Post Tensioned Beam to Column Interface Parking Garage

[camber]

I am attempting to evaluate a 24 year old parking garage for the addition of a lightweight garage for servicing a helicopter on the upper deck. I am currently checking the interior columns and am having difficulty where the beams frame into the interior columns at slightly different elevations due to the ramp. The location I am considering has a 3' difference in beam elevation from one side to the other. I am using ADAPT and when I input a short 3' upper column and a7' lower column a large amount of moment is being "sucked" into the column. Is this a realistic approach or am I missing something and should model each beam with the "typical" 10' tall column above and below which is the floor to floor height for the ramp. Any help or comments would be appreciated as we are looking at adding a fiber wrap to the columns to address the issue. I am using the ADAPT reactions for dead load and live loading and adding in the moments due to lateral froma risa-3d model. Thanks

 

[MikeT14]

camber

I recently had a similiar question regarding the design of a new PT garage. According to ADAPT Tech. Support, you should use the actual unbraced height of the column in your runs. I'm sure you noticed noticable difference in the moments going to the columns, and consequently the PT force, when modeled using the actual unbraced height and the "ideal" or "typical" column heights. If I understand it right, the short column height assures that the correct PT force is used, as a shorter column is stiffer and can therefore do more of the work of "holding up" the PT tendons. Using the "typical" height may result in using too high of a PT force which will effect the moments and shears at the column face.

 

[camber]

Mike T14

Thanks for the information. It appears that this was not considered in the original design as the reinforcement of the column does not reflect this larger moment due to the short column action, now I am in a quandry. Unfortunately I may have to tell an owner they cannot add the additional load do to a deference in how the original ramp was designed. Thanks for the help.

 

[14159]

Is this an existing column and beam? If it is, then you have to ask yourself the question: "How has it worked all this time?" The answer is probably that the original designer ignored beam end restraint from the column because they didn't have any good way to account for it. The beam tendons and rebar were probably overdesigned because of this simple support condition. I've never seen or heard of a column actually failing because of this and suspect that they probably just crack a little and have some steel yielding, therefore relieving this theoretical huge moment.

I've run into this kind of thing a lot in new construction. For example, we had a 70-some foot long PT girder in a hospital. The moment that was theoretically cranked into the column was 3000-4000 kip-ft. Making the column larger just makes it worse because it sucks more load over to the column. Every time this came up, we just made sure that the beam could work fine with no restraint from the column, use a large amount of column reinforcement and then stop worrying about it.

If you go to your architect and just tell him it doesn't work, then they'll probably ask the same question: "How has it worked all these years?" Their common sense won't be swayed be talks of EI/L, etc.

DBD

 

[camber]

DBDAVIS

Thanks for the info. It is an existing column that we are adding additional load to due to the owners request to put a "roof" over a portion of the upper garage. In a new design I could see the approach you outlined but they are asking me to perform a study on the existing structure which we cannot modify. Any "cracks" they happen to notice that are due to this in the future will be a phone call to me wondering if the new roof structure is the problem. A lot of liability for a small fee to study the existing structure. Thanks for your thoughts.

 

[14159]

Another thought:

For problems such as this, I often find myself not only looking at the moment applied to the column but the INCREASE in moment applied to the column.

Consider this example, assuming that the column is showing no distress. If your analysis shows that 800 kip-ft are theoretically there now and you'll have 825 kip-ft after your addition, then that's a lot different than if the current loads should result in 400 and you're increasing to 825. If the load increase is very small, a very large change in behavior should not occur in most cases.

The whole liability vs fee question is very real and something that irritates us all -- I feel your pain. I also hate questions like this because you can bet your life that some other firm will say that it's ok and nothing will probably ever happen regardless of whether they're right or wrong.

DBD

 

[AggieYank]

Your analysis sounds right. The fact that it has stood up for 30 years shouldn't mean you "let it slide", as I'm sure you know.

If I'm checking the building as you are, I'd definitely note that the structure with the additional load would require modifications to withstand the extra load. I'd also mention that your analysis showed that the existing column wasn't acceptable as-is, and recommend obtaining a fix in your professional opinion.

DBDavis, as far as you say:
"The whole liability vs fee question is very real and something that irritates us all -- I feel your pain. I also hate questions like this because you can bet your life that some other firm will say that it's ok and nothing will probably ever happen regardless of whether they're right or wrong."
-I wouldn't worry about the fact that another firm will issue a report saying it is fine. In fact, I would think that any decent architect would recognize the fact you did a thorough and correct analysis and respect you more than the guy who said it was fine, even though the other guy will save some money. They'll be happy that guy saved them money, but for bigger or important jobs, who are they going to go for.