working with existing concrete structures

huangyhg

working with existing concrete structures
how do you guys approach modifying/adding load to existing concrete construction where existing drawings aren't available?  is it common practice to assume the beams and girders have at least the minimum amount of reinforcement in them or do you get them x-rayed?  x-raying seems overkill, but who know's how over or under reinforced the members are.
i woould base on min steel of time of design. may be different from current. alternatively in past where this has shown structure to be inadequate i have carried out a back analysis on likely applied loading and compare forces generated by 'old' and new loads.
can you do a load test?  aci has a procedure for load testing. we have the same problem with a floor slab in an existing building.  no drawings but the architecturals. the ll on the architecturals is 300 psf.  the floor is 12" thick.  they want to convert it into a parking lot.  the slab is too large to x-ray cost effectively and too large to do destructive testing like chipping the bottom of the concrete cover to expose the rebar.  load testing is the only option.   the load testing procedure is in aci 347 "strength evaluation of existing concrete buildings."  since you are looking at a beam, a cost effective way to determine the rebar is to chip away the bottom or top cover and measure it. you can then patch the damage. be careful to use either a chipping hammer by hand or a 15 lb. max power chipping hammer so as not to damage the rebar and create microfractures in the concrete.  x-raying is expensive and i don't know if they can x-ray overhead.  also, many times you cannot see clearly enought to distinguish size of rebar.  call your local testing firm to see the options.  aci 364.1 "evaluation of structures prior to rehabilitation" has all of the concrete test methods and their applicability listed.  also alexander newman has a good book; "structural renovation of buildings: methods, details, and design examples." mcgraw-hill publishing company, 866 pages, 2001.
if there are no signs of structural distress then it is a reasonable conclusion that the structure was designed and built properly to the codes of the day.
you can then check the stresses of the existing use versus the proposed use.
the one catch is that if loads increase by more than 5% then the ibc says that they need to be brought up to current code standards.
be careful of cracking due to changed support/load conditions. i will try and warn the client that some minor cracks/additional deflection may result.
in particular be mindful that old slabs rarely have compression steel at mid span.
fire rating may be an issue depending on the typical cover at the time of construction - check the relevant code for that time.
it is not an easy task, and is more detective work than engineering.
after doing all this, if there are still a few critical   
atomic 25, as a responsible engineer, you should not assume anything about the structural elements that you cannot visually verify. if you assume anything, assume that there is no steel in the bottom of the beam at all.  if you can justify no increase in load, then you can determine that the beam is adequate based on historical performance.  if there will be additional load on the beam, you should reinforce it.  one effective method of reinforcing an existing beam is with carbon sheeting. see
i wouldn't assume anything either, not even minimum steel.  you don't know who, what or why the beam was designed and constructed.  becareful with x-rays because like i said before they are costly and don't always give you what you need.  chipping the concrete away or load testing are the best methods. just don't stand under the beam when it is load tested. some someone you don't like.
well, it looks like there's varying opinions out there.  obviously each situation is different, but i tend to side with csd72's approach because it blends the overall knowledge of a structural engineer with practical risk.  
stevemort, i'm not sure using the term "responsible engineer" is correct.....maybe "conservative engineer"?  i cant see the footing.  i cant see the column reinforcement.  i cant see the concrete strength.  carbon sheeting isn't going to provide connection rigidity..etc
will definitely try to check out those books.
thanks guys
i would certainly consider how much additional load you are looking at relative to the current load.  i agree it would be scary to assume even minimum steel if you are looking at more than 5-10% increase.  and if you make that assumption, where will you assume the reinforcing is??
atomic25,
if it helps, i also agree with csd72.  the change in use is the main thing.  if it is being changed from an office building to a residential building or a parking garage, no problem as long as you don't add too much dead load.  but if it is being changed from office to storage, it would be a no go unless you can prove it somehow, and that would be unlikely.
there are cheaper options than x-ray, covermeter survey for instance.