Keeping track factored and unfactored

[COEngineeer]

Usually I feel that it is very hard to keep track load factored and unfactored load. You will need unfactored load for shear diaphragm for steel deck, soil bearing foundation, etc. Then you need factored load for your steel and concrete design. On the top of that, you have so many load combinations. Do you think one day we will use strength design on everything (i.e. soil) so we dont have to know the service load anymore? I am fairly green and I find this very overwhelming even with computer software like RAM where it will generate LRFD and ASD load combinations for you. Comments and thoughts?

Never, but never question engineer's judgement

 

[civilperson]

Never! What loading do you check deflections and drift with?

 

[csd72]

I worked with a guy that had designed offshore drilling ships in the UK, he said that they dont use factored loads in that industry, only the old working stress (ASD) code.

Why? Probably something to do with keeping track of the 100 plus load cases and thousands of members being complicated enough.

 

[COEngineeer]

how to check deflection? I dont know.. maybe put another reduction factor like phi

Never, but never question engineer's judgement

 

[Lion06]

The nice thing about RAM is that in the gravity beam module it designs for strength based on your preference (ASD or LRFD) and checks deflections based on service loads.
The foundation module has different load combinations. It uses ASD combinations for soil bearing (sizing the footing) and uses LRFD combinations for the design of the footing for strength.
Also, in RAM Frame, you can show the service diaphragm shear at each story.

 

[UcfSE]

Bridges are often designed now with LRFD combinations for the soil. Still, serviceability checks may keep us from ever truly leaving ASD behind. Who knows what could happen in 50 years though.

 

[COEngineeer]

strucutralEIT, I just recently tried to use Ram Foundation. Yes it does 2 different load combinations (LRFD and ASD)for foundation. But for some reason it is trying not to exceed my soil bearing pressure with the factored load! Click view update on one of your footing, you will see.

Never, but never question engineer's judgement

 

[connect2]

In Canada even Geotechnical is now, (2006), required to provide 'Limit States Design' parameters. For the past 20 years concrete, steel, masonry, and everything else has beeen limit sates but the 'dirt guys' have hung onto to the 'allowable WSD' concept.

In anycase it's a huge bookkeeping problem. At the end of the day, load factors aside, WSD aside, we still run the actual load from the top to the foundation as ONE of our runs.

By the way in our latest code the load factors for serviceability limit states design, ie deflection/drift etc, re wind and snow are 0.75 and 0.9 respectively for 'normal' buildings ... we don't even use the actual loads, they are factored down!

Factor up, factor down. Predict failure.

It's a game of statistics and probability that doesn't make much sense to me. Twice in the last 10 years we have meet or exceeded the specified 30 year (now 50, as of 2006) design snow loads in our area.

Go figure.
Lets not talk seismic.
The Ontario Building Code is the largest regulation in this province of Canada.
We don't charge enough.
Its a wonder anything stands.
One to many beers.
Got to go.

 

[Lion06]

I don't believe that to be true. Check your settings and defaults, something might be out of whack. I actually just went through the program pretty thoroughly for spread footings and found that it works pretty well. It does have some issues assigning extremely small amounts of top steel to some lateral columns and not meeting the minimum steel in footings (I haven't figured out why yet).
I didn't have any issues with the soil pressures. I had a allowable of 3ksf, and it stays under that for footing plan sizing, but was routinely higher than that for the strength combinations.
Is there a chance you have a minimum footing size in there such that the strength combinations are giving bearing pressures below the allowable?
Print a report and look at the max unfactored pressure and the max factored pressure.

 

[COEngineeer]

my max unfactored was 2.0 ish. Max factored was 2.3 . I set the soil to be 2.5. I ended up doing it on spread sheet.

Never, but never question engineer's judgement

 

[Lion06]

What size was the footing? I know there is a default for a minimum footing size as well as minimum dimensions from face of column/baseplate. That may have been controlling the design.
Look at some other footings and see if this is common or if it's an isolated case.
Also, try setting the allowable lower - make it higher than the max unfactored, but lower than the max factored and see if it bumps up your footing size. Be careful, though, because if it's a lateral column you might get a higher size due to overturning considerations.

 

[COEngineeer]

all gravity columns. All of my lateral loads are taken by the core walls. Do you have good luck with combined footing? (using continuous footing from column to column). How about mat foundation underneath the core taking lateral?

I also hate the fact that there is no place to turn off punching shear. If I have grade beams over the column, there should not be punching shear. I usually do not model the wall, I am not sure if it will do it correctly if I model the wall also.

Never, but never question engineer's judgement

 

[dougantholz]

Okay, beyond a discussion of a possible bug in RAM, I am with COEng. I learned using the silver 2nd LRFD edition in school (ASD wasn't taught) but on my first day my boss told me to buy the green book. I've been using the green book for 10 years. I've passed the SE2 first try so I don't think it has caused me irreparable harm. I like to design in allowable stress - it makes more sense to me as an engineer. I can do both seamlessly, but it does bother me some to jump back and forth. If I am doing a simple structure with limited concrete design I will sometimes use a single load factor of 1.4 or 1.5 to scale up my total service to total ultimate. Its sorta cheating, but my boss pays me to be quick, conservative and to move on.