Cantilever Floor Induced Load Path Concerns

IBC Section 1604.9 requires structural members, systems, components and cladding be designed to resist forces due to earthquakes and wind, with consideration of overturning, sliding and uplift. It also states that a continuous load path be provided for transmitting these forces to the foundation. Seems obvious to engineers that a continuous load path is needed, but it’s still nice to have the code say so.

But what happens if your structure’s upper and lower story walls do not stack? How do you create the required continuous load path? As engineers, we try to steer the architect towards eliminating the offset, making things line up, and keeping construction simple. But architectural requirements cannot always accommodate simple, and non-stacking walls occur all the time.

Specifically, let’s consider an exterior wall where these offsets are usually due to architectural set-backs or cantilever conditions. Small offsets occur quite often in residential construction in areas that use a brick façade at the lower level, and siding or stucco is used at the upper level. Even this small – typically 5 ½” – offset can create havoc in shear and uplift load path connections. See below.

 

 For a stacked wall condition, the uplift load path connection from the upper level to the lower may be as simple as a single strap connecting stud to stud. Add in a small cantilever, and the uplift load path requires four to five different connections to create to transfer the same uplift load. A longer cantilever of several feet would require a joist design considering both bending and shear induced into your floor system from the uplift as well.

Shear load will obviously need to be transferred from floor to floor as well, and again more steps are required to create this load path in a cantilevered floor system. See below for an example of shear and uplift load path connections with a cantilevered i-joist floor system.

The detailing of this load path also changes based on what type of floor system you have (floor truss, I-joists, solid-sawn joists). Check out the Simpson Strong-Tie Connection Solutions for Cantilevered Floors technical bulletin for more information on connection solutions for cantilevered floor conditions.

How have you handled these load path questions in your structural design? Please share a comment and let me know!

 – Paul

Paul McEntee

Author: Paul McEntee

A couple of years back we hosted a “Take your daughter or son to work day,” which was a great opportunity for our children to find out what their parents did. We had different activities for the kids to learn about careers and the importance of education in opening up career opportunities. People often ask me what I do for Simpson Strong-Tie and I sometimes laugh about how my son Ryan responded to a questionnaire he filled out that day:

Q.   What is your mom/dad's job?
A.   Goes and gets coffee and sits at his desk

Q.   What does your mom/dad actually do at work?
A.   Walks in the test lab and checks things

When I am not checking things in the lab or sitting at my desk drinking coffee, I manage Engineering Research and Development for Simpson Strong-Tie, focusing on new product development for connectors and lateral systems.

I graduated from the University of California at Berkeley and I am a licensed Civil and Structural Engineer in California. Prior to joining Simpson Strong-Tie, I worked for 10 years as a consulting structural engineer designing commercial, industrial, multi-family, mixed-use and retail projects. I was fortunate in those years to work at a great engineering firm that did a lot of everything. This allowed me to gain experience designing with wood, structural steel, concrete, concrete block and cold-formed steel as well as working on many seismic retrofits of historic unreinforced masonry buildings.