kids' Wooden Fort Shakes!

[littlehomer]

Hello,

I built a swingset with a Fort and I am (begging) asking for some help.

I built a 6 foott by 4 foot Fort using 4-4x4, 12 foot long PT wood. I cemented about 1.5 feet and the rest is obove ground.
The wooden floor is about 4 feet from the ground. I used 4 - 4x4 PT wood for the Floor.

On the left side I have attached an A-Frame for 2 swings using a 10 foot 4x6 beam. The A-Frame is also cemented.

Problem:
When my kids use the swings the entire Fort shakes. It shakes more when they use the swing closest to the Fort.

At the top of the Fort I have screwed and connected all 4 4x4 post with 2x4 wood.

What else can I do to prevent the Fort from shaking?

Please help.

Thank you.

TonyB.

[wizendwizard]

I would really suggest going deeper that 1.5 feet with the support post. And hopefully you used a wide base when pouring the concrete to hold them in place. I suggest on a 4x4 at least 80lbs per hole.

[mrcaptainbob]

Cross brace them. Fasten a 2x4 from the high point of one 4x4 to the low point of another. A giant X on each side will greatly stiffen it.

[littlehomer]

Wizenwizard I'm past that point. I should have a wide enough base to prevent it from tipping....just not enough support to prevent it from shaking

Mr. captainbob...So you suggest creating an X on all 4 sides?

Do you think It would also help If I created a 5th X on top of the 4 posts? It should create diagonal support..right?

The Fort will look a bit Tacky once I screw X's on all sides....Oh....well better than having it shake.

TonyB.

[HayZee518]

What does the shaking show? something is loose. wooden construction moves all the time. sometimes you hear your rafters creaking - right? this is caused by thermal expansion and contraction. wood stuff gives a little. build two uprights and put in a cross member. now move it. it moves. half way down this span, put another cross member. now move it again - it moves. now put in two cross braces. one corner of one side to the bottom of the other side - now try moving it - doesn't want to move but still gives a little - right? same concept of your project plus you are putting extra weight at the top of the structure which offsets its center of gravity. under neath your deck, bolt in some steel angle and see what happens. makes it stiffer.

[littlehomer]

Thanks for the reply. I have a more of a technical question.
The plan is to screw X's on all four sides. I believe this is what you mean by "cross member".

Now, If I can have you imagine that one 2x4 cross member will be installed diagonally. The other will be overlapped either in front or behind of the first one in the opposite direction.

My question is, can I place one member diagonally, then butt the other member against the first 2x4. It will still create an X using 3 seperate pieces of 2x4. One long one, and 2 half sizes. This is to avoid overlaping.

The alternative is to measure 2-2x4 long pieces of wood and cross them. This would cause an overlap right at the center of the X, creating a 3" depth rather than a 1.5" depth.

Thank you and any response is appreciated.

TonyB.

Originally Posted by HayZee518

What does the shaking show? something is loose. wooden construction moves all the time. sometimes you hear your rafters creaking - right? this is caused by thermal expansion and contraction. wood stuff gives a little. build two uprights and put in a cross member. now move it. it moves. half way down this span, put another cross member. now move it again - it moves. now put in two cross braces. one corner of one side to the bottom of the other side - now try moving it - doesn't want to move but still gives a little - right? same concept of your project plus you are putting extra weight at the top of the structure which offsets its center of gravity. under neath your deck, bolt in some steel angle and see what happens. makes it stiffer.

[HayZee518]

ok, in answer to your question about cross bracing. one continuous piece end to end will transfer weight from one vertical top to the other vertical bottom. another continuous piece the other vertical to the opposite bottom will transfer the same weight but by onehalf the wiehgt because of the opposition force of the other 2X4. if they were split and butted up against the cross center of the long one the other's weight would be transferred to the midpoint of the long cross brace NOT to the bottom of the other vertical. same as if you continued this butted piece with anothe diagonal. two seperate cross braces not cut will do the job. if you want, bolt them together in the center too!

[mrcaptainbob]

Hayzee...would it not do just as well if only a single 2 x 4 were used diagonally on a side? It can hold compression and tension and do the duty of two. One on each side I think would secure it quite well. What's your opinion on this? Also, corner bracing placed at the 20% mark from the top of the vertical may also do just as well. Can't imagine a 4x4 would take an 'S' shape of any magnitude to be noticeable on something like this. Would certainly be more aesthetic.

[HayZee518]

I agree with you on the one piece cross brace. a span of 4 feet isn't THAT much. as far as gussets at the corners - eh..... wouldn't matter that much. even if the whole thing was welded steel it would give a little anyways.

[littlehomer]

Thank you all for your assistance.

I think I have all the information necessary to finish my project. Your advice and suggestions has been extremely valuable.

Littlehomer.