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ANSWERED on Wed 19 May 2010 - 2:48 pm UTC by redhoss

Question: Can my porch frame support a concrete deck

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 18 May 2010 18:21 UTCTue 18 May 2010 - 6:21 pm UTC 


I would like to know if the steel frame that I have built is strong enough to support a 4” poured concrete deck. 

I have a poured concrete walkout basement that measures 30’ x 40’.  The wall thickness is 8” and the height is 10’. 

I have constructed a steel frame for the first floor patio (over the walkout side of the basement).  The first floor patio will measure 10’ x 48’.  The frame is constructed with w10x12 wide flange beams welded to steel posts made from 4”x4”x.250 square tubing.  A post exists every 16’.   Floor joists are constructed from 6“ c-purlin welded into receiver channel  on 12” centers.  The joists bear on top of the wf beams and the basement wall.  I was originally planning regular 2x decking but would now like to have a 4” poured concrete deck.  This would include laying some corrugated metal and pouring on top.  I am curious to know if this structure is strong enough to support the concrete plus a standard load.  Or should I choose another decking?

Best Regards,





 19 May 2010 14:48 UTCWed 19 May 2010 - 2:48 pm UTC 

Hello Pete, I understand your problem. You don't exactly specify the 6 inch c-purlin used. I tried a 6x2.9 (6x2.75x0.06) and it works perfect for your original design as does the W10x12. Using the additional 52psf load for the concrete, the W10x12 still works. However, the purlins I assumed will not work. Do you have the exact specs for your purlins. It is possible that they are much heavier than what I used. Let me know and we can look at it again.




 20 May 2010 14:19 UTCThu 20 May 2010 - 2:19 pm UTC 

Hello Redhoss

Thanks for looking at this.  The purlins are standard 6x2 made from 14 gage steel.  Based upon your calculations, how far off are these purlins?

My options are to find a lighter concrete or split the 10' span with more beams.  I would prefer to use lighter concrete. Or are the joists so far off that more beams would be required?

Thanks again




 20 May 2010 15:50 UTCThu 20 May 2010 - 3:50 pm UTC 

I have some great news for you. I usually check my hand calcs with a beam program. I was in a hurry to go mushroom hunting yesterday and I didn't check. I made a mistake and calculated a required Ix of 11 in^4. I just found my error and the Ix needs to be at least 2.18 in^4. Your existing 6x2x14ga purlins have an Ix of around 4 in^4. Sorry for putting you in stress overnight. You can pour your 4" concrete on your existing structure. Be certain that you connect the bottom flanges of the cee purlins with at least two runs of light angle (you probably were going to do this anyway). Please ask for a clarification if you have any questions. Sorry again for my error.




 20 May 2010 17:14 UTCThu 20 May 2010 - 5:14 pm UTC 

Wow.. Thanks for double checking.  I was already starting to plan more weekend welding projects.

Great service by the way!


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