Decks, Patios, Porches and Docks - How to stop sway in second story deck

We recently had a second story 30' X 15' deck built. We notiiced that it sways quite a bit, and feel it should be reinforced somehow. What is the best way to take this sway out? Any suggestions would be appreciated as I want to have a little more info. before talking to the builder.

Thanks, Kieron

You will need some sort of diagonal bracing. The type and location will depend on the height anf size of the deck.

Dick

"Second story deck" -- How tall are the posts? How BIG are the posts? Are they cross braced? Is the deck attached to the house or free-standing?

It is a 30'X15' redwood deck attached to the house, and spans over the driveway (two car) and front walkway. 6 4x4's and I believe they are 12'. Each support beam has a 2' fork at the top. The 2 center supports are skewed 5' to the left of center. I hope this information makes sense. Thanks for your help.
Kieron

I saw a number of decks similar to yours braced to prevent sidesway. They had the long side parallel to the house.

Because they did not want to block or create any obstructions on the long side, they used horizontal diagonal bracing attached to the bottom of the joists.

I haven't done any engineering in California for quite a while so I can't speak to the details and availability of seismic hardware and the current code rquirements, although I am assuming yours was built to code. - I saw one bracing system in Brazil that used steel strapping and seemed to work well.

You apparently have a horizontal problem and not a load bearing problem.

Just a thought to keep in mind when you talk to your contractor.

Dick

Six posts under a 15 X 30 deck. I'm assuming that there are 3 posts about 7' to 7-1/2' out from the house, and the other 3 are 14' to 15' from the house. That's fine, as long as the joists are 2X8 or larger.

But your problem is the post spacing (and support) in the other direction. The center and outer beam span 8' to 10' on one side and 18' to 20' on the other side of the center posts? What was used for the beams?? Was this deck built with a permit?

The only thing that will reduce the sway and allow you to keep the driveway usable will be to use steel 'els' attached to the 4 outside posts and the beams that rest on them, and steel 'tees' at the 2 center posts and the beams that sit on them. These 'els' and 'tees' need legs that are 3' to 4' long, and thru-bolt them to the posts and beams. Don't use lag bolts.

If any deck is braced to act as a diaphragm and is attached to the house, it cannot sway or rack irregardless of the column spacing or attangement.

This is basic and simple structural mechanics, just like wind bracing. The principal is used frequently in both roofs and walls in many structures, especially those subjected to laterial loads and seismic forces.

Longer columns (steel or wood) provide little lateral reisistance if they are attached to the deck with a non-moment transfering connection. That is the reason for the knee braces you see.

You can reduce the deflection of the post (and reduce the deck racking) somewhat with stiffer materials like steel, but the connections to the deck should be improved if you want to get maximum benefit.

Dick

Thanks for all the suggestions, I would like to take care of this asap. Also, one more question. Do you think that this is something the contractor should be responsible for, because it seems like he should already know these things. Even more so, given the area we live in and the earthquake factor, which you mentioned? Just an afterthought.

The sway is all about the lack of proper bracing. That's one issue. I have suggested some possible solutions.

Then I questioned the spans of the girders. That has nothing to do with the sway that you are experiencing. But there is NO dimensional lumber that is going to span 20' -- if I'm correctly following what you posted.

BOTH issues are on the builder.

I'm with Dick (Concretemasonry) on this. Install a treated 2x4 nailed (flat) to the underside of the joists running diagonally from ledger (at house) to beam. This method is far superior to ANY cross or 'X' bracing of the posts! Ideally the 2x4 brace should be fastened directly to the ledger and beam and to EACH joist. Solid blocking between the joists at the bearing (beams) will further strengthen and stiffen the deck.

The bracing that senor-mouse is suggesting will take the sway, or almost all of it, out of the deck. But I am still concerned about what was used for beams and how far they span.