Decks, Patios, Porches and Docks - can I remove bounce after deck is built??

I am building my own deck for the first time. Deck runs perpendicular to the house 30' and is 14' wide. I used all 2x12 PT for the framing. joists are 16'' on center with blocking half way down the joist. The Decking is 5/4 x 6 boards. The first beam is 15' out and is made from 3 2x12's, the next beam is another 15' and is 2 2x12's. This is per the local building inspector. I just finished the decking and will leave the rails and stairs for next year. I notice some slight bounce in the middle of the joists. Can another beam be added at this point or is that improbable or what else could be done?


John

John,

You should install another beam/posts to get rid of the bounce. Trying to double up the 2x12's is another option and this is probably easier depending on the room you have below it. Sometimes solid blocking may help, staggered between center span.

Personaly the distance between beams is too far even though you are using 2x12's. Addding another 2x12 between existing is another option. Granted, what you have is code approved but I never like to build just to code as it meets minimum requirements but in application, things like "bouncing" is not mentioned in the codes.

Hope this helps!

Fasten a 2x4 flat on the bottom of each joist with 4 inch screws 12 inches OC. You are making a flange to strengthen the existing beam.

Brickeye,


I don't understand what you mean. DO you mean attach the 2x4 to the joist mating the 2" side of the 2x4 to the 2" bottom of the joist so that I really have a 2 X16??

alpine951,

What I think brickeye is suggesting with the 2x4 is to place it on the bottom. Virtually making an "Upside Down Tee" with the 2x4 attached on its wide side to the bottom of the 2x12.

I would opt for the solid blocking staggered, placed in the middle of the span. A 2x4 flat is great for lateral stiffness but not up and down where you are having problems.

Hope this helps!

The 2x4 makes a flange. The 4 inch face goes against the bottom of the joist. A 2x4 on the bottom of a 2x10 this way doubles the joist strength. The same as adding another 2x10. The key is adequate fastening to spread the shear load over the fasteners and not crush the wood. No book handy to re-run the numbers for a 2x12.
If this was indoor and dry you could use 1/8 inch thick 1.5 inch wide steel straps. When a beam deflects the bottom edge gets longer, a curve is always longer than a line. 2x4s are very reluctent to stretch. This is the same reason an I beam is used in structures. The flanges give maximum deflection strength and the web just transfers loads.

The joists allready have blocking between them at the mid point of the joists. I think I understand what you mean about the 2x4. Fastening the 2x4 to the bottom of the joist with adequate screws will help to prevent the flex in the joists.

Hey,


Do you know anything about how cross bracing is attached? See my other post on this. A lot of views but no responses.

alpine951,

The ONLY way to cure the bounce is going to be to add another beam, as Doug suggested. 15' is simply TOO FAR to span with 2X12's!!

When I build a deck, I never span the joists more than 5' -- and I use 2X6 joists!!

You didn't mention the post spacing under your double or triple 2X12 girders. How far did you span THAT?? You may need to add some posts as well. (My 4X6 girders never span more than 6'!)

Engineering is usually cheaper than brute force. Adding a 2x4 flange on a 2x12 joist increass the strength by around 55%. Not as much as doubling up the joists, but probably enough to improve a code compliant but bouncy span.
If you want a more technical explanation the flange increases the polar moment if inertia of the beam. This is the value that is used to compute the strength of a non rectangular beam. The botttom of a beam is in tension, and as the beam flexes must increase in length. Adding stiffness to the bottom edge puts the additional strength in a position to increase beam strength (and hence stiffness) with a minimum of material.

alpine951,

You are still overspanned, even with the 2X4's that brickeyee is suggesting.

A mid-span beam, at 7-1/2' out from the house, is a half dozen pier blocks, some concrete (redi-mix) for the footings, and and 60' of 2X6's (or 30' of 4X6's if you can get them), and your bounce will go away!

alpine 951:
I read this post yesterday and have seen this problem with bounce in the past, so I was quite interested. Yesterday I put a call in to the structural engineer I work with; he was in meetings all day, but I hope to hear from him today. Bear with me. I am really interested to hear if brickeyee's idea will work; if so that would be a real easy fix. Based on the span of the 2x12's, I'm assuming the bounce is truely in the 2x12's and not in the beams that support the joists.

Bruce

There is no bounce at the beams. It is only at the mid point of the joists.

The post spacing for the 6x12 beam is one right smack in the middle and two six feet on either side of the middle post. These posts a re 6x6's. The outside beam is similar spacing with 4x4 posts.

OK, I just got off the phone with the structural engineer.

He said the 2x4 on the bottom of the joist is difficult to quantify. What he recommended is to try it on 4 or 5 joists and see if it works; it will help, but difficult to tell how much. He recommended screwing at minimum 12" o.c. AND glueing with a good construction adhesive like PL400. Be sure to give the glue time to set up before trying the bounce test.

The other alternative he had (other than another beam line at mid-span) is to sister up a 2x10 next to each joist and nail it to the 2x12's, 3 nails @ 16" o.c. along the entire length so they work together.

Bruce

These are simple beam calculations to perform. The only thing that is different is that wood is anisotropic, and has different ratings for flexural modulus and crush modulus that must be accounted for. PL400 is not a structural rated glue and is not designed for structural loads. Using it adds something, but how much is a real unknown.
Wood has a great advantage over steel in fastener use. In a multifastener steel joint only a few fasteners bear the load unless the fasteners have an interferance fit in the holes. When the bolts deform under load to more evenly share loading they loose some strength. In wood the material is soft enough to crush slightly to evenly share loading without loosing appreciable strength. Fasten with 4 inch screws 12 in OC. The loads on the screws can be computed the same way as the welds for a flange on a built I beam.
I have blocked a number of decks and houses this way using both wood and steel strapping to stiffen joists. Most commonly to allow the installation of tile floors with larger tiles without bulding the deck up excessively high. Steel straps and the floor becomes very solid. I have only measured deflection once, but it was around 1/1800 of span on a 2x10 floor with steel strapping. A very stiff floor.
Nobody makes an I beam thicker in the web to strengthen it. The strength is increased by a larger flange cross section. Put the material where it will do the most good.

Hey brickeye: One of my passions is collecting old manuals and guides related to construction; and FYI that technique you've described - using 2X4s, appears in three old manuals I have just dug up and re-read after seeing this thread the other day. The first one, which was put out by the United Brotherhood of Carpenters and Joiners is titled "Journeyman's Guide To Scaffolding" and shows how large area platforms for scaffolds may be built for extra load and without bounce and without dangerous vibration using your method. The second is a US Navy SeaBee guide for building temporary piers, stairs, and etc. Again it suggests your method, especially when larger dimension lumber is unavailable or where wood is scarce. The third, an Army Enginering Manual suggests your method for quickly building temporary bridges where the cadence of marching troops can cause bounce and vibration. Each manual states that strength is increased; the Army issue says strength is dependent on the length of the span. All I can say is brickeye ... You da man!!

I had a little problem in a customer's house who wanted those large ceramic tiles. She begged and begged for a way to stiffen the floor. I looked at the basement with a ton of wiring, plumbing, and duct work and no room for sistering. Talked with my mechanical engineer, put up the steel straps. Sistered with a 2x2 (had to leave a few gaps) to create an edge for drywall. Floor like concrete. I did wear out a brand new electric screwdriver putting in 3 inch screws. The steel strap is around a 14x16 inch timber on the bottom of the joist.