Limiting effective width of composite steel beams

[NJonesUK]

I'm working with a modelling program called Building Designer and I've been playing with some of the factors to give myself an efficient design.

When looking into a particularly heavily loaded building I've noticed an issue regarding balancing getting the required number of shear connectors for full shear connection and overloading the slab with longitudinal shear.

However by limiting my effective width to less than the maximum L/4 I can attain the required shear connection and not overload the slab with longitudinal shear, but how do I achieve this in reality or will my building do this without my intervention through the means of upper bound theorem?

Not sure this made total sense, if there's any confusion I'll try and explain better. If not, any input is greatly appreciated.

Regards,

Nick

 

[dhengr]

I suspect the confusion is mostly your’s, and for you to explain, to you. Damn these computer programs, when we don’t know how they handle and manipulate fairly common structural concepts and phenomenon. The structure knows how it is going to act, once built, and it will act that way, irrespective of what you or I think, or what some darn computer programmer did. And, it’s for us to figure out the way the structure will act so we design, weld, space shear connectors, reinforce, etc. properly. Then it is for you to determine exactly how the analysis and design programs you are using account for some of these structural and material characteristics and phenomenon.

Look up shear flow and shear lag. The limit L/4, effective usable slab, is basically set by shear flow and shear lag; for a normal length beam and slab thickness, the slab can only distribute a limited amount of shear flow per foot of beam length, and it takes some beam length for this stress to distribute, in width, out into the slab.. Thus, you must space shear connectors in accordance with what the slab can accommodate and distribute out into some (L/4) width of slab. I think that what you are talking about is that you can’t reconcile the number of shear connectors per foot of beam with the concrete shear strength in that same distance. Obviously, you shouldn’t overstress the slab in shear, thus the slab must be made stronger or the shear connectors not spaced so tightly. You must provide a means of distributing any concentrated loads, to the slab, in such a way as to allow the shear connectors, shear transfer, a chance to develop through shear lag and shear flow, between the steel beam and the conc. slab. And, your computer program is playing with these concepts.

 

[paddingtongreen]

@NJonesUK, I think you must clarify what you mean by, "I've noticed an issue regarding balancing getting the required number of shear connectors for full shear connection and overloading the slab with longitudinal shear." Which shear plane is overloaded?

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

 

[BAretired]

NJonesUK,

I don't get it! Why don't you leave the width alone and simply reduce the percent shear connection to something less than 100%?

BA

 

[EnricoNespolo]

NJonesUK: that's an interesting question!
I work for a company that produces small connectors fixed by shotted-pins and since these connectors may have small shear resistances this questions came to me many times.
This may happen even if you use partial shear connection. And the softwares aren't involved at all.
In a simply supported beam in complete shear connection the number of connectors in half beam is dependent not on loads (or better on stressings) but is dependent on resistances of steel and concrete. If max compression in concrete is lower than max traction in steel the number of connectors will be related to the width on collaborating slab, to the height and to the class of concrete.
So the bigger the width will be, the bigger the number of connections.
This is not dependent of stressings!
And if you use partial shear connection you may find the same behaviour, since minimum number of connection in partial shear connection if a fraction of the number of connectors of complete (the minimum ratio is usually 0.4).
So if, for some reasons, you have big steel profiles and concrete slabs (that means no problem on flexion resistance verification even in minimum partial shear connection) you will find that if you compute a smaller collaborating width you'll have less connectors.
Sometimes this happens, maybe you have to design following beams with long spans and short spans: the short ones will have this behaviour (and you will find yourserf with many connectors to be placed in short beams!) or as well, maybe you have a big composite beam since you allow very very little deflections, or for estetic reasons!
But I wonder this: is it allowable to use an effective width less than L/4? The EN1994-1-1 doesn't say anything about the eventuality.
This is not dependent of sollecitations!
And if you use partial shear connection you may find the same behaviour, since minimum number of connection in partial shear connection if a fraction of the number of connectors of complete (the minimum ratio is usually 0.4).
So if you have for some reason big steel profiles and concrete slabs (that means no problem on flexion resistance verification even in partial shear connection) you will find that if you compute a smaller collaborating width you'll have less connectors.
Sometimes this happens, maybe you have to design following beams with long span and short span, and the short ones have this problem (and you will find youserf with many connectors to be places in short beams!) or as well, maybe you have a big composite beam since you allow very very little deflections, or for estetic reasons!
But I wonder this: is it allowable to use an effective width less than L/4? The EN1994-1-1 doesn't say anything about the eventuality.

 

[JAE]

BAretired has the right direction in mind in my view.

Use a less than 100% composite. You rarely should use 100% as this is not cost effective.

 

[dcarr82775]

NJonesUK,

You only have 3 choices for the controlling strength:
1. As*Fy of the beam
2. Compressive strength of the slab over its effective width
3. Shear strength of shear studs (which AISC keeps changing much to my annoyance)

Whichever of the 3 is least controls and sets the limit on the upper bound strength available for use in the other 2. I believe your dilemma comes from not understanding the design principals of a composite beam, and isn't related to the software.

 

[NJonesUK]

I'd have to disagree for the most part on my 'lack of understanding' but I appreciate your input, thank you.

I have since resolved this issue, however what I was really trying to get at is - why does the software allow you to change the effective width of the beam in question if in reality the effective width is a fixed factor in the beam design.

Cheers for all those who input into the thread. Most of what was written above is really useful stuff. I'm still pretty new to this engineering game and it's always nice to have some wise heads to turn to when something doesn't make sense. Ta.

 

[apsix]

Perhaps the software allows you to change the effective width of the beam for locations where the slab edge or beam spacing is closer.

 

[a2mfk]

apsix is absolutely right- I remember doing this in RAM, you either have to put in the spacing to the next beam, the slab edge, or the max possible effective width to each side. For edge beams it may only be a few inches to one side...