Non Composite Beams - With Shear Studs 1

[UKOS8878]

Quick question,

I have designed a suspended floor system using CSC Building Designer, where the most economic design was for the beams perpendicular to the span on decking to be designed as 'Composite' with shear studs, and the beams parallel to the span and edge beams to be designed as 'Non-composite'. However, the contractor has installed shear studs on all of the beams--even those designed as 'non-composite'. Will this be a problem...i am assuming that if the beam is adequately sized as non composite, adding shear studs would have a positive effect?

When I do a design check on the original (non composite) beam size now with the number and spacing of shear studs that have been added on site, the design shows that there is not enough shear studs for the beam to act as composite, and therefore shows as a failure? Can it be assumed that if there is not enough shear studs, the actions will just bypass the shear studs and act as a normal slab sitting on a non composite beam as i orginally designed for?

any help would be greatly appreciated.

thanks

 

[dik]

It should have no detrimental effect; it will improve stiffness for deflection. From way back when... There's a fundamental Lemma by Feldberg that states, "Other than stability issues, if you strengthen a system at any location, the resulting system will not be less strong than the original."

Stability is not likely to be an issue, either.

Dik

 

[a2mfk]

DIK- That sounds like a Yogi Berra type quote...

There are few times when less deflection can be a bad thing, but it is something you may want to at least have a brief look at...

 

[JoshPlumSE]

UKO -

It should be fine. Like others have said, you should start out with an increased stiffness. The biggest issue that I can think of is that when the floor is heavily loaded, one of the shear studs may fracture. I've heard of this happening before.... though I cannot say how common it might be. While there is no reason for you to fear structural failure, the occupants find this somewhat disconerting and you can have "perception" issue related to the safety of the design.

 

[dik]

Way back when (30 years past) I went through Vol 1 of Massonette and Save's (sp?) text on Plastic Design, they made rederence to this lemma... and I looked it up at the time... Yogi's dead, and his surviving next of kin has the last name "Bear". It's still one of the best texts on plastic design, ever written.

If shear is an issue, then the original beam, by stiffness, will likely accommodate the load before there is sufficient displacement for the shear studs to fail.

Dik

 

[msquared48]

Make shre you document in the as-builts the size and spacing of the shear studs so any future calculations for any changes to the structure will be correct.

You might want to do a few checks on a few beams to see if the stud size and spacings create full or partial composite action, and note such too.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[UKOS8878]

thanks for the responses.

It makes sense in my head that if the beam is designed as 'non composite' it will therefore be oversized for its 'composite' equivalent(even though i will not have full composite action as there is not enough shear studs on all the beams in question).

 

[steellion]

I don't like to mix composite and non-composite steel beams on the same diaphragm. Therefore, I will always add a few studs at the top of every beam on the floor, even those designed as non-composite, to "lock" the system together. For short beams, sometimes I will get the same "cannot fit enough studs for composite action" error in RAM, so I simply design as non-composite and add about 1 stud/foot to the drawings.

 

[msquared48]

I Agree with steellion.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[WillisV]

Agree it shouldn't be an issue - the only time that it could be an issue is if the beams you designed non-compositely are drag struts and just a few studs were used to transfer the shear out of the diaphragm. In this case the studs don't know they are there for lateral load only, and if there are too few of them they could get overloaded under just gravity load. For this reason it is best to design any drag strut using shear studs for a minimum of 25% composite action.

 

[keenaseng]

You probably don't have enough studs to achieve full composite action, which is probably why you are getting the failure message. However, you are likely to get partial composite action (depending on the number of studs), as other posters have stated, and as such your stiffness will be increased.

Assuming you have enough studs restraining the top flange of the beam against lateral torsional buckling, your partial composite beam strength capacity will be somewhere between the non-composite steel section capacity and the fully composite beam capacity. I don't think this is a linear increase proportional to the number of studs either - you pick up a fair bit of additional capacity with minimal studs. I think I remember reading somewhere that reducing the number of studs required for full composite action by 50% only reduces the composite beam capacity by like 15-20%. Not entirely sure of that however...

Did you assume you have full lateral restraint to the primary beams when you designed them originally as non-composite?