The side leg shape seat

Previously I discussed two design approaches, a tilted leg design and a straight leg design. These designs were very so, much different. The tilted legs showed some harmony on the design and looks great, the straight legs log resistant but no so much beautiful.

Today I will analyse the side leg shape seat.

L shape seat
L shape seat

From all this approach looks more stressed. The problem is that the table is supported by two legs instead of four plus it is taking all the torsional stresses generated by the table and weight. In regards to the design it looks that it will fall apart and challenges hour notion of safety. From all the designs this will be the most demanding regarding the material strengths.

Side Legs Chair
Side Legs Chair

Using a steel shaft unibody to support the upper seat table. The seat table is where we sit on. In order to make this type of approach it’s very important to use a very resistant material that will not break, can flex and is light weight. Not any wood could be used for this design since it is heavily stressed compared to the other two alternatives.

 

 

The table case and the open source quest

On a previous post http://numerica.pt/2017/07/31/chair-legs-and-ikea/ I introduced the table/chair dilemma, a thing I looked at on a  trip did,  I found a very nice table that I wondered if I could use it as a decorative seat (http://numerica.pt/2017/07/21/tilted-leg-table-curiosity/). I showed that there  are a big variety of leg supporting types (leg configuration) for seats and that this arrangement was just one of many.

4 Straight Leg Chair Design
4 Straight Leg Chair Design
Side Legs Chair
Side Legs Chair

My initial target design was as follows:

Tilted table
Tilted table

I modelled the behaviour of this table to understand possible weaknesses. The model was as simplified as possible. Also material properties used were very far from the real behaviour of my target subject.

Table with tilted legs
Table with tilted legs

The weight support on this design is concentrated on the center. However I can imagine if we lean forward the bulk of our weight shift to the front and produces a bending effect on the table this in turn will stress highly the leg to table connective zone since it has liberty to rotate in that direction. For this design the top location of the legs are more stressed due to their angled design.

My conclusions: To use this table as a seat the material used in the legs should be very strong specially to deal with the bending moment generated due to the tilted angle and centered position of the connection table to leg. The connection interface between legs and table is a critical interface for this design since it will have to deal with the torsional moment that we generate when we lean front, back, left and right.

Remarks: This analysis was performed with FreeCAD and images were retouched with imaging software like gimp. Although these open source tools don’t allow for major analysis we can still learn something. If I were to study this in detail, commercial software would be much easier and powerful to use and understand exactly what was going on with the design.

So many types of things we can sit on

When we think of something to sit, we think on something we can rest on. These things are around since forever. However for the same function we have imagined a diverse set of designs that get things done. Every design has it’s advantages and disadvantages, probably material strength will play an important role here. The stronger the legs are the thiner they can be and more weight they can withstand. Bellow are a few representations os seats or tables (why not?) in 2D. Some imagination is important from here on. On my office I have I Rise Legs approach and a side legs design, on my living room I have a more classic straight legs design. Probably during a days work I will view many of these designs and probably you will too. Many reasons will determine the design from cost, function and requirements but I will be focusing on the open legs and straight leg variant. Differences? Maybe many, maybe not…. From intuition I feel that straight legs will be stronger but with 120 Kg on top….

Types of tables
Types of tables