Adding Studs to Existing Composite Beam
Adding Studs to Existing Composite Beam
(OP)
I am adding a large stair to a steel composite building. I am having difficulty getting the existing beams to work. The girder that the beams frame into was overly designed while the beams were designed just below failure at 100 PSF LL. With the additional load even adding a WT section under the beam does not keep the stresses below both 0.66Fy and 0.9Fy checks. Has anyone added studs to an existing beam? I cannot find any information about this and was wondering if it had been done before. Thanks.
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
www.tbastructures.com






RE: Adding Studs to Existing Composite Beam
RE: Adding Studs to Existing Composite Beam
Hg
Eng-Tips policies: FAQ731-376
RE: Adding Studs to Existing Composite Beam
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
www.tbastructures.com
RE: Adding Studs to Existing Composite Beam
If you aren't a terrible amount of load you should be able to get a WT to work. I had some existing (non-composite) WF beams where the space was being changed from 40 psf LL to 100 psf LL and I needed a relatively large WT, but I was able to get it to work. What kind of additional loads are you looking at? Can you get creative with adding a WT with a channel on the bottom flange of the WT?
RE: Adding Studs to Existing Composite Beam
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
www.tbastructures.com
RE: Adding Studs to Existing Composite Beam
The most appropriate paper for your situation might be article in Second Quarter 1996 AISC Engineering Journal, "Strengthening of Existing Composite Beams Using LRFD Procedures". That says "This paper deals with the addition of either a flat steel plate or a WT section to the bottom of the beam to moderately increase its load carrying capacity. For a major increase in capacity,the method described in this paper could easily be extended to other reinforcing shapes".
RE: Adding Studs to Existing Composite Beam
Just core the holes large enough for the stud welder, and at least four times the diameter of the head of the stud for grouting purposes, and be certain that you use a non-shrink grout (at least equal in compressive strength to the concrete slab) to fill the hole. Stud size should be as normally designed for new composite beams.
Also, if you have excessive deflection under the proposed load (or even under the existing loads if the beam was mistakenly underdesigned), jack up the beam slightly before adding the additional studs to transfer the dead load to the composite section.
Strengthening by adding cover plates to the undersdie of the beam is also a relatively simple procedure, which we have done in numerous existing buildings. For economy in field welding, try to use a plate wider than the flange so that the welding is performed in a comfortable position, instead of overhead. Also, design the cover plate and the welding to suit the loading conditions, do not be overly conservative by specifying a continuous weld, and cut-off the ends where approriate. A full length cover plate is never required.
Note that jacking of the girder before welding the cover plate will also transfer the dead loads to the composite section. (Place the plate in position, then jack against the plate, clamping it against the underside of the beam.)
RE: Adding Studs to Existing Composite Beam
I would second epr's comments. Been done before. But I would first try to add steel to the bottom of the beam as its probably much more cost effective.
RE: Adding Studs to Existing Composite Beam
That (and that fact that the ductility required of the studs to perform efficiently at 25% composite is extremely high) is the reason why I rarely go much less than 40-50% composite. $15 worth of shear studs saves a lot of headaches later.
RE: Adding Studs to Existing Composite Beam
Percent composite being controlled by AsFy is only true if the PNA is at or above the top of the top flange, correct? If the PNA is in the steel section then the percent composite is related to Ac(f'c)?
RE: Adding Studs to Existing Composite Beam
RE: Adding Studs to Existing Composite Beam
That is exactly my problem. When adding steel, it increases my required studs for full composite and decreases my percent composite.
StructuralEIT,
In any calculation I have about composite it only shows the caluclation of 0.85f'cAc/2 and AsFy/2. It doesn't seem to correlate to any other calculations.
CowboyBill74,
The client is already upset about having to add 8' W8's between the existing W14's. We are at the 15th floor of an existing building and getting the steel in for the stairs is going to be a job in itself. I'm trying to reinforce in the simplest(also somewhat smallest) way possible.
epr,
Do you have any papers, research supporting the methods you use/describe? Can you please let me know where I could find them. Thank you.
Thanks for all the input.
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
www.tbastructures.com
RE: Adding Studs to Existing Composite Beam
RE: Adding Studs to Existing Composite Beam
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
www.tbastructures.com
RE: Adding Studs to Existing Composite Beam
RE: Adding Studs to Existing Composite Beam
When you get some resolution on this matter, please let me know the references you have acquired. i would be very interested in reading through them.
RE: Adding Studs to Existing Composite Beam
or add a new beam along side to take the stair loads.
This may end up cheaper than all those cored holes and welded studs.
RE: Adding Studs to Existing Composite Beam
It's not necessarily a matter of cost, its more of ease of construction because right now they are not sure how they are getting the steel for the stair up to the 15th floor, and it is only 11 foot in length. These are 28' spans. I think coring a few holes and adding some studs would be easier.
We'll see what the owner thinks.
StructuralEIT,
I've copied the email I received from AISC below:
"RCraine,
Although the AISC Specification and Manual to not address it, one can add additional shear studs to increase composite action. Refer to AISC FAQ 4.5.5 at www.aisc.org/faq. We are not aware of any specific testing, so it must be approved by the EOR after considering the strength of the existing concrete and new grout (used to fill the newly added stud holes.) Additionally, one may consider adding plate reinforcement to the bottom beam flange to increase the steel beam capacity.
Sergio Zoruba, Ph.D., P.E.
Senior Engineer
American Institute of Steel Construction, Inc.
866.ASK.AISC"
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
www.tbastructures.com
RE: Adding Studs to Existing Composite Beam
Hg
Eng-Tips policies: FAQ731-376
RE: Adding Studs to Existing Composite Beam
Thanks for the logical advice. Its sometimes overlooked.
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
www.tbastructures.com
RE: Adding Studs to Existing Composite Beam
RE: Adding Studs to Existing Composite Beam
Although I have core drilled an existing slab and added studs to non-composite beams to increase its capacity, this was done in an industrial environment. It sounds like you are in a commercial environment with your description of a 15th floor. I would probably avoid core drilling because of the difficulty in dealing with water dripping and leaking thru to the floors below. I would elect to add or reinforce beams non-compositely.
RE: Adding Studs to Existing Composite Beam
RE: Adding Studs to Existing Composite Beam
That is my greatest concern. However, lucky for me, there are 14 studs on a 28' span. My guess is at 2' o/c. 50/50 chance, and if we hit one, we can guess where they are. 1 stud won't hurt us if all of this takes place during demo/renovation work.
jike,
I'm confused on your concern about water leaking through levels. Water should not be on any of these levels to being with, and would be a problem regardless if I cored into the slab or not. Is there something I'm missing?
WillisV,
Yes, that is true, if you look at most calculations, the steel is controlling at least 2:1.
Thanks for all the input and feedback.
RC
All that is necessary for the triumph of evil is that good men do nothing.
Edmund Burke
www.tbastructures.com
RE: Adding Studs to Existing Composite Beam
We have done this in cases where they tried to laser screed a composite floor and added up to 2 inches too much concrete (even buckling the deck in places before they realized something was wrong.) Adding studs worked just fine.
Some one suggested adding non-composite beams, which sounded like they would place these parallel to the existing to pick up some loads - not connecting it to the existing overloaded member.
You may be able to splice these new beams from pieces short enough to get up to the 15th floor, although at 28 foot span, it sounds like relatively heavy steel sections will be required.
RE: Adding Studs to Existing Composite Beam
As far as bringing in the beams, you can always bring it in in 10' parts and get it welded together on site. 10' should fit in the diagonal of the elevator (get someone to check).
The point regarding water is also very valid.
RE: Adding Studs to Existing Composite Beam