Lateral bracing for metal deck

[Buleeek]

Hi everyone,

I'm about to design a metal deck over existing building. It will be a deck with 4" concrete over metal deck over steel joists. The joist will rest on beams and columns. The whole structure will not be attached to the existing building. Live load 100 psf. See attached sketch.

I have a few questions regarding the design:

1. Bracing. I do not have any problem to install vertical X-bracing between the supporting columns, however I do not have a possibility to install anything in direction parallel to the joists (due to existing building). What would you recommend to do in such situation?

2. Bracing. Can horizontal bracing located right under the deck joists (running diagonally from one opposite corner top of column to another) be considered as a proper lateral bracing?

3. What would you suggest to assume for the lateral load caused by "people"? There are wind and seismic, but no information about lateral load caused by walking/partying people.

All extra advice or recommendations on designing of such decks will be greatly appreciated.

Thanks,
Buleeek

 

[phamENG]

1) Braces in one direction, moment frames in the other.

2) You have metal deck topped with concrete. Use it as a diaphragm and avoid horizontal bracing.

3) What is the use going to be? That will help guide your decision. Do some searching here as it's come up quite a bit. There's a paper out there (and a Structure Magazine article based on it) that looks at lateral loading on and lateral load resistance of backyard wood decks that may be helpful. The resistance part not so much, but the loading can be. They did several tests that included synchronous jumping, and they found that a negative feedback loop developed and limited the lateral live load to just over 25% of the live load. Vibrations and sway sync'ed up with the occupants and created a very uncomfortable situation and they all slowed and stopped in response. It came out to be 12psf lateral live load for a 40psf vertical live load. The only codified lateral live load that I'm aware of in the US is for stadiums and bleachers. They require 24plf parallel to the rows of seats and 10plf perpendicular to the seat rows. If you figure each seat and the space in front is usually 2 to 2.5 feet, that works out to be close to the 12psf stated above. If you're not building an elevated workout platform for a platoon of soldiers, or a viewing stand for a sports arena, you may be able to back of that some. For something of this size, general stability may cover it. If you're going to have lots of people moving in different directions, the forces tend to cancel one another out and the net effect on the overall structure is very small.

One thing I will say is that the "no room between wall and column" may not be a good idea. The frame will move. If you put them tight together it will press on the building below and could cause damage. If you have to consider seismic loading, look closely at required seismic separation.

 

[Buleeek]

phamEng,

1. Can you explain "moment frames in the other (direction)" ? I only have beams and columns on each end of metal joists.

2. What will make this deck to work as a diaphragm ? Is it concrete thickness, welded metal deck to joists or studs ?

3. The live load will be 100 psf. It is going to be a roof top bar with tiki bar and tables. 20% of 100 psf = 20 psf which is a lot for columns to carry/resist especially with no bracing in that direction.

Thanks,

 

[XR250]

Are you actually spanning 64 ft with your bar joists?

...and your columns are 15' tall?

 

[Buleeek]

Unfortunately there is no other way. Any advice is appreciated.

 

[XR250]

Walk away?

Not sure you are going to be able to get a bar joist to span that far with that kind of live and dead load...
What are you planning on using for joists?

 

[Buleeek]

Spacing and depth to be designed by the joist manufacturer. LH - series should do just fine.

 

[phamENG]

XR - you can get them. They'll be a minimum of 32" deep, but they can do it. Buleeek - make sure you do a vibration analysis on this floor.

To your last set of questions:

1) Ah. I see. At each end, use a steel beam in lieu of a joist and design a moment connection, or work with the joist manufacturer to provide a joist that can be fixed to each column.

2) No, concrete isn't the diaphragm, though it will serve to stiffen the diaphragm and increase the capacity. You need a copy of the Diaphragm Design Manual from SDI - DDM04. Link

3) For this application I wouldn't be too concerned. Wind and seismic will likely control. Maybe do a 5psf lateral live load if it makes you feel better. If you're concerned they might use it for synchronized dancing in the future, check it for a 12psf ultimate load to make sure it won't collapse, but don't consider it for serviceability.

 

[XR250]

Spacing and depth to be designed by the joist manufacturer. LH - series should do just fine.

Have you checked the tables to get a handle on the depth and spacing before you delegate the design?

 

[phamENG]

Spacing and depth to be designed by the joist manufacturer.

Why? If you're using standard joists, you should be picking them from the table. As I mentioned above, you need to do a vibration analysis on this floor. To do that, you need to know something about the framing - like the spacing and stiffness. This is also being installed over an existing building. I'm guessing there's an architect that has set a finished floor elevation and detailed stairs to access it? What happens if they assumed a 28" deep joist, but the joist manufacturer comes back and says a 48" is the most efficient. Your deck just had to move up 2 feet and your architect's stair layout is shot...along with lots of other details...not least of which your braced frame design.

 

[Aesur]

With typical joist spacing you will be at least 48" deep for the joists, most likely deeper. When checking vibration there are many contributing factors, one of which is joist spacing. Too close together, and they are horrible for vibration, too far part and the joist are too deep and/or require a deeper concrete over steel deck. Like phamENG said, you will need to analyze the joists to properly design and optimize this structure. Additionally, given your height of 15', joist deflection should also be considered, wouldn't want the joist to deflect such that it is sitting on the other structure.

Do you have a plan view you could share, even a sketch showing the columns in relation to the existing structure? How tall is the existing structure? What seismic design category is the structure located in, can cantilever columns be utilized? There are a few ways to frame this structure laterally, a better picture of the project is needed to offer better advice than what has already been offered here.