The house I own was built in 1920 and is missing a five-foot section of the main girder in the basement. The first floor joists rest atop these two pieces. Posts prop up the two sections of girder but there has been unwanted movement in the both sections that is affecting the upper super structure. The girder was originally 20 feet long before it was cut. The wooden girder is about 9 inches square and is made up of three 3 x 10’s nailed together.

I want to replace the damaged wooden girder with a steel I beam and use the basement foundation to support the ends of the I-beam. Based on the original wooden girder what size I beam would I need to install? Can the steel beam be installed in two sections and bolted together via sister plates without having to add support to the middle of the beam?

Toolmonger

You have a few concerns to think about. Many folks think that a steel girder will be smaller then the wood it replaces. In many cases that is not correct. The Steel often ends up being much larger. Also when removing the center columns which carry some load, you now place the entire load on the edge of the foundation. Which in most cases may not be designed to hold the additional weight. Most likely you will need to dig next to the foundation and increase the footing size to carry the additional weight you will pose on it once the center columns are removed. Also you should consider using a microlam beam to carry the load. You will find that it will be smaller in size to carry the same amount of load then the steel beam could with similar sized dimensions.
I strongly suggest that you consult with a engineer to size this as in many cases the original beam may also be undersized and simply sizing a beam because of a pre-existing beam size is asking for trouble.

Good Luck