Masonry Structures Design

[Poul75]

Are you familiar with the CSA S304.1-04, the Canadian Standard on Design of masonry structures? Not me.

In table 4 (Specified Compressive Strength normal to the bed joint f'm for concrete block masonry), there are values of f'm for mortar type S or N given for different compressive strenght of block.

I want to know why the value of f'm for mortar varies with the compressive strenght of the block.

exemple:
For 15 MPa concrete block, f'm for a mortar type S will be 7.5 Mpa for grouted. And for a 20 MPa concrete block, f'm will be 10 MPa.

Does it meen that it not necessary to have a 15 MPa grout for a 15 MPa concrete block because f'm will be limited to 7.5 MPa?

Thanks for your help!

 

[concretemasonry]

I do not have a copy of the standards you cite, but there was Canadian contributions made when ACI 530 was written. The commentaries for the code and specifications do an excellent job of relating the thinking behind the standards.

Under ACI 530, the masonry strength (f'm) can be determined by two methods: 1.a conservative table or 2) actual prism tests. The table is a conservative correlation of tests including mortar, masonry units and prisms. It is probably similar to your Canadian table.

Generally, a change in mortar strength has little to do with f'm. The reason for this is that the strength you see on a mortar test has a different aspect ration (shape, dimensions) than the actual mortar that is a thin layer bonded to the masonry units. I have made/tested 4500 psi hollow prisms using 2500 psi mortar, which correlates with an extrpolation of the table numbers.

When you look at a prism failure after testing, you usually see a shear-type failure of the conposite assembly independant of the mortar.

You can specify a high mortar strength if it makes you feel comfortable. Be advised that the ASTM C270 appendix recommends using the weakest mortar possible to carry the load because weaker mortars possess other properties that are advantageous.

To answer your question about the relationship between block and grout strengths, from a technical standpoint, it is not recommeded to use grout to increase the comprssive strength of the prism. The grout strength usually should be equal or slightly higher than the net CMU strength (some standards limit the grout to being 15% higher than the masonry unit strength). The reason for this is that if you are using a lower strength masonry unit and grouting it with a souped-up high strength grout, you will get a misleading high f'm, but the outer fibers may not be strong enough to resist the outer fiber compressive stresses from the combined vertical and out of plane bending loads.

From a practical standpoint, it is cheaper and more reliable to specify a higher CMU compressive strength that to use excessive grout unless you are heavily/closely reinforced.

I saw a practical result of the use of the mortar/CMU strength in a loadbearing masonry project of 10-15 buildings (15-20 stories) where the contractor chose to use only one mortar for the project (central job mixing of pre-proportioned mortar) and five different strengths of 6" hollow block. No site prisms were made since the block were pre-qualified and marked - the actual block testing was done before the block were identified/marked at the factory according to strength grade.

Masonry design is always a challenge because you are using a combination of manufactured and site mixed materials assembled to create a structural element (the wall) rather than combining steel and site mixed concrete to combine a similar structural element.

Dick

 

[drile007]

Which software do you use for (earthquake) design? What method does it use for calculating the base shear resistance?