability, School of Engineering, The University of Newcastle Newcastle, NSW, 2308, Australia, ,Mark.Stewart@newcastle.edu.au

ABSTRACT

A reinforced grouted concrete block wall/beam, composed of cast-in-place concrete beam and reinforced grouted block wall, is common in mixed retail/commercial and residential construction in China. Often the ground floor houses a shop and commercial storage and residential flats are built on the floors above. In such buildings the ground floor is built as a reinforced concrete (RC) frame with or without shear walls and the upper floors are built with walls of reinforced grouted concrete blocks. According to Chinese code, the RC beam depth to span ratio for wall/beam composition should be greater than or equal to 1/6 for seismic design. Compared to a RC beam with unreinforced masonry (URM) wall on top of it, the reinforced grouted concrete block wall/beam structure has greater stiffness. The higher stiffness can help reducing the cast-in-place RC beam depth to span ratio, and thus increase the height of the ground floor. In this paper, test results for six different wall/beam compositions are presented. The structural capacity, the load-deformation relationship, force-transferring path, and failure mode are examined. The experiments show that the reinforced block wall/beam acts like a deep beam. It was found that the RC beam depth to span ratio can be reduced from 1/6 to 1/10.5.

KEYWORDS: reinforced grouted concrete block, wall/beam, depth to span ratio

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