## 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.

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.

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.

## Straight leg seat design for a bit of fun

Remembering when I was small, where i played countless hours with lego. Built many interesting constructions and tested the strength of many designs. That was a form of modeling the behaviour of a bridge, house or any structural  analysis that I could think of at that time. We all model things every day since we need to live in this world and figure out even if very basically the behaviour of objects. Think that when you hold an object you use the right force to hold it. Some how we know exactly the necessary configuration to execute physical task even without any though what so ever. When we look at a design and it challenges our notation that it will work it is because our underlaying protective system already is calculating and giving us feedback that it is dangerous to use. However this feeling can be wrong and the structure can be very safe, however most of the time our body is wright :).

I tested two designs on the simple table to understand what would happen.

This design present complete load transfer to the legs. The connection between table and leg is compress. Very little to none torsion is present. This design looks very stable, simple and is the classic, maybe the only simpler alternative is to use a solid cylinder instead, but that would make the seat very heavy. This design probably will hold as much as what the material used (wood, iron) can handle.

Comparing one design with another we can see that the behaviour to pressure  is completely different. The straight legs approach is stronger and would deal better with increased weight on top.

## 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.

My initial target design was as follows:

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.

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.

## Chair – Legs and IKEA

Yesterday I went to the mother of chairs and found so many design approaches that I got overwhelmed by it. It’s about purpose, function and creativity, ahh and some color.

Real examples of my previous figure were there! That was expected! I know….

Since I am a visual person I will show it any way… If you look closely 4 legs the central seat but pay attention to how the legs connect to the seat. This is the primary detail that holds everything together. Although materials change from metal to plastic, shaft diameter changes also but also important is how we consider this connection and how we simplify it. And which one will I study? It looks easier compared to the spider but not similar to the small table I shoed before.

Heres another shape.

Another design approach that has the advantage of folding.

Here the typical IKEA office chair. This chair can have different height levels.

The purpose of this post was to show there are many designs available and all of them try to solve a particular problem. There is no such thing as bad in any of the designs, maybe more a question of taste and function.

## 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….

## 4 leg chair the chalenge

Continuing my divagation through chair design and analysing the previous chair/seat post.

The purpose of this design is to serve as a support for objects. All 4 legs are tilted in a square fashion pattern at the bottom and are closer to each other near the table surface.

It’s not my intention to replicate the design of the table completely so my first simplification is the junction of the legs to the table.

Every design has it’s strengths and weaknesses. It’s purpose and function. This design is particularly sensitive to edge force since leg is supporting the table center. For a table this size the table will work comfortably with the objects we place on top, however as a seat considering an average weight of 80Kg.

So here starts the challenge:

• Withstand 80 Kg to 120Kg
• Use a clean, light design

Everyone probably already has an idea and the first one that comes up my mind is a straight leg chair (the simplest form of this design).

So why do we bother inventing new ideas??? The answer is that not all people are made equal. Some like simple chairs, others like edge design chairs, some others are in between and the rest probably don’t care.

Looking for over the edge designs, challenge our notion of possible or not possible. And maybe let us feel in some way better either for simplicity or because of blending in room design…

## Tilted leg table curiosity

I spent a weekend at a Rock hotel. Well there was a lot of rock on the room, hallways, reception, everywhere! Well it’s a rock hotel! 🙂 The hotel was quite nice with a very good panoramic roof top with chill-out music (at least one place without rock in the hotel). On my room I noticed a table with tilted legs. I wondered if I sat on it, it would break.

The design is wonderful with black glass on top, however if I changed inclination of those legs even further would it break?  Could this design be used for a chair? Could I use 3 legs? could these legs be longer? These questions got in my mind.