trench footing
trench footing
(OP)
Does anyone have experience in designing a concrete trench footing? If so, I could use a little guidance...Thanks!
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RE: trench footing
Your question is more complex than you might have first thought... But if you want simple:
1. Determine the load in kN/m or kips/ft
2. Determine the Phi UBP (Factored Ultimate Bearing Pressure); This could also be the ABP (allowable bearing pressure) if you're working to the old-fashioned working stress methods.
3. Load over bearing capacity yields required area (in this case a required width, since we're inputting kN/m over kN/m^2)
4. Width required known, and assuming you're loading concentrically, consider the footing to each side of the width of the wall above as cantilevering out accross the soil from the face of the wall (I often consider the length of the cantilever as extended in 1/6 of the wall width to be conservative; or fully half of the footing width if I'm not confident in the builder's ability and would prefer an over designed solution).
5. Now you should have the bending moment required in the footing, as well as the shear load. The footing thickness should never be less than 200mm (or 8") as a general rule, and your reinforcing steel should generally have a minimum of 75mm clear cover to the soil you're sitting on (check your code requirements for specifics).
Quick question: Are you a student or an EIT? Because you're not supposed to use this site for homework, but everyone's more than happy to help a noob; That's how I got started on eng-tips. And if you're an experienced engineer who I've just insulted, please be careful to be more detailed with your questions, as I would think most people would make the same mistake as I just did. If you do happen to be a student, I will caution you that you probably don't have the experience to know what portions of the advice you're receive here don't work for you: There are a number of different methodologies in Structural Engineering, and if you present the wrong kind to your Prof, your having gotten someone else to do the solution will be obvious.
Either way, good luck,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: trench footing
RE: trench footing
As for the 1/6 reference in youngstructural's post - he is saying to not design for the moment at the face of the wall, design for the moment at a distance of 1/6 of the wall width to the inside (this will give a higher moment).
Note, however, that if this is a CMU wall, the critical section for moment is halfway from the center of the wall to the face of the wall (or 1/4 of the wall widthto the inside, not 1/6).
T&S steel goes in the longitudinal direction, just design the transverse steel for bending at an appropriate critical section.
RE: trench footing
RE: trench footing
Be that as it may, given the applied vertical loads, and transverse loading and/or overturning moment at the top of the wall, you can design this wall in a per foot basis using the methodology found in any sheet pile design manual. There is a great reference, probably out print, called Sheet Piling Design Manual by USS from July 1975.
You need a good handle on soil properties before attempting.
You get the required depth of embedment for the trench wall (substitute sheet pile wall), and the maximum moment in the wall so you can design the wall's thickness and reinforcing.
Does this sound like your project?
RE: trench footing
Thanks.
RE: trench footing
You need to more fully describe what you are trying to do. We all have different images since you are using terms of which some of us are not familiar. Again, I am assuming that there is NO SPREAD FOOTING beneath the bottom of the wall. The contractors digs a trench of some wideth, let's say 16 inches wide and let's say 10 feet deep. The wall reinforcing is lowered into the excavation and concrete is placed. Is this what we are talking about?
That is my image.
RE: trench footing
IF I understand your last post correctly, you want to design a wall with a footing that does not have any specific formwork required; IE: Contractor just pours concrete into a trench and vibrates the concrete to consolidate, after which it is simply allowed to cure. looks like this:
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This is only possible in quite cohesive soils, because otherwise the quality of the concrete will be diminished by loose soil and biologicals being mixed in by vibration. You can still reinforce such a footing, however the maximum labour savings is only attained if you don't. I would personally strongly recommend reinforcing in the long direction (thus giving at least some beam action), but am often quite comfortable to not reinforce in the lateral direction (across the footing section as drawn). If you don't reinforce in the longitudinal direction, you greatly increase the risk of differential settlement and cracking in the wall above (both concrete and CMU walls will typically crack when place on longitudinally unreinforced footings). I would NEVER omit longitudinal reinforcing for a footing supporting CMU or clay fired brick.
If you do reinforce laterally, it's as per previous post, PLUS StructuralEIT's comment re CMU wall effective cantilever (good catch buddy), and it's really just a matter of leaving a code-stipulated clear cover around your bars (again, min 75mm).
If you don't reinforce the section laterally, you must consider the effective shape to be:
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Essentially you're considering any concrete which would need tensile forces to be engaged as cracking away. The remaining concrete can be engaged by thinking of a 45 degree slope where the concrete is forced into compression through the stress distribution from the concrete directly above it. Be careful, as the 45 degree angle is not always true (depends on concrete properties), is sometimes stipulated by code (check your concrete code's section on the design of unreinforced concrete), and often you can find a more permissive angle. That said, typically 45 degrees is considered to be conservative.
Thanks for not being offended by my comment. You really need to bear in mind that online no one sees you, and we can only go by what you type.
Good luck,
Let us all know how it goes,
YS
B.Eng (Carleton)
Working in New Zealand, thinking of my snow covered home...
RE: trench footing
RE: trench footing
RE: trench footing
Works very well, just need good field quality control and a tight spec.
RE: trench footing
RE: trench footing
They are also called
slurry walls
cut-off walls
and a few other names that I cant think of at the moment.
The term 'Trench footing' can mean a number of different things depending on where you come from.