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Civil - Structural Engineering Question

  1. Jul 13, 2011 #1
    There are quite a few canalised streams in my area and I was watching one riparian owner's contractor rebuilding his stream retaining walls.

    The top 2 metres of these walls were being constructed in 225 (9") dense concrete blockwork, three leaves thick making a total wall thickness of 675 width of wall.

    Watching the work I said to the owner "Is there going to be any cross bonding between the leaves?"
    I didn't see any, nor did I see any ties being installed.

    Comments by budding Civil Engineers?
  2. jcsd
  3. Jul 15, 2011 #2
    Disappointing lack of response there was a learning opportunity here, especially if certain contractor's brickies are members of PF.
  4. Jul 21, 2011 #3
    Cross bonding between the three leaves will increase the strength and robustness of the walls, but whether it's necessary depends on how the wall is intended to function.

    If it is designed solely as a mass structure, cross bonding the leaves will have no effect. If it's designed to resist bending, cross bonding will increase its strength if the individual leaves would otherwise "delaminate" under lateral load, for example, if the perpends between the leaves were not being filled with mortar as the work progresses. If there is sufficient "mortar bond", cross bonding will not add anything at normal working stresses, but would probably add something as failure approached.

    The other aspect is accidental damage. If the "stream face" was potentially subject to damage from floating objects, an unbonded leaf would be much easier to remove and replace than a cross-bonded one.

    It's horses for courses really - have to wait and see what happens to it I guess!
  5. Jul 21, 2011 #4
    Welcome to Physics Forums Bpseco.

    Cross bonding is needed to transfer shear to make the three leaves act as one mass in a gravity retaining wall.
    If the ground behind the wall becomes active the inner two leaves will simply add to the load on the outer leaf, rather than working in conjunction with it.

    go well
  6. Jul 26, 2011 #5
    what are the ground conditions?
    does not sound like a great idea to me to use 3 skins of block with no ties every 3 courses. In fact if I was constructing it out of block I would use expamet brick mesh in the courses and fully tie the leaves together so that they all act as one mass thus achieve the min. wall thickness required to resist the soil active earth pressure and other lateral loading/surcharge/water pressure etc
    what was below the block? RC wall?
    have they put on any finishes lower down yet? waterproofing etc..?
  7. Jul 26, 2011 #6
    How do ties transfer shear?

    Surely all the do is stop the walls separating?
  8. Jul 26, 2011 #7
    the ties wont help with shear transfer they will just all a modicum of continuity between the layers of blockwork.
    You would need dowel bars to transfer the shear..
    BS 8002 (Earth retaining structures) only allows unreinforced masonry walls up to 1.5m high without the need for buttressing or steps.
  9. Jul 26, 2011 #8
    Yes indeed, so ties are no substitute for proper cross bonding.

    Of course the wall could have been post tensioned, or even just tied, to earth anchors, but they weren't.
  10. Jul 26, 2011 #9
    yes I was going to say the 2m span for unreinforced concrete block is a little risky. overturning and sliding are jumping out at me right now!

    Proper cross bonding would have been preferable always - that way at least you have a honeycomb x-section and 'proper' stiffness in both planes and there would be no delamination.

    Still RC is always the preference of choice, at least for the majority of work I do
  11. Jul 26, 2011 #10
    No sheet piling then?
  12. Jul 26, 2011 #11
    Incidentally I should say welcome to Physics Forums, there's plenty of room for structural engineering experience here.

    The annual round of high school balsa wood bridge design will soon be in full swing.


    Have you explored the learning materials section of PF?

  13. Jul 26, 2011 #12
    It has been awhile since I last designed any sheet piling - usually a contractor job that one.
    I am familiar with the theory behind retaining wall design, we do a fair amount of underpinning and retaining walls in the office. The learning material section looks good though, concise and clear.

    Ah good stuff, I do like a bit of truss design. I believe I built one myself a long time ago as an undergrad. Balsa wood compression members and steel wound wire tension bottom chord, worked a treat for point loads at the apex points of the truss.

    Now if anyone fancies designing a steel beam to span 9m with 150 RC planks spanning into it with a loading width of 3m along with 5m line load of 3m high masonry and a couple of point loads from incoming beams, all this whilst limiting deflection to span/360 it would be most appreciated and ideally section depth to 305UC max! :)
    what a fun evening ahead
  14. Jul 26, 2011 #13
    I do remember, some years back, doing a specialist underpinning project on an ancient monument, in the middle of a Water Authority scheme to rebuild a section of river walls.

    The WA Engineer, an old boy about to retire, said to me "you can't beat mass concrete in water engineering".

    go well
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