Concrete filled HSS column to flat plate moment conn.

[OCI]

One of my current projects is an irregularly shaped work platform or mezzanine. I have designed other more regularly shaped mezzanines using a steel braced frame and composite deck that worked well. Because of the shape of the current platform and limitations on column / brace locations the one way nature of composite slabs is not going to work well and I'm looking into other structural systems. The client wants concrete for the floor surface. I would like to use concrete filled HSS columns and a reinforced concrete slab with rigid connections to the columns if I can get it to work. If not I could try steel columns to composite beam & slab moment frame. The latter system is common and well documented. I havent had any luck finding any info on filled HSS columns to concrete beam or slab connections though. Any idea's or resources to suggest? Thanks.

 

[ishvaaag]

Depending upon the loads, even a top plate with 4 connectors may work, plus ordinarly reinforcement for punching. Here is common this late times to use greek patterm for the reinforcement against punch action embracing the reinforcement that cares of bending. For a more competent transmission of moment action the ordinary shear heads made of standard shapes embedded in the flat plate also works, and ACI has enough within to design the shear heads. The Nilson´s book on reinforced concrete also explained the ACI way of computing these connections. The shear head uses to have 0.5 to 0.6 of the gross depth of the slab and uses to have 2 to 3 ft arm projections; continuous, its contribution to moment strength can be taken into account by equivalent section, or you can check it works well within the slab by the simple trick of adding it to the model as 4 members in cross shape within the plate, that uses to be fine, and has the advantage the plate elements show the actual effect of the embedment for reinforcement, plus you having forces to design the members.

 

[OCI]

Thanks, that re-affirms my thinking. What type of lateral system would this be considered (for asce7 R value)? Concrete moment frame? The other situation is that I am in seismic zone D in california, so I have to detail as a SMF and adhere to the column to beam stiffness ratios which may be hard to make work.

 

[OCI]

Here's a picture of the platform. It's small (20' x 20'). I was hoping to keep the colums small, like HSS 8"x8". We'll see though.

 

[OCI]

Because this is not a building it would fall under ASCE7 table 15.4-1 "Seismic Coefficents For Nonbuilding Structures Similar To Buildings". I just found the last row in table 15.4-2 which is for "All other self supporting structures not covered above, similar to buildings" R = 1.25, ouch! Could this be considered a concrete moment frame even though the columns are composite? Any input would be appreciated. Thanks

 

[ishvaaag]

Well, respect qualification according to your codes I can't attest since I have not them and well, composite structures of every kind are made anywhere and *** should *** be amenable to be dumped in some categories in the codes. I think our latest standing code CTE that rules everything structural and mechanical for buildings here (Spain) quite candidly forces to ensure every part, steel or reinforced concrete, be it the case and whatever its position, take what it must to take to behave well. So in the absence of clear category, might check for worse cases of envelope models.

Now, composite structures in general are not the better for strong motion or blast. There's always then something hard and strong, the steel, punching its way throug the weaker concrete. I always remember how in the 1st bombing of the basement of WTC the slabs were cleanly spitted off the shearhead by the explosion.

In your context, however and anyway, a weak shear-head may still work. Sometimes even here that we have only less than .04g surface EQ accelerations I have used horizontal connectors to the shearheads to enhance connection. Furthermore the shear-head will help for inversion of moments; AND, since you are required to calculate the structure for the condition of strong motion, ascertain well its degraded stiffness, this might help to comply with an enforced strong column approach.

Also, if seemingly noncomplying, I would look if compliance on fundamentals of engineering is permitted by the code; for, frankly, for a structure like yours, any adscription of such a thing as a "beam" looks as imagination. This is a irregular plate on columns; does the code gives specific procedure to analyze such structure? prohibits it? If not, you may analyze it according to the science of construction, and showing compliance with all those requirements that are clearly still applicable should suffice.

 

[OCI]

You were right, ACI318 has detailed design procedured for using shear studs to transfer shear and moment from columns to slabs and beams. I never noticed them before. For now I'm going to assume R = 3. Thanks

 

[OCI]

Thanks for the detailed reply. After looking at this a little more I don't see any reason why I can't make this work. I can thicken the slab at the beam strips and taper out to the cantilevers. If necessary I could use a 4" W section for shear heads. With the reduced I from cracking the weak story might not be a problem. The big question is what R value to use. I'm going to assume R=3 for now unless I get direction from my superior (I'm an EIT still) to use a different value. I'm thinking I could even use smaller HSS for the columns, like 6". Anyway I'll let you know what I come up with after I crunch some numbers.