Am I reading Code Incorrectly?

[Apexpredetor]

Good Evening All,

Quick question about Residential Code, Section %502.5(2) - Girder Spans and Header Spans For Interior Bearing Walls... Since I have a good depression on the side of my house I was thinking about making a garage under a prefab cape code type addition...

I am looking at some modular houses plans (continuing my escapade with home addition/garage). I keep seeing girder spans utilizing (4) 2x10's with spans over what, I interpret, is the maximum span of 6'-2" (supporting two floors, 28ft width).

I hope some of you are familiar with the trusses that come with modular construction? They are bolted together at the midspan. Maybe I am incorrect in assuming that these set-ups are supporting two floors since there is a pre-engineered roof truss supporting the second floor (Cape Code type modular addition) as clear span?

 

[EngineeringEric]

You lost me on the description... maybe to early today. do you have a section by chance?

the 28ft width probably isn't the actual joist bearing since that is a long span and would not be doable with non-engineered floor joists.

 

[TXStructural]

You will need an engineered solution. You cannot span these distances with dimension lumber. The issue is primarily how the loads get to the ground, and what you describe is not workable. Assuming the addition has not been constructed, you could have it designed to span the garage, or potentially you could have supports between lanes in the garage.

As a reference, Texas requires any members spanning greater than 24 feet to be engineered, and I am sure your state is similar. Spans of 28 feet can be done with plate-connected wood trusses about 24 inch deep. This gives you the added advantage of giving you space within the structure to run drain lines, ducts and electrical.

If I were building this, I might consider a concrete flat plate/slab over the garage for fire resistance (and because I am partial to concrete structures, being in tornado alley.) Another option would be steel trusses, either hot rolled or cold-formed steel. These have an added advantage of being pre-assembled and light enough to be handled by a couple of guys.

 

[nac521]

Without seeing the modular plans you are describing it is difficult to comment on the disparities between the modular plan girder layout and the tables in the IRC. Keep in mind the modular design is an engineered solution and won't exactly match the prescriptive IRC tables.

 

[woodman88]

The 28' (in the table for Girder Spans and Header Spans For Interior Bearing Walls) is the building width, not the joist spans. Your floor joists could 10' one side and 18' the other side of the 4-2x10 header for a 28' building width.

Garth Dreger PE - AZ Phoenix area
As EOR's we should take the responsibility to design our structures to support the components we allow in our design per that industry standards.