Footing Design Guide 4

[erpic]

Hey all!

Newbie structural engineer here. (25 years old, masters degree, been working for a firm for almost a year now straight out of school) I've been doing a lot of foundation design for pre-engineered buildings in the past couple of months using RISA Foot. I'm comfortable using it, especially with help and guidance from my colleagues, but I don't have a solid grasp on how RISA Foot is actually coming up with the design. The majority of the time it seems like I am increasing footing sizes and thicknesses just to satisfy the overturning moment while the bearing stress ratio of the soil hovers around .3 or .4.

I'm looking for a quick and dirty design guide geared towards structural engineers. I don't care much for geotech and I didn't do so hot in my foundations design courses in school (mostly because I didn't think I would need it, curse my logic) so I have a VERY rough understanding of the design processes involved. I should note that I work in the northeast and making frost protection is one of my primary concerns during design.

Any help would be appreciated!

 

[bridgebuster]

Try this web site, a lot of sample calculations on various structural elements.

http://www.ce-ref.com/index.html

 

[BAretired]

There have been a number of threads on this forum on the subject of footings with overturning moments. Try doing a search of the structural forum.

BA

 

[spats]

By far the best reference for design of metal building foundations is "Foundation And Anchor Design Guide For Metal Building Systems" by Alexander Newman (2013), published by McGraw-Hill. It's up-to-date and has a lot of good information.

 

[bridgebuster]

I've attended several webinars by Newman; the man knows his stuff.

 

[hawkaz]

I don't mean to sound like an old S#*t, but until you know why the computer is doing what it is doing, please turn it off. If you only have a rough idea, you may not catch "stupid" answers that the computer spits out. These calcs aren't very complicated. It's best to do several by hand so you fully understand what is happening.

 

[JedClampett]

hawkaz, I was going to say the same thing, but I just had a little rant about computer designs of trusses on another thread and thought I had hit my limit.
I guess I don't think the proper order of learning design is to do one (or a bunch)on a computer, get comfortable with the program, then get out a textbook and learn about it. What happened to the old fashioned way of having someone explain it to you and then doing one by hand?

 

[BAretired]

I agree, hawkaz. I would not feel comfortable using software which provided answers which I could not confirm with a few simple calculations.

BA

 

[BAretired]

Also, it is not enough to get the right answer. You have to know why it is the right answer and be able to explain it to the contractor when he says "Why are the footings so darn big?".

BA

 

[KENAT]

Gee guys, at least erpic identified there is an issue and is looking to address it - that's more than a lot of posters seem to do.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[BAretired]

Really?


BA

 

[erpic]

Thanks for the responses guys. Thank you spats, I think that textbook is exactly what I was looking for.

Just to clarify to everyone, i DO have an idea of how Risa Foot is doing the design and I would not use software that I wasn't comfortable using. I was merely seeking guidance on finding a design guide so that I could find a more suitable way to design footings for these types of buildings. Most likely a spreadsheet of some sort. I don't think this is unreasonable considering what the Risa Foot help section says about the OTM calcs:

"RISAFoot calculates this value differently than the way many engineers would traditionally approach a hand calculation. The RISA method is a bit more complicated, but will provide a more realistic representation of the true safety factor versus overturning whenever uplift loads are present."

Thanks for the 2 cents though. I've been like a sponge for the past year trying to learn everything I can from my bosses, i'm young and I realize I don't really know shit yet. However, I love my job more than anything and like Kenat said I know this is a problem of mine and I am working to fix it.

Cheers!

 

[spats]

Does RISAFoot explain how they calculate "the true safety factor"? How do you know you agree with them? Maybe it's way conservative... maybe not. Without the proper technical documentation, I wouldn't use it. I hate black boxes, and that goes to the point the others were making. As a young engineer you need to know that there is no substitute for understanding what you're doing.

 

[hawkaz]

erpic,
Good luck in your endevours as a structural engineer. Please keep learning and keep asking questions.
When you get to the point where you are going to take the PE exam, I give the same advise as above, turn off the computer and run calcs by hand. This is the best way to make sure that you undertand where everthing is coming from.

 

[steellion]

If you are designing a footing that has a high moment and relatively low axial force, then you will end up with a large footing area controlled by overcoming overturning. Are you sure that you have moment on the footing? I am used to seeing PEMB frames with pinned reactions at the base with a vertical and horizontal reaction to resist. Then, the horizontal forces may be addressed with tie beams between footings. Maybe yours is different.

You really need to learn how to design foundations by hand to verify the computer output. RISA is a reputable company and I'm sure their calculations are fine, but if you don't know how the program works, it could be Garbage In, Garbage Out. Just start with P/A +/- M/S; that is how all of the footing equations are derived. I'm surprised you thought that you wouldn't have to design foundations in practice. Unless you work for a very specialized company, you will be designing foundations throughout your career.

 

[AELLC]

steellion

What if the resultant is outside the kern of the footing- then you can't use P/A +/- M/S

 

[BAretired]

If the resultant is outside the kern, you calculate the effective length of footing and note that pressure varies from 0 to 2*average pressure on the reduced area.

BA

 

[JoshPlumSE]

Spats -

There is a good example in the help file / general reference manual about the difference between the RISA method and the traditional hand calc method. But, I can give a quick summary here.

A traditional hand calculation for OTM safety factor would be something like the following:

Step 1: Resolve your loading into an axial force (P) and moment (M) at the center of the footing.

Step 2: Calculate the Stabilizing moment as: Ms = P * L/2

Step 3: Calculate your Safety factor as: OTM safety factor = (P*L/2) / M

The problem with this is it always assumes that axial force is a stabilizing force. Think of the case where you have wind uplift on your column. The De-Stabilizing moment caused by wind forces would actually be P_wind*L/2 + M_wind.

If you look at each load case individually and decide if they are stabilizing or de-stablizing then you can have a better sense of what that actual safety factor is against overturning. That's what RISA calls the "true safety factor" method.

Step 1: Look at each load and decide if the total load is stablizing or de-stabilizing.

Step 2: Sum up the destabilizing moments and sum up the stablizing moments.

Step 3: Calculate your safety factor as: OTM Safety factor = stabilizing moment / de-stabilizing moments

Let's say you start with an OTM safety factor of 2.0. This should mean that if you multiple your wind force by a factor of 2.0 then you end up with an OTM safety of 1.0. That doesn't work for the traditional method, but it does work for the method RISA uses.... Which is the reason why we call it the True Safety Factor method.

 

[AELLC]

JoshPlum
I use SF(against overturn)=2 for retaining wall situations where overturn is due to water or soil, but use SF=1.33 for wind situations - and I am using ASD calculations.