Replacing an Enclosed Back Porch Foundation

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I have an enclosed back porch with a concrete block foundation that must be replaced. It`s foundation is separate from the main home`s foundation as it ajoins at the ends where the main foundation exists and accessed with a door cut out from main foundation.

The porch is 8 feet wide by 18 feet long, single story with pitched roof,asphalt shingles containing nothing more than a deep freezer, closet spaces.

If I assume the structure`s weight as 50 lbs per square feet (?) what size steel I Beam is safely adequate to use to support the weight of the porch while I remove the blocks and install solid form concrete walls? W6X20, W8X28?
 
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333: It sounds like a span length of 5.5 m is too long for even a W8 x 28, because it sounds like you do not have lateral bracing near the compression flange at midspan. Therefore, the compression flange could possibly slip laterally, and be unable to carry the load. I do not see an easy way for you to provide lateral support at midspan. Maybe you have ideas. Therefore, could you instead provide a vertical support at midspan? This would reduce your span length to 2.75 m. You can indeed have a laterally-unbraced span length of 5.5 m, if you wish, but it would require a larger beam size (expensive). Let us know.
 
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