Masonry: Questions from our OnlineSeminar10. September 2020
Versatile, flexible and the perfect counterpart to structural concrete. Masonry is without a doubt one of the structural all-rounders. But planning, designing and implementing anchors in masonry occasionally creates its own challenges. We addressed this construction material in our OnlineSeminar series and provide answers to participants’ questions below.
Fixings solutions with an approval must be used if planners and structural engineers want safe fastenings in masonry. The ETA (European Technical Assessment) approvals are commonly used for this purpose. The loads according to the ETA are examined and determined through tests.
The physical construction properties of masonry provide thermal and sound insulation.
Identifying masonry stone
Participant question: How do I proceed if am unable to clearly identify the hole pattern of the stones?
A wide number of various stones fall under the umbrella term masonry, which is why it is important to be able to precisely identify the stone. When it comes to fastenings in masonry, it makes a significant difference whether the stone has a dense or less dense structure. It is important to know that the stone ultimately decides the maximum load. Once the masonry stone has been determined, a comparable stone can be looked up in the ETA which can serve as a reference stone for the required tension tests. If the exact stone is contained in the ETA then the corresponding loads can be used.
Participant question: How can I tell where the joint is if the masonry is plastered?
If the masonry is plastered and you are unable to see the stone then you need to investigate further. If the joints aren’t visible through the plaster then part of the plaster must be removed to determine their exact location.
Participant question: A stone that has been identified but is not covered by the ETA
In this case you must look up comparable stones in the ETA and use the load specifications of the similar stone. If planners or structural engineers come across perforated bricks or if the solid brick cannot be compared with other stones from the ETA then they must carry out tension tests. We can’t provide any load specifications if we don’t know which subsurface we’re dealing with.
The influence of joints on fastenings in masonry
Participant question: To what extent can fastenings be attached in bed joints?
To begin with, there is a distinction to be made between a bed or a butt joint. Butt joints can be mortared, but don’t necessarily have to be. If they are not mortared then the load must be reduced by 0.75. If the cmin cannot be complied with when dealing with an unmortared butt joint, then the load must likewise be reduced. If we have bed joints or mortared butt joints then we can insert an anchor, as the loads given in the ETA apply to the overall masonry including the joint.
Participant question: How are interlocking butt joints assessed?
Interlocking butt joints are treated as normal unmortared joints. Further information on fastenings in joints is provided in the previous question.
Participant question: Are there universal adhesives and perforated sleeves for the variety of stones?
The FIS V injection system is a universal system for masonry and concrete. But the perforated sleeves should be chosen individually as there is no single sleeve that covers all eventualities.
Participant question: Why are two single loads with a short edge distance sometimes larger than a group load with two anchors?
So-called group factors are applied when anchor groups fall below the characteristic spacing distance. Interpolation is not possible in this case, and this value must be applied consistently between smin and scr. Depending on the stone it can also be lower than 1, so that 2 or 4 plugs will have less load-bearing capacity than individual plugs. These group factors are stone characteristics that were determined through tests.
The situation is similar when it comes to the edge distance: Full load, when c >= cmin, otherwise 0.
Participant question: Upon identifying the stone’s hole pattern, would it be useful to place the drill holes or anchors so that they always hit the stone’s inner wall?
If we are talking about the internal partitions, then it doesn’t matter where the plugs sit as the least favourable position is always assessed when determining the load-bearing capacity. From a building practice point of view, it is also barely possible to determine the precise position.
Participant question: How many plugs can I use per stone?
As a matter of principle the spacing distances are positioned so as to permit one plug per stone. But additional tests with shorter spacing distances are carried out with some stones. In these cases, more than one plug can be placed for each stone, but the load reductions (group factors) must be taken into account. In unfavourable circumstances this reduction can be lower than 1, which means that 2 or even 4 plugs will have less load-bearing capacity than one individual plug. This is only done when it is required from a constructive perspective. The reduction factors are absolute, interpolation is not possible between min and max.
Participant question: The centric tensile strength is tested during on-site pull-out tests. How does this transfer to the shear capacity?
The shear loads are determined by the tensile loads as follows:
The value determined through the tension test is transferred to the shear load with the same ratio as the estimated reference stone. If the shear load of the reference stone is higher than the tensile load then the shear load is capped at this value. If the test determines higher tensile loads than with the reference stone, then they have to be capped at the value of the reference stone and used for further calculations.