Need more information to actually be able to help... Describe what you mean by a trench footing, and give your area (different geographies need different footings; particularly with regard to depth and insulation requirements). Water table will also be a concern, and the bearing capacity of the soil.
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...
I am not a student. The only types of foundation systems I have experience with are spread footings, piles, caissons, and piers. I did not realize my question needed more detail, because like I said, I have never designed a trench footing before. However, thank you for your help, this gives me a good start. I'm not sure if I completely understand your comment about the cantilevered portion of the footing being 1/6 of the wall width...How will that give you your necessary bearing width? To my understanding of a trench footing, it is similar to a spreadfooting, except it is not formed and the contractor basically digs a "trench" down to the needed frost depth. Am I correct?
What you seem to be referring to is just a wall footing. You design it the same way you design spread footing, but now you only have bending in one direction, and the loads are per foot of length. You design for a 1 ft length, similar to designing a concrete slab as a beam of 1 ft length.
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.
No, I believe a wall footing is a little different than a trench footing. A wall footing is similar to a spreadfooting, where often times you have a foundation wall coming down into it. However, a trench footing has no foundation wall, and the masonry wall (or whatever type of wall you have) simply frames into it. Almost like a really deep beam, I think. However, based on Youngstructural's post, it sounds like you design a trench footing in a similar way as a wall footing....I think.
I am assuming that the trench footing nomenclature being used here is similar to the bathtub wall used when they built the WTC basement wall. In that case, the walls were tied back with soil nails to keep the hudson river out of the excavation. I only bring this up because that wall did not have a conventional spread footing under the wall.
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.
I think all of you are talking about continues footing. Why you people put different names, other than those you learned in school. The deviance from standard, just make life harder.
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?
Sorry Structuralnerd, looks like I'm going to complicate all of this again...
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...
Thanks for everyone's helpful posts, but I think I have figured out how to design this trench footing after speaking with some other structural engineers around my office. And, yes, it is just like what minorchord is describing, only not quite as deep. I am putting longitudinal reinforcement as well as some vertical reinforcement. Thanks again everyone!
minorchord... is your 10 ft depth for discussion purposes or have you really had a 10 ft deep x 16" wide footing? If so, how did the contractor prevent the side walls from collapsing?? The deepest trench I've worked with is 48 inches and that was tough to keep from sloughing the sides.
archeng: The excavation was filled with slurry as the trench was dug and the concrete was placed using tremie methods. As the concrete was placed, the slurry flowed out of the excavation into a nearby prepared pit. Of course the rebar mats were configured before and lowered by a crane and supported from outside the trench excavation.
Works very well, just need good field quality control and a tight spec.
