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-   -   How far can I span 2 - 2 x 10's between columns? (http://www.diychatroom.com/f19/how-far-can-i-span-2-2-x-10s-between-columns-183743/)

walt1122 07-17-2013 11:31 AM

How far can I span 2 - 2 x 10's between columns?
 
Hi All, I had a small porch roof built off of the side of the house. It extend out about 5.5 feet and currently has 4 - 4x4 P.T. posts across the the front supporting 2 - 2x10 P.T. syp and the roof framing made up of 2x6 16 O.C. and 5/8? probably 1/2 plywood covered with ribbed metal roofing material. I think it is about an 8 on 12 pitch. I really don't like the two innermost posts and would like to remove them leaving a total span of 20 feet 6 inches. The 2 x 10s are made up of 4 pieces. Each section is made up of two 14 foot and then the 4 foot and 6 inch piece. Section one starts with 14" and ends with 4'6" and the second is the opposite to alternate the splices. Construction adhesive and galvanized 10 penny nails used to put them together. I'm about to cover the 4x4's with 3/4 1x6 P.T. pine and figured I had to ask before I started. My computer is old and camera even older! I have no way of sending pictures. Hope this description is enough or please if you need more info I will try to explain further. I'm thinking this is a push... haven't looked at any charts. Came to the experts first.

thanks

Walt

tony.g 07-17-2013 01:20 PM

In removing the centre two posts, deflection of the main beam will be the critical parameter.
The deflection will partly depend on the maximum load the beam will be supporting (eg load of roof structure plus snow load).
Limits on deflection are usually stipulated in local Codes, but for a porch roof, something like span/150 should be acceptable, or about 1.6" in your case.
Remember also that you will be putting considerable extra load on the remaining two posts. Will they -and the connections to the beam - be up to it?

gregzoll 07-17-2013 02:57 PM

You remove anything from that span, you are either going to be woken up to it crashing down, someone will be on the porch when it collapses, or it will happen while you are gone, then come home to it down on the ground, and most likely the local rescue sqaud checking for injured, along with the building inspector tagging the house, due to the collapse.

walt1122 07-17-2013 03:00 PM

Hi tony.g, yep that is the issue. Not much of a snow load here in Tennessee and the 8 on 12 slope would let any accumulation on the metal roof slide off. So yes I know there are charts that explain how far you can stretch (span) a deck and just wondering if someone is aware of how much deflection if any I'm looking at here. I'm hoping there would be no issue but I don't know?

Does anyone? How can I find out?

thanks

Walt

walt1122 07-17-2013 03:08 PM

Thanks for the input gregzoll. You are assuming the middle posts are an integral part of the structure and I am wondering if that is true. They could easily be cosmetic. I don't know and I am just asking if there is anyone who does! Do I need them or can I, without doing damage or injuring anyone, remove the two middle posts to make the porch look the way I would like to see it.

thanks

Walt

tony.g 07-17-2013 03:18 PM

Walt; there are on-line calculators which help you to compute the maximum deflection of timber beams under uniform loading.

The actual formula for maximum deflection under a uniformly-distributed load is:

(5 x W x L x L x L)
(384 x E x I), inches.

where W = total load supported (lbs);L= span (inches); E = modulus of elasticity (lbs/sq in.) and I = moment of inertia of the cross-section of the beam (in^4).

'E' depends on the species of timber, which you can look up, or assume a low value if you don't know the timber;
'I' can be looked up in tables which give values for different sizes of beams.


But do remember that deflection is only one aspect which should be checked. You also need to check the maximum bending stress in the timber (though that might not be the critical aspect). Similarly, the fixings to the posts will be taking increased load; and the 4x4s themselves will be carrying more. The deflection of the main beam will itself apply a bending moment to the posts.

In short, you might be within the permitted deflection limit, but don't blame me if the whole thing comes crashing down!

walt1122 07-17-2013 03:38 PM

thanks tony.g, I hear ya'. I'm only guessing about all this but it didn't look like too much weight and although the span seemed pretty far I thought I would give it a try and ask. Somehow the contractors were unable to make the columns equidistant or even off-set them from the door at the same distance. Just a visual thing! Just an inch or so but still...Figured if I could get rid of the middle ones then all would be well.

Thanks for the equation and the information.

thanks

Walt

gregzoll 07-17-2013 04:00 PM

Quote:

Originally Posted by walt1122
Thanks for the input gregzoll. You are assuming the middle posts are an integral part of the structure and I am wondering if that is true. They could easily be cosmetic. I don't know and I am just asking if there is anyone who does! Do I need them or can I, without doing damage or injuring anyone, remove the two middle posts to make the porch look the way I would like to see it.

thanks

Walt

They were not placed there for grins. If the architect or builder knew that they were not needed, they would have never placed them there to begin with.

But hey, it is your home, do what you want, but don't come back crying when the porch roof collapses, after you remove those two columns.

Willie T 07-17-2013 04:45 PM

1 Attachment(s)
I take it this is the change you are hoping to make?

Willie T 07-17-2013 04:56 PM

1 Attachment(s)
..... and not this.

tony.g 07-17-2013 05:07 PM

I suspect it is the 2nd sketch:(


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