With a little research on the internet, and/or a few hundred dollars in Bamboo books, you can find a lot of information on different ways to connect bamboo. Most modern bamboo structures use a bolting system, and/or rebar cemented into the last node or two to connect to the foundation, or another bamboo. There are also real fancy steel end caps. These look very nice, but we are looking for cost effective solutions.
As seen on The Bamboo Cabin Prototype and the Isla Meurta project, both cemented rebar and bolted connections were used. Bolts are quite expensive, and in places with serious structural loading, the penetrated node spaces need to be filled with a stucco mix so you can tighten the bolts without crushing the bamboo. A bolted building system generally uses more bamboo than the hybrid system we are employing. Bamboo is a beautiful material, which is a big reason to choose a bamboo house, and it looks better with more pieces. However, it can still look beautiful with fewer pieces using this hybrid system. This becomes a matter of personal preference and budget.
Our hybrid system
Our system of connection seems to be unique, in that I have not seen it published anywhere as of yet. It involves welding steel pins together in various forms to connect bamboos. It allows multiple bamboos to be connected together in ways that reduce the quantity and can make for a more simple and elegant design. It is a take-off of the very widely utilized system of cementing a steel pin or bolt into the end of a bamboo.
We are using the idea of embedding a steel bar, and or all-thread into the end of a culm, and cementing it in place to fix one bamboo into another firmly. We can connect multiple bamboos together using this system. Our octagon corner posts use seven pieces of bamboo, connected together in a way that they all bolt to the floor and the beams and roof sections as a single structural unit. We utilize molds and jigs to set our bamboo posts and roof sections. We pre-weld steel pieces to insert into the ends and cement them in place using a rich stucco mix and an accelerant to reduce curing time and increase strength of the joint. All-thread is utilized to bolt down the pre-fabricated roof sections and the post bases are designed to be firmly bolted to the foundation using bolts set into the floor before the concrete pour.
This system can be adapted to meet the parameters and structural needs of the situation. For instance, the roof sections are held together with a few all-thread bolts, but the majority of the connections are made with ¼” rebars set at an angle and stuccoed in place. These pieces are further held in with the T&G and roofing.
We can utilize this system to prefabricate the bamboo structural components of the building. Posts, walls, and roof sections can be prefabricated and stacked until needed in the assembly process. Working at table height improves quality and efficiency.
Once a building is designed, the joint splines can be mass produced to specification.
Appropriate technology for appropriate problems.
Below is another idea that we have prototyped but not done extensive work with as of yet. This system could be used in ultra light structures such as greenhouses and low income housing.
It is important to note that it is against codes to weld rebar for structural reinforcement due to its carbon content. For greenhouse structures, and other very lightweight structures, this is probably not a problem.
I have used welded rebar for these types of structures for years, and in many non-structural ferrocement structures, where the screen is the primary reinforcement, and the rebar is more of a form to attach the screen. For permitted structures that will have significant roof loading, etc, there is round & square bar and flat steel stock that can be welded for this purpose.
After the culms are pre-drilled with holes for filling the cavity and the ends are cut to join the other bamboo pieces, they are slid over the splines and secured in place. The bamboo joints can be filled with cement. Where the loading requires, steel pins can be placed through the end to help in the bamboo onto the cement.
Here is an idea we came up with that will significantly improve the holding power of the cemented joint onto the culm without bolts or pins. Rather than put one hole of ¾”-1” in the section to be cemented, why not put a few at different places around the circumference of the bamboo. The bamboo has a wall that varies from ¼” to ¾”. If we tape over all the holes except the ones on top where you are filling the hole, you end up with more than one filled hole that has a relatively large surface area to the cement. In order for the bamboo to pull off, or twist around the cement plug, you would have to shear off the ¼”-3/4” concrete plugs filling the holes in the wall. This simple, low cost, method will mean less/or no bolts, and a very secure connection. Color can be added to the cement for aesthetic purposes. The cement also fills the spaces between the joints in an attractive manner.
Please keep in mind this is a test for strength and viability of the concept, not an exercise in fine carpentry!
One important note: you cannot cement into a green bamboo. The shrinkage of drying will cause it to crack! See the Bamboo Treatment page for drying times.
In all the books I have seen, and all the many bamboo sites I have visited, there is nothing like this idea. I hope that this idea will be of use to others working in bamboo to be able to expand on their creative ideas and make better use of this resource.
Here is a photo of a bolted joint. The plugs where the hole saw was used cover the stucco mix inside the joint.