Flag Pole Design for Seismic

[dursunlutfu]

Hi Folks,

I am working on a residential project where I need to design the wood deck also. The deck will be connected to the existing house ( not cantilevered )from one side and the other side will be supported by columns for both lateral and gravity. I will design the columns as flag poles- cantilevered/fixed at the bottom 3" dia. std pipe cols. I am looking for a reference. Has anyone of you folks come across with any reference ?

Thank you for your time.

 

[BridgeSmith]

Forgive my unfamiliarity with residential deck design, but why would you design the columns to take lateral load, rather than laterally bracing the deck and designing the connection to the house for the lateral loads?

 

[dursunlutfu]

The client does not want to have a brace below because there is a floor below. @HotRod10

 

[BridgeSmith]

Maye I'm not understanding the situation, or possibly you're not understanding what I proposed. I was envisioning diagonal braces attached across the underside of the deck, attached to the bottom edge of the joists, that would keep the deck from racking. Would that be aesthetically unacceptable?

 

[dursunlutfu]

@HotRod10 Thanks! Yes, I thought you meant cross braces between columns. I have never done that before but it sounds logical. I need to think about it and try to find if there are any references about it.

 

[BridgeSmith]

The only tricky parts as I see it is providing enough shear capacity with the screws where the diagonals are connected to the joists and rim board, because of limited thickness, and providing a connection between the joists and ledger with sufficient tension capacity.

The second issue could possibly be solved by using a wide deck board at the wall screwed to the ledger and the joists, or using angles to attach the joists to the ledger. I just don't see typical joist hangers providing enough capacity.

 

[dhengr]

Dursunlutfu:
HotRod10's cross braces in wood or as HDG stl. straps will work just fine, assuming normally proportioned decks, except their flat surfaces hold water for some time. I’ve also used light cable with appropriate fittings and turnbuckles. You really shouldn’t need some grand reference for this, it is pretty basic statics and engineering judgement and design. Take a look at some Wood Design Texts, at the NDS and IBC or IRC. AFPA has a design guide for decks, #6, “Prescriptive Residential Wood Deck Construction Guide” and USP/MiTek or Simpson have some hardware and details for some of this. Then, you take the gravity loads into the ext. wall of the bldg. through joist hangers and a ledger properly bolted to the rim jst. and you take the lateral loads through the rim jst. and several joist bays into the int. floor diaphragm, as required by the loads. The rim jst. should be properly/adequately nailed to the sub-flooring along the diaphragm edge, and some blocking perpendicular to the int. joists should also be used to get the concentrated lateral load reactions into the int. fl. diaphragm. The floor jsts. in this region should have some extra nailing too. These lateral reactions should not be bolted just to the rim jst., that is generally not adequate. The ledger should be thru bolted to solid ext. wall sheathing and the rim jst., not thru insul. board or siding. And, this entire ledger pocket should be very well flashed, behind and under the ledger, and the also over the top of the ledger and deck, to keep water out of the wall.

Edit: The flashing behind and under the ledger, that is, the flashing of the ledger pocket, should also include end dam flashing to direct water out of the pocket at the ends. Many of these quality details and proper nailing, blocking, connections, etc. are fairly easy to do on the original design and construction, and almost impossible to fix after the fact, or on a deck installed at a later date.

 

[BridgeSmith]

dhengr, thank you for correcting and 'fleshing out' the concept I proposed. I am, after all, a bridge design guy by trade, so out of my element discussing a residential deck.

Although I've done several DIY projects on my own house, my exposure to residential construction techniques (and terminology) is obviously limited. If you wouldn't mind, would you catch me up on the terminology a bit? I think I get from your response, the rim joist attaches directly to the house framing and the ledger is 'sistered' to it, is that right? Then you have joists running perpendicular to those 2. Is there a more specific name for those? What would you call the beam at the outer edge that supports the other end of those joists? (That's what I was calling the rim joist, but that was obviously not correct)

 

[dhengr]

HotRod10:
The rim jsts. frame the edge of the fl. framing and fl. diaphragm, the plywd. or osb fl. sheathing. They sit on top of the foundation sill pl. or on top of the double top pl. on a stud framed wall, and distribute wall loads from above continuously. They may be parallel to the regular fl. jsts., just another jst. of the same height; or they may be perpendicular to the regular fl. jsts., in which case they are side nailed thru and into the ends of the regular jsts. The IRC has some prescriptive details and nailing reqm’ts. for this framing, and production framers or DIY’ers. can be kinda lazy about doing this properly all the time. This min. prescriptive framing/nailing is usually not enough to properly take the concentrated loads from a deck. You can pull the rim jst. right out of the bldg. if the deck is ever fully loaded laterally. The ledger is either sistered directly to the rim jst. or more commonly to the wall sheathing plywd./osb on the outside of the wall and rim jsts. The ledger should not be applied, over insulation board or siding which are too compressible and cause too much cantilevering of the ledger bolts, which are not sized for this canti. bending. The entire connection is ill suited for the canti. bolt and compressible action. The bolts basically canti. from the face of the rim board/jst. The outer deck beam is just that, ‘the outer deck beam,’ and the deck jsts. may frame into its side or canti. a couple feet beyond/over the outer deck beam. The deck jsts. and outer beam should preferably be covered with a self healing flashing tape so they don’t suck and hold water under the decking or btwn. the beam plys.

Obviously, a deck which is 15’ wide, along the face of the bldg. and hangs out 30’ perpendicular to the bldg. wall, is quite a different animal than a deck which is 30’ wide, along the face of the bldg. and hangs out 12-15’ perpendicular to the bldg. wall, as relates to our current framing scheme. The deck corner forces, or our diag. bracing forces, must take the deck lateral loads well into the interior fl. diaphragm system to support the deck properly. If the interior fl. jsts. are perpendicular to the deck ledger, you can usually increase their nailing to the fl. sheathing and fix your corner hardware to those jsts. to get the deck loads up into the fl. diaph. system. When the interior jsts. are parallel to the deck ledger you have to put blocking btwn. the int. jsts., and into the fl. system several jst. bays and then nail this blocking to the fl. sheathing to transfer the deck loads from the corner hardware.

 

[BridgeSmith]

Thanks dhengr. Your explanation is very helpful and all makes sense.

 

[dursunlutfu]

Thank you both !