retaining walls are, as the name suggests, any wall that is designed to retain any material which
could be earth water solid waste or anything else that needs to be retained. If you're an engineer
inspecting a retaining wall or you designing one or if you're a builder constructing a retaining
wall you should stick around because in today's video i'm going to cover the correct location of
the steel reinforcement in a retaining wall and its footing as well as i will explain the theory
behind it in an easy way to understand. There are a variety of different retaining walls in this video
we'll look at one of the most common ones in australia which is the block work retaining wall
i'll be covering four scenarios first case is a cantilever block work where the retained soil sits
over the base the second case is also a cantilever where the base sits in front of the wall the third
scenario is a wall pinned at the top with the retained side opposite the footing and finally
the fourth type of wall is pinned at the top with the retained side on the same side as the footing
a retaining wall should be built entirely within the boundary of one of the properties and not on
the common boundary like a fence you might find yourself in a situation where you have to use a
wall type number one this kind of wall may be more economic especially on sandy soils because the
backfill itself acting on the base is employed to provide most of the required dead weight
for this scenario we can draw a plane frame like this one with the vertical
bar representing the cantilever stem and the horizontal bar representing the footing
the 10 kilo newton's point load in x direction is the resultant force of the active pressure due to
the soil the 10 kilonewtons per meter uniform load in y direction is the weight of the soil above
we usually neglect the pressure loading from the soil below to design the heel if we look at the
bending moment diagram there is a negative bending moment displayed on the tension side of the bar
in simple terms if the bar is bent the inside of the bent is typically experiencing compression
while the outside is under tension since the steel is there to provide tensile strength if we look
at the deformed configuration we can easily tell where the steel reinforcement needs to be placed
going back to the drawing the vertical bars should be placed on the tension face of the wall
which is the inside face maintaining adequate cover
the footing reinforcement follows the same principle with the main bars on the tension
face the starter bars coming out of the footing should lap with the vertical reinforcement and
it's a good practice to first tie the bars with the starters and then lay the blocks
the wall type number two doesn't have a heel therefore the retained soil is not able to hold
down the footing as a consequence usually overturning and sliding control the design
the deformed shape of the wall is the same as the previous scenario
however since there's no soil on top of the footing the applied load will be upwards from
the soil pressure and thus the tension face is at the bottom of the footing
the vertical bars will be placed again in the inside face of the wall
nonetheless the main bars in the footing will be placed at the bottom
face a mesh can be placed at the top face of the footing for crack control a footing key
which is a deepened portion of the footing might be necessary to provide greater sliding resistance
and it can be placed right below the wall as i show here or at the far end of the toe
number three wall type is the case of basement or foundation walls for residential construction
and larger structures a concrete slab depending on the stiffness will provide a prop to the top
of the wall the connection between the wall and footing can be pinned or fixed depending on where
you place the rebars and the size of the blocks in this case i'll consider a pinned connection
at the bottom and the steel reinforcement for the stem will be placed in the outside face
of the wall and this happens because now we do not have a cantilever retaining wall
as the top of the wall is pinned as we can see there will be a positive bending moment therefore
positive reinforcement will be used for the stem footings are usually designed in bending
as wide cantilever beams supporting uniformly distributed soil pressure however since the
wall lies eccentric to the center of the base the footing will experience unbalanced stress
along the base and this also applies to the previous walls where there is an eccentricity
i'm just going to copy the footing rebars over from the type number to wall as they would be
pretty much the same configuration make sure you have enough lapping and development length
as we saw in the 2d frame analysis the location and magnitude of the bending steel reinforcement
is dictated by the bending moment distribution note that the function of the reinforcement is
to provide the tensile force required by the moment the main vertical bar is now
placed in the outside face i'm going to lap it and cog the bar into the concrete slab
and finally the number four type of wall in this scenario the retaining
wall doesn't have a toe and the top of the wall is laterally restrained by the concrete floor
instead of going through the frame analysis which follows the same principles as the previous walls
i'm gonna make a couple of comments that i believe are important
the first one um it is critical to construct block walls with the correct cover to reinforcement
and in a future video i can show you how much the moment capacity of the block wall change
as we increase cover also the block wall must not be backfilled before the concrete floor at the
top of the wall is in place otherwise your design is not valid or you can prop the wall if you want
since the slab is restraining the upper end of the wall the conditions for development of the
active state would not be satisfied because of inadequate strain in the soil therefore at rest
lateral earth pressure should be used to design this type of wall
and finally the wall should be provided with a granular material for backfill as well as a
perimeter drain to ensure a dry basement and all you know avoid future problems
so that's it for today i'll leave you guys with a little homework and you can leave the answer in
the comments below also leave a comment if you'd like to see a complete retaining wall design
thanks for watching i hope you like the video and i'll see you next time