Block Wall Strut-Tie Reinforcement

[Engineering05]

Hi everyone,

I am designing a reinforced block wall to behave as a deep beam (it is about 7m deep x 8m span). In our masonry code (Australia) there is a provision that states:

The main reinforcement in the direction of the axial load...

...If the reinforcement does not comply with these requirements, As shall be taken as zero and the member designed as unreinforced masonry...

Because the angle of my strut (using strut-tie analysis) is not coincident with the rectilinear reinforcement in the block wall I am forced to design the wall as if it is completely unreinforced which is quite punishing...

Bottom line is that there is a significant financial cost if I can get this wall to work in block. Some of the other engineers here have quoted that is the angle between my compressive strut and vertical reinforcement is less than 15 degrees I can use the primary vertical reinforcement in calculating the strut strength. Does this ring a bell with anyone? Is there some provision in a foreign code or well referenced textbook that allows me to do this?

Cheers,

Engg05

 

[BAretired]

It doesn't sound as if there should be any problem with a depth almost equal to the span, but 8m is a healthy span so I would be inclined to use plenty of reinforcement, both vertical and horizontal. I'm not sure why the Australian code would suggest it should be designed as unreinforced masonry, in fact I'm not sure how you could design it as unreinforced masonry.

BA

 

[Engineering05]

Thanks BAretired,

I just updated the italicized text to include the statement about the reinforcement being aligned in the same direction as the action. Is there some relationship between the angle of the action and reinforcement used to restrain it against said action?

 

[hokie66]

I believe the quoted provision is in the section "Design of Members in Compression", and I don't think the provision is directly relative to your situation. But as far as I know, there are no current provisions for strut and tie design in the Australian Standard for Masonry Structures. So if I am correct, and you want to design strictly to Australian Standards, you are out of luck. Perhaps others can clarify the situation with foreign codes.

 

[Engineering05]

Thanks for the input hokie66,

This is leaning to one of those problems that in my experience need some rationale applied rather than explicit interpretation of the standards i.e. a fundamental approach. This feels like a problem that may require Morhs circle to solve. I guess the question I need answered is: what is the effect of having reinforcement at an angle to the line of action? Perhaps this is entirely the wrong approach...

I feel a bit out of my depth attacking this problem from a first principals approach. Can anyone offer a place I should start?

 

[hokie66]

I think that reinforcement being parallel to struts is not an issue. You don't do it that way in reinforced concrete.

 

[BAretired]

I agree with hokie respecting the direction of reinforcement. Deep beam theory is different from standard beam theory; plain sections do not remain plain in a deep beam, but with such a high depth to span ratio, some conservatism is possible without significant cost. There are several articles available on the internet respecting design of deep masonry beams.

How thick is the wall? How much gravity load will rest on top? Is the deep beam exposed to lateral forces such as wind or seismic? With a gap of 8m and a height of 7m, lateral forces may require framing the opening with pilasters and a lintel spanning horizontally as well as vertically. Bond beams will likely be required at about 2400mm intervals. Perhaps the lower ten or twelve courses could be considered a deep beam and the upper portion would be simply a bearing wall.

BA

 

[KootK]

Because the angle of my strut (using strut-tie analysis) is not coincident with the rectilinear reinforcement in the block wall I am forced to design the wall as if it is completely unreinforced which is quite punishing...

I don't know AU codes well and I've never had the opportunity to review this particular clause and the surrounding clauses first hand. So keep that in mind in what follows.

This clause sounds very much like STM (Stut-Tie Modeling) provisions in my jurisdiction which allow STM struts to be reinforced as structural columns if desired (longitudinal reinforcing and ties). The idea is pretty simple: if you can swing it, turn your strut into a good old fashioned column. With this interpretation in mind:

1) I interpret the clause as meaning that, if the reinforcement is not aligned with the strut then the strut must be designed as un-reinforced masonry. I would not think that the rest of the system would need to be designed as un-reinforced masonry. It wouldn't make much sense to use strut and tie modeling but not be allowed to have tension ties.

2) Obviously, one would want the "column" aligned with the strut. Yeah, I can kind of see the argument for the 15 degree business. There must be some small angle at which a reasonable engineer would call it good enough. I wouldn't do it personally however. If the struts on a member of these proportions can't be made to work un-reinforced, I'd be looking for another solution.

Can anyone offer a place I should start?

Start by recognizing that, in an STM deep beam design, the critical thing is usually the anchorage of the ties into the struts at the supports. The rest isn't terribly critical and, as BA mentioned, conservatism will come cheap. So:

1) Check your struts. Or just look at bearing at the beam ends and be conservative.
2) Check your ties. Or just use something like the reduced lever arm method and be conservative.
3) Investigate your nodes where the struts meet your primary tension tie at the bearings.

3a) How much bearing length do you have available?
3b) Are you able to hook your tension tie or does it need to work as straight bar anchorages?
3c) Will your wall/beam thing terminate in vertical control joints at one or both ends?

A couple of questions to satisfy my own curiosity:

1) Are you spanning a lower level opening or some kind of below grade foundation issue?
2) Will the beam/wall thing remain fully shored during construction?



I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.