25% composite action - In existing beams?

[rharting]

I'm working on a project where we are checking an existing composite floor system for additional equipment loads. The existing infill beams are W16x26 with only 10 shear studs on them. Given 3/4" dia, weak stud the beam would be less than 25 % composite action. Adding more steel would only make this worse and the owner would like to continue using the floor system.

Questions to all:
a)25% min composite action is a standard practice and in many texts it says that composite action below 25% are not accurate. Is there anything in the code that states the 25% min rule?
b)when determing the moment capacity of the W16x26's without any additional reinforcement, would you consider the less than 25% composite action or simply design it as non-composite?

Thanks to all.
thread507-182835

 

[PMR06]

Just to double check (I had this happen once), are you sure it is 10 studs over the entire beam length and not 10 studs each side of center, so 20 total?

 

[rharting]

PRM06,
unfortunately it appears as if it's across the beam (although it doesn't specifically state that). the number of studs is noted in a box and the note says "indicates number of 3/4" dia 4" high shear studs"

even if the engineer expected that to be on each side, i can't imagine a contracter reading that note and knowing that is half the total amount required.

 

[ishvaaag]

For actual capacity one counts anything of value. To comply with the code what the code allows. You seem to be considering some growth of loading hence it seems you need essentially to find if it will comply with the code, by the code's rules. You might (in a generic case) find that you have enough capacity to cover the demand, but still not comply with the rules in the code, asking full development of the capacity of the beam, a situation in which an engineer is expected to say to the owner, we need to reinforce this, it doesn't work as composite as per the code specs.

 

[slickdeals]

The answer to your question regarding 25% is that there is no minimum composite action required by AISC. More information is on Page 16.1-310,311 of the AISC 13th edition.

We are Virginia Tech
Go HOKIES

 

[paddingtongreen]

I know of no such limit. If there is no limit, adding a plate, or better still a tee to the bottom of the beam actually moves the neutral axis down and reduces the load in the concrete. I don't see how reducing the load in the concrete can be anything but an improvement over what is there now, regardless of percentage.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[ishvaaag]

For what I remember the AISC I most once read (maybe LRFD 1993) demanded stud or equivalent shear transfer devices to ensure 100% development of the capacity of the beams used in composite design. However more detailed reading on the issue may discover such is not the case.

 

[ishvaaag]

Other interesting point is that if we design the connectors for capacity we may expect (actually and according to the code) good behaviour under the maximum service level loads even if somewhere the number of connectors is less than the elastic demand.

For partial composite action, the lower number of connectors may make that the difference between the actual elastic demand present and what provided is as high as to produce the failure of the connectors even at service level condition, then also failure in composite action, and less than required limit strength.