Message - Dug-out roof design

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Posted by  Arthur on December 10, 2003 at 13:55:38:

In Reply to:  I-beam atttached concrete blocks need ideal posted by Dave on December 06, 2003 at 06:53:30:


I am a structural engineer in California and, although not licensed in the state of Tennessee, I am licensed in six other states and I am willing to give you a hand with your dug-out roof for free. I have a three year old who will some day play baseball, and I figure that what goes around comes around. I assume that the dugout is 30 feet long and 12 feet wide and that the walls are concrete block, likely 8" wide block. If you only want one beam across the 30' open side of the dug-out then you need a roofing material that will span 12 feet. The simplist way to do this is to use corrogated metal deck. A 3" deep metal deck section will span 12 feet. This deck is typically specified as a "N" deck and typically comes in 24" wide sections. You will need a minimum of 20 Gage deck. I calculated out a W12x26 beam for the 30 foot span. Set this beam on the block one course down from the top of the wall and center it 12 inches in from the end of the 12 foot long wall (this should be the second block cell in from the end of the 12 foot long wall. The beam will stick up above the top of the wall approximately 4". The deck should span from the beam to the back (30 foot) wall of the dug-out and should have 4" of pitch to allow the roof to drain. The simplist way to attach the metal deck would be to attach a 2X6 wood nailer to the top of the beam with threaded studs and to the top of the back wall with anchor bolts. The metal deck can then be screwed directly to these wood nailers. I would attach it at some increment of 8" and align the bolts/threaded studs to fall within the "up" flute of the metal deck. 24" on center would be close enough. The beam should be centered on the second block cell in from the end of the wall and should have some means of anchorage to the wall. I would suggest that a 5/8" x 12" anchor bolt be placed in the wall to attach the beam (through a hole in the bottom flange of the beam. The hole in the bottom flange of the beam may be oversized if a 1 1/2" square x 1/4" thick plate washer is provided. Typically I request that this plate washer be tack welded to the beam flange after the beam is secured to the anchor bolt. The block wall should be reinforced and fully grouted below the beam. I would suggest that the first three cells from the end of the wall (one either side of the beam bearing) be grouted and reinforced with a full height #4 rebar. This assumes that the wall is not much more than 8 feet tall at the beam bearing. The remainder of the wall should have #4 vertical rebar in grouted cells at a minimum of 32"o.c. and horizontal dur-o-wall reinforcing at 16"o.c. in the bed joints.

There you go. If you would like, I would be willing to go over any of this with you or your steel supplier as needed. There are other ways to frame the roof out of wood, but you are already working with the steel guy and steel deck is the easiest solution. Some of this is a little conservative, but hey, its off of the top of my head and its free. I figured a 40# live load on the roof to account for people walking on it or storing things on it. (I've seen both.)

Play Ball! (Just had to say that)

Give me a call or drop an e-mail with any questions.

Arthur Fellows, S.E.
Fellows Structural Engineering
Tahoe City, California

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