Lateral load resistance of slab on metal deck

[faromic80]

I am involved in the design of a 5" total depth concrete slab on 1 1/2" metal decking. I am designing the slab on deck and someone else is desiging the steel framing. My questions are regarding the lateral load resistance of the floor. The floor is being designed for 10% of the DL of the floor system. If the 5" total depth of concrete (although only 3.5 inches will be effective in resisting shear) can resist the end shear due to the lateral loads, do the deck connections (fastener pattern and sidelap screws) to the steel need to be designed for this same load? I'm unsure if the shear will actually be transferred to the deck since it's being used a form deck during construction. I spoke to the person designing the steel framing, and he also designed for this lateral load, providing continuous bracing around the perimeter to take out the lateral loads. I was always under the impression that the concrete (if designed to do so, which it most likely could since it's rigid) resists the lateral load and the steel is designed from gravity load and no bracing would be required. What I'm understanding here is that he expects the lateral load to be transferred from the concrete to the deck and from the deck to the steel through the puddle welds in the 36/7 pattern I designed. Is this really the case? I didn't think so. I never thought the steel saw that much of the lateral load.

Another issue is the bracing of the top flange again LTB. If the connection between the metal deck and steel framing is based on the lateral shear capacity, and not the unbraced lengths of the beam, how do you know how much lateral force is required to prevent the beam from LTB? I know of Appendix 6 is the 13th ed. of AISC. Is the criteria you would use?

 

[faromic80]

I want to add something more that is important to the story. It is reasonable in this case that the steel take the lateral load. It's an existing building and we're adding this floor in between existing floors. The lateral load resisting system is braced frames. The concrete on deck does not transer any lateral loads to the superstructure. In fact, there is a 1/2" gap between the concrete edge and exsting interior of the walls.

 

[msquared48]

Usually your design catalog for the metal deck system you use will give the allowable shears for the deck system, composite or otherwise, to include any side lap welds, button punching or otherwise needed to develop the strength of the system. As the concrete is taking the shear, I don't think you have to worry about transmitting the same shear lload through the deck. These are usually tested and rated proprietary systems.

Mike McCann
MMC Engineering

 

[haynewp]

The concrete is taking the shear but you of course have to get the lateral out of the concrete and down into the steel beam somehow. So, if this was big rigid concrete floor on composite (or noncomposite) steel deck I would use minimum weld and sidelap fastenings everywhere for the deck itself (I think it is 2 or 3 per deck span, check Vulcraft catalog) and assume the lateral shear is carried in the 3.5" of concrete. Then I would provide shear studs on top of the beams that are at the braced bay locations to take out the lateral from the concrete and into the steel bracing system.

 

[faromic80]

The floor is noncomposite. haynewp, you would provide shear studs only on the beams that frame
into braced bays? There are some issues here with this floor. The design is for a new floor in a power
plant. They are not wanting to use the typical method of puddle welds to attach the deck to the steel because they do not have a process to do so. The power plant industry is much more stringent on this type of thing so I have to go along with it. I am also hesitant to use TEK screws as I need a 36/7 pattern and dont want to drill so many holes through the beams and girders. My boss has suggested using headed studs in a non-composite manner. The steel
framing is already designed and there is not enough time nor funds to redesign it due to the schedule of the
project. One issue with adding headed studs would be that the slab and beams would act at least partially
compsitely since studs are installed, right? Also, some general questions: if the slab is non-composite, how is the shear transferred from the concrete to the deck, friction? What capacity would I use for the shear studs? Could I just calculate the stud capacity per AISC ASD 9? The max shear loads at the ends of the slab is 447 plf. Assuming studs at 24" o.c., each stud will have a shear load of 900 lbs on it. This obviously will have a large design margin, but I have to do a calcualtion for this. Additionally, I would probably provide studs uniformly everywhere to reduce the possibility of construction error and the studs being installed in the wrong locations.

 

[dik]

Many cold formed steel catalogues/standards have weld values for attaching sheet material. Just a matter of checking the capacity vs. actual shear... you can even increase the average shear by 50% to account for parabolic distribution.

Dik

 

[haynewp]

I don't design the form deck as a diaphragm. I like to use the concrete when it is thick enough, then place shear studs on the beams that make up the lfrs and if there is any other locations I judge need them. These studs get the lateral out of the concrete and into the steel frames, but yes they will also act like any other composite beam shear studs and could shear off under gravity loads only.

It is a safety issue too, you need the deck attached in all areas to function as the form for concrete and walking surface. I have never used anything but puddle welds or shear studs when attaching the deck to the floor steel. If the flanges are not too thick, I guess you could use tek screws but seems like a tough day for whomever is using the drill. For your special situation, I would agree with using shear studs everywhere but you should look to make sure they can take any gravity load induced shear in the locations you also need them for lateral.