Girder Shear Stud Distribution 1

[BadgerPE]

I am working on my first building design in which composite action is used between the steel framing and the concrete deck. Governing codes are IBC 2009/AISC 13th. Currently, I have the building modeled using RISA Floor and I am working to verify results by hand of a few of the girders/beams. My question is related to the placement of shear studs for a girder subjected to point loads. Please see attached document for two cases I am considering (diagrams are arbitrary and are assuming SIGMA_Q_n are the same for each girder).

General considerations:
f'c=4 ksi
Deck = 18 ga 2" deep
Overall slab thickness = 6.5"
Shear stud diameter = 0.75"

Case 1:
Girder subject to 3 point loads of approximately equal magnitude.
The way I understand section I3.2d.(6) of AISC is that I should have the full amount of shear studs, required to develop the moment at the concentrated load, from the concentrated load to the point of zero moment. In between the two outside concentrated loads shear studs should be spaced at the min(8t, 36"). This seems to be pretty straight forward and it is how RISA calculates the shear studs.

Case 2:
Girder is subjected to 6 concentrated loads of varying magnitude.
This is where I get a little confused. I am still applying multiple concentrated loads to the girder. However, RISA provides shear studs at uniform spacing across the entire member. Is there something in the code/DG that provides this method of stud distribution? About the only thing that I see is in Comm I3.2d.(6) the first paragraph reads "Uniform spacing of shear connectors is permitted, except in the presence of heavy concentrated loads." I take it RISA's interpretation of this is that when you increase the number of concentrated loads, you start to get away from the "heavy concentrated loads" segment of the Comm because the loading is more spread out on the girder than when there are only 2-3 loading points.

After re-reading the Comm and typing this up, it makes more sense in my head. Can anyone verify that this is the approach they would take? Also, how many point loads on a girder would it take to go from stud spacing as indicated in Case 1 to stud spacing in Case 2?

Thanks!

 

[JoshPlumSE]

RISAFloor actually has an option to use "segmented" stud arrangements. See the composite tab of the global parameters. Though I believe the assumption is based on framing, so your point loads would have to come from other beams framing into this girder. At least for RISAFloor to want to switch over to segmented studs. The idea behind segmented studs is to place more studs in the high shear regions towards the end of the girder and less at the mid-point of the member.

When set to uniform stud spacing RISAFloor uses a concept of "maximum stud density" based on your applied loads. The program then looks at the moment demand at each location (maximum moment and each point load). We calculate the number of studs required between that point and the end of the beam and compare that to the length between that point and the support. Whatever location comes up with the worst case stud density controls and that's what RISAFloor will use.

 

[BadgerPE]

Josh,

Thanks for the info! Could you expand a little on the procedure to calculate the quantity of shear studs required from M=0 to M @ concentrated load assuming M @ concentrated load is less than Mu?

 

[JoshPlumSE]

Crackerjack -

Calculate the moment capacity at the maximum moment location (call this Mu_max) based on the shear that can be transferred between beam and slab. Controlled by the minimum of the a) sum of the shear stud strengths, b) the concrete capacity in compression, c) yielding in the beam. Usually, the shear studs are going to control. Let's say you need 30 studs over a 15 foot length. That's a stud density of 2 studs per foot.

You do exactly the same thing at locations of point loads. The only difference is that you use the moment at the location of the concentrated load (call this Mu_point_load). Let's say you need 21 studs over a 7 foot length. That would be a stud density of 3 studs per foot. If RISA uses a uniform spacing then we would specify a stud layout that gives us our 3 studs per foot.

This is somewhat simplified because other issues arise when the stud capacity changes. Such as when you go from 1 stud per row to 2 studs per row.

 

[johnbridge231]

The count of studs can also be calculated in a plastic way, ie independently of the applied loads. In this case, the studs are designed with an overstrength in order to resist the longitudinal force that is developed when the girder yields. The pros of that method is that is load-independent and that it assures great ductility of the composite member.

Analysis and Design of arbitrary cross sections
Reinforcement design to all major codes
Moment Curvature analysis

http://www.engissol.com/cross-section-analysis-design.html

 

[TDIengineer]

If using Risa Floor I strongly recommend to NOT use segmented shear stud placement. I prefer a uniform placement method. When looking at the composite slab over the beam as a rigid body, each stud will carry the same load from VQ/I over the length of the span.. Placing more studs at the end span where the shear is higher is correct based on theory, but with the slab as a rigid body it makes more sense to take your shear flow out as a uniform stud placement, imo.

I believe there were some articles about segmented versus uniform in one of the engineer publications.

 

[JoshPlumSE]

The AISC has code commentary stating that it's the number of studs that matter not placing them in the high shear regions. So, that supports TDI's argument about uniform stud placement being as good capacity wise as varying the spacing per beam segment.

I am not personally aware of any technical arguments against the segment stud option. It might just be that you don't save all that many studs and that it may not be worth the added construction complexity.

 

[MJB315]

I would suggest uniformly distributing the studs as well, especially from a construction perspective. Trying to specify 14 studs on one beam, 19 studs on the next (with 7 studs between the end and first beam connection and 5 in the middle), 20 studs on the next beam, to 14 studs on the next...may lead to more effort and confusion in the field than is really necessary. It may be best to just specify 20 studs on each beam in that bay and keep moving.

You do want to avoid being overly conservative of course and adding more way studs to the project than is needed, but at approximately $2 - $2.50 stud (price check?), you can usually take away a lot of confusion for short money. And when putting together documents, taking away confusion, to the extent possible, is half the battle.

"We shape our buildings, thereafter they shape us." -WSC

 

[steellion]

I typically use uniform spacing of studs unless it is a girder with 2 concentrated loads at third point. In that case, the majority of studs go at the end thirds, with minimum studs between where there is no change in shear.

 

[PUEngineer]

I typically use a uniform distribution of studs.

I hate to hijack this thread, but our drawings and specifications require that a shear stud layout drawing be submitted for review. This almost never happens without a huge fight. Does anyone else that does a lot of composite construction require this? And usually get something worth a darn?

 

[steellion]

PU, I usually see the stud layout on the steel deck shop drawing. I've never had to really fight for it.