StruCalc vs. EnerCalc

[medeek]

I've recently been testing software for the design of residential and light commercial square and continuous footings. Has anyone had experience with either of these software products in this capacity?

I am liking StruCalc's very simple interface however the one thing I am noticing with StruCalc for a continuous footing (when reinforcement is enabled) the callout for a min. #4 bars placed transverse to the stemwall. In residential construction I have never seen this done except for point loaded square footings. Typically I've seen 1 to 3 longitudinal #4 bars. Most continuous footings for single story residences that I see are typically 12" wide and 6" deep with (2) #4 bars horizontal cont.

Okay, another observation, in the print preview screen of the StruCalc app. in brackets next to the transverse reinforcement: (unnecessary).

Anyhow, feedback or comments on these two tools would be helpful.

A confused student is a good student.

 

[KootK]

I've used Enercalc and found it useful. No experience with Strucalc though. Normally, I'm first in line to recommend some shiny software package. In this instance, however, my recommendation is this: don't bother with software. I say this for two reasons:

1) Residential foundations are pretty straight forward.
2) You're rather adept at spreadsheet programming.

Make your own stuff. A strip footing and pad footing spreadsheet will cover 95% of what you do. A retaining wall, concrete beam, and footing overturning moments spreadsheet will take care of the remaining 5%. The time you'll spend verifying the software -- and I know you will -- would be better spent making your own that works just the way you like.

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.

 

[XR250]

I agree with Kootk. I don't use any software besides 2D frame and a retaining wall programs. Everything else is spreadsheets or HP11C.

 

[medeek]

I think your right KootK, I've been going through the detailed report from StruCalc for both stemwall and isolated column footings. They've been so kind as to list all of the checks they are doing and I can actually follow the concrete calcs for once (unlike trying to decipher the ACI 318-11 cold). I've also grabbed a copy of "Design of Reinforced Concrete" by McCormac and Nelson for further interpretation and guidance. I probably need to read this book from cover to cover.

No magic going on here, just a few basic calcs and checks. I'll have a new spreadsheet shortly.

A confused student is a good student.

 

[KootK]

If you get the opportunity, I recommend obtaining a CRSI Manual from the concrete reinforcing steel institute. In addition to a zillion canned designs, the intro to each section provides some great commentary on the methods used. They write programs to generate their tabular designs so the methods tend to be presented in very linear, mechanical formats. Buying a new copy is very expensive. You can get 98% of the benefit out of a copy one or two cycles back. If you hop on ebay, you can probably get one for $20 including shipping.

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.

 

[CBSE]

ENERCALC has quite a few benefits and uses for the price. I recommend it to anyone wanting to design in multiple materials. I have a spreadsheet for just about everything I need for the top 4 materials, and am now using ENERCALC and WoodWorks for pretty much everything, unless it's a custom one off calc.

I do really like the combined footing program.

 

[medeek]

Working on a new spreadsheet for footings and I noticed that in a certain situation Load Case 4 will govern:

4.) D + .75L + .75(Lr or S) ASCE 7-10

Note that I am only calculating the bearing pressure here and none of the strength design stuff (one-way shear and reinf. moment calcs).

However, I noticed a discrepancy with the output of StruCalc. Upon closer examination it appears that StruCalc is not factoring this load combination and is instead applying the full loads:

D + L + Lr or S

ACI318-11 Sec. 15.2.2 states that the base area of the footing should be determined from unfactored loads. So does this mean that the ASCE 7-10 load combinations for the calculation of the bearing pressures does not apply?


A confused student is a good student.

 

[KootK]

In the US, I believe that soil bearing stresses are still checked using allowable stress methods. I would think that the companion load factors in the ASCE 7-10 allowable stress load combinations would still apply. Another good reason to keep on programmin'...

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.

 

[medeek]

This is what I have for a stemwall footing. Now I need to modify it further to include overturning due to shearwall action.

The Excel file is here if anyone is interested in testing it out:

http://design.medeek.com/resources/footing/

A confused student is a good student.

 

[frv]

I used to love EnerCalc for being very simple and easy to use, especially for relatively simple things. However, in my experience, Enercalc has become almost unusable in the last few years. It crashes ALL the time and there are constant upgrades that only destabilize it further. Perhaps I have a unique setup on my machine that conflicts with Enercalc, but just giving you a heads-up.

 

[medeek]

The updated stemwall calculator and the square footing calculator:

http://design.medeek.com/resources/footing/2014-021_P1_SQUAREFOOTING.xls

http://design.medeek.com/resources/footing/2014-021_STEMWALL_FOOTING.xls

The output of the square footing calculator looks like the following:

Going through the exercise of creating these two spreadsheets and comparing them to StruCalc's Output finally got me motivated enough to finally open up the ACI 318. I can't say I'm totally comfortable with concrete yet but at least I've got me feet wet. With my square footing calculator I was not able to completely reconcile my values for the applied beam shear and the applied bending moment with StruCalc's values (their values are slightly higher), otherwise everything else seems to checks out. I've sent an email to their support requesting more information on how they are arriving at their values and hopefully I will get to the bottom of it.

The next step is to take the stemwall calculator and create a version that deals specifically with lateral loads from shearwalls. The additional items to check are probably:

1. Overturning of the entire shearwall/foundation. My question here is should one ignore any dead load contributions of walls and foundations that are connected to the ends of the wall. Theoretically these would offer more resistance to the overturning.

2. Check the soil bearing pressure of the stemwall footing with gravity and lateral loads. I'm going to assume symmetry for now so lateral loads to the right or left will produce the same result.

3. Check the footing for Shear and Bending: Vu, Mu

Beyond this there should also be some sort of check to make sure the stemwall is thick enough and high enough to resist the internal moments caused by the shearwall/holdown action however I don't have any good examples of dealing with that yet.


A confused student is a good student.

 

[KootK]

It's valid, and efficient, to count any dead load resistance that you feel that you can confidently rely on.

Your stem wall is really just an ordinary concrete beam. Textbook examples abound. What will be unique will be things like:

1) the odd applied loading
2) whether the wall is stiff enough to be considered rigid
3) whether or not you'll rely on return walls for hold down resistance.
4) localized detailing issues as we've discussed above.
5) whether or not you'll rely on stem wall beyond the strong walls to resist OT.

Because of these case to case idiosyncrasies, I suspect that the stem wall design may want to remain a hand calc item. I'll be interested to see what you can pull off.



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.

 

[medeek]

Malone and Ambrose's books both have examples of footing calcs for isolated shearwalls. Spent the better part of last night understanding kerns, cracked sections and the wedge method. Seems relatively straightforward. However, I do have a few questions and observations.

1. I've noticed that for the overturning calcs Malone is applying a safety factor of 1.67 (.6 inverted) and Ambrose is using a safety factor of 1.5 both of which seem reasonable and I have used 1.5 and 1.25 myself for platform overturning in some my Aerospace work. Where does this safety factor arise from and is it part of the code somewhere (ASCE 7-10, IBC 2012 etc...)

2. For overturning typically the load combination to look at would be ASCE 7-10 load combo 7 or 8:
.6D + .6W
.6D + .7E
If I do use .6D then is the safety factor already built in or do I need to take another 1.5 safety factor on top of the .6 reduction?

3. In Ambrose's book, Example 12 (page 128) he simplifies the problem by only considering dead loads however the soil pressure check and the structural checks of the footing for bending and shear should also include all of the dead, lateral and live loads factored appropriately using the load combos of ASCE 7-10. So in a sense there are two checks of everything, one without the lateral loads and one with the lateral loads. As mentioned above the overturning check only involves the dead loads and the lateral loads.




A confused student is a good student.

 

[JedClampett]

1) For retaining walls, the 1.5 factor is in 1807.2.3. I'm not sure where else it is, but it's very customary.
2) If you're using a .6 factor times your dead load, you're essentially using a 1.67 factor of safety. No further F. S is required.
3) I don't have that book, but it makes sense. The soil loads usually have a significant factor of safety built in, plus they tend to redistribute to accomodate local high stresses. But I put in live, lateral and other loads in my soil pressure calculations. Why not? It doesn't cost me anything to make the foundations bigger.

 

[medeek]

After giving it more thought I've come to the following conclusion. The footing for a stemwall foundation is typically not designed to be a grade beam. In other words you check the bending and shear only for the transverse direction. The longitudinal bars do provide some strength/redundancy to help span over soft spots. If you need beam action from the footing (ie. slab on grad foundation at a garage door) then the footing needs to be designed as a grade beam.

For standard stemwall foundations with typical footings I am going to assume the stemwall acts as a rigid beam. This simplifies the internal stress calculations. If this assumption is made then the pressures at the footing will not be affected by the details of the shearwalls above. The only thing that affects the pressure distribution is the overall lateral load or moment and the vertical or gravity loads. This can be seen with the diagrams shown below that all have the same gravity loads and lateral load. Notice that the pressure wedge required to establish static equilibrium is the same in all 5 cases. What is different is the placement of the shearwalls and the resulting shear and moment in the stemwall (beam).

Now, how to get this into a spreadsheet???

A confused student is a good student.

 

[medeek]

And for good measure:

A confused student is a good student.

 

[medeek]

I am probably reinventing the wheel here and there is already decent software out there that can handle the bending and shear on a stemwall under shearwall action, please direct me if this is the case otherwise I'm going to try and spreadsheet this.

Or perhaps the beam action of a stemwall is not considered since in reality it is probably best modeled as a semi-rigid beam so the bearing pressure of the soil actually smooths out the higher shear and bending moments as shown above.

It just seems to me that one should check the beam action of the stemwall under shearwalls with high uplift forces (ie. the uplift forces using a Simpson Strongwall)

Another observation is that reinforcing bar should probably be placed in the top and the bottom of stemwall and not just the top as often shown in prescriptive details.

A confused student is a good student.

 

[KootK]

Agreed on all counts. As for wheel reinvention:

1) this part might be where Enercalc becomes the way to go.

2) I developed a closed form general algorithm for this. Happy to share.

3) For beam on elastic foundations, see this guy's stuff: Link

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.

 

[medeek]

Wow, I can learn a thing or two from SteelTools spreadsheets, he's got some nice stuff.

A confused student is a good student.

 

[KootK]

Yeah, you two are birds of a feather.

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.