Building Homes or Extensions/Extension of concrete beam

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Expert: - 2/18/2011


Question

Drawing
Hi Bruce, I have an existing car porch about 8 feet wide and is supported by 2 concrete pillars. One beam connects the 2 pillars and another connect the pillar to the wall.
I want to widen my porch and at the same time, intend to remove the middle pillar so that I can have an open space of about 20 feet wide.
My contractor plans to hack part of the concrete beam supported by the 2 existing pillars, attach a new rebar to the existing rebar in the existing beam (about 3 feet in, add a new pillar to support the extension beam, and also add another reinforced concrete beam from the new pillar to the wall. Upon extending the beam, the contractor would then remove the existing middle pillar.
I just want to know if this is the proper method. I worry if the new structure would crack in the middle causing the beam to collapse.  Please let me know if you require additional information. Thank you. Sorry for the very bad drawing. I attached a gif file, but if you cannot view, then pls try opening with windows picture & fax viewer.

Answer
Hi Maziah, I suppose it can be done but I am a little leery of this attempt to extend the beam for a couple of reasons:  First of all it's only an 8 foot beam to begin with.  A typical concrete beam to span 8 feet doesn't contain the same dimensions or reinforcing steel a beam that spans 20 feet will require.  So by just tying into the existing beam you are adding more stress to the old beam than it was designed for.  Secondly, if the contractor is going to remove 3 feet of the 8 foot beam to tie into it.  Why not just take the remaining five feet of old beam out of there completely and replace it with a new, fully functioning, properly designed concrete beam?  If the idea is to keep the middle (at present the outside side beam) beam to support the roof so that you can tie into the roof without redesigning or removing the old roof structure then I would keep that beam in place and dowel into that beam enough to tie the new steel into a new beam across the front.  Your contractor is creating a pivot point in his design that will be potentially ineffectual in holding up the added roof weight and now the center weight of the old side beam hanging on the middle of it as well.  I think you need to have a local engineer review this design before proceeding any further.  Look up a qualified PE in your area that specializes in structural concrete and have him design a proper fix for your needs.  I hope this information helps please feel free to write again regarding this or other matters, sincerely bruce e johnson..bejohnsonconsulting.com
    Questioner's Rating
    Rating(1-10)Knowledgeability = 10Clarity of Response = 10Politeness = 10
    Commentthank you so much bruce. your advice is very useful.


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Bruce E. Johnson

Expertise

I can answer any construction related question in regards to carpentry, concrete, drywall, masonry, structural elements of any type of building, residential or commercial. Interior or exterior.

Experience

Custom Commercial and residential buildings. Churches, theaters, schools and auditoriums. Most recently I am working with the Catholic Church on several design build committees. I have a website related to scheduling and project supervision. Although my expertise is more related to multimillion dollar commercial, educational and theatrical projects my generous credentials in residential and remodelling construction make me a viable source of information regarding all forms of building questions.

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