Covered Porch Header design 20ft Span 2

[Apexpredetor]

I am trying to size a header for a covered porch but I am having an inner struggle with how to determine the size.

Any span tables for this specific use?

IT is a 20ft clear span 6.5/12 roof pitch, shingled roof. 40-50 GSL. The designer has a double 2x12, but I am a bit hesitant on that being that garage door headers require 3 2x12 at 18ft clear span (As per SYP span tables).

Thanks all.

 

[ztengguy]

Have you checked the load and what the stresses and deflection are? if higher than allowables, might need a different design. I have never seen span tables for this, but could be out there.

 

[XR250]

(2)2x12 won't come close to cutting it. Deflection will likely control any beam you use.

 

[a2mfk]

Is this an engineering question? You say the "designer has (2) 2x12s" , so you are not the design engineer? Pretty simple calculation, no inner struggle necessary...

Watch your wind loading also, in my opinion most porch roofs act as big overhangs (or at least partially as overhangs) and thus you should consider using overhang uplift pressures for at least a portion of the roof.

 

[boo1]

use a Gluelam and have a truss shop size it for you

 

[JAE]

I used to have inner struggles years ago.

 

[Apexpredetor]

Thanks for the responses.

I am not the designer, but I am reviewing the drawings (So I end up the designer in the end when/if I stamp it). The drawings were wrong. IT is not 20ft clear span. more like 6ft supporting 20ft clear span trusses.

I got the same thing, failed in deflection horribly under my first calc, but the drawing was incorrect.

Thanks all.

 

[XR250]

Huh, this is really basic stuff. Are you qualified to be reviewing this (no offense)

 

[ztengguy]

Im lost, you didnt design or draw it, but you are stamping it? Why not make the designer stamp it?

 

[EngineeringEric]

This is probably going to be steel or at least LVL. You are supporting some 400plf LL and 250plf DL (guess, not accurate). I hate to reiterate, but if you are uncomfortable with a design then you should not be performing the design. Get help or do not take the job, I don't do civil cause i am not comfortable with it. If someone else has designed it as a (2) 2x12 I would make sure they are aware of the loading conditions maybe they misread 20 ft span as 2' span... :\

 

[a2mfk]

ZTENG- I bet this is US residential home design. The dirty truth is most homes in the US are "designed" by a CAD tech, who does some prescriptive design using tables and stuff, and generally gets it close to minimum code requirements. There are businesses and websites with thousands of house plans that you can buy and then have "engineered" for your location. In most jurisdictions, you need an architect or engineer to sign and seal the drawings for permit. Many times this is blatant plan stamping, other times the engineer/arch may run a few calcs and actually do some engineering work.

Most people would be shocked if they really knew how this whole system worked and what gets built, since most homes have little actual engineering performed. I have seen some really, really bad "designs" that were obviously plan stamped. During the big real estate building craze in Florida a few years ago there were archs/engineers signing hundreds of drawings a week. You may guess about how much review and design they were doing.

I am not accusing the poster of doing one thing or the other, just saying in my experience that his is how the residential market works. I am a little concerned that the poster is not qualified, since they were just looking for some tables in regards to a very simple problem (one I had in my timber design class in college). That is not engineering, that is prescriptive design. BE CAREFUL!!! You are the engineer of record in the eyes of the law and you cannot blame the "designer", cad tech, or anyone else for anything you are signing and sealing.

 

[msquared48]

This is the danger of "prescriptive" design, and the design of this beam falls outside of this.

Mike McCann
MMC Engineering

 

[Apexpredetor]

Let me reiterate. This is not a "STAMP FOR HIRE" job. I check everything on the plans down the spelling of the door and window schedule! I know that this does go on out there thought. Scares me actually. This client comes to me becasue the last architect he used was exactly that, a stamp for hire that didn't check anything, just stamped it.

I do not "design" the building. I am reviewing the designers plans. In my definition of a designer, they are above drafters, with intimate construction and knowledge and experience with oprescriptive code, but not trained as an engineer. Very typical of residential type construction here. IF it were to be engineered in its entirety at over $100/hr bill rate it would never get built. Most of it is prescriptive code. This span caught my eye due to the long span of 20ft and only 2 2x12 header. I checked it for deflection first (FAILED). I wanted a gut check so I looked for some span tables. Closes span table I could find was for garage door headers.

It ended up being an incorrect dimension. It is more like 6ft span supporing some trusses over a cover entranceway.

Thanks for all of the help and consideration guys. I appreciate the help.