From Exp 1 it is clear that we need to figure out a better way to make our centering independent of our structure to test form stability.
AIM:
a. To test whether the vault is stable under its self weight with our designed form (with the designed span: rise ratio) without centering support.
b. To test whether our centering design can be independent from the structure such that it induces no significant stresses in the vault in the process of and after removal.
This experiment is NOT a test for strength but a precursor to our final experiment. It is meant to be quick; hence, quick setting but low strength PoP was used between all layers and the arch is intended to be demolished after success to begin our final experiment.
EXPERIMENT 2
Step 1:
Fixing centering such that it can be dismantled without disturbing the structure.
Step 2:
Laying 1st tile line in PoP mortar, supported on the centering from one end. 20mm of the 75mm tile width rests on the centering and the remaining tile cantilevers.
The full tile line from one end support to the other end support is laid.
PoP mortar mixing and tile laying similar to the method in Experiment 1
Step 3:
Laying 2nd and 3rd tile lines cantilevering from the previous tile line, with staggering joints.
It is important to note here that each successive line is laid only after the preceding line is completed from support to support to form its own arch. Once an arch is complete, it bears its own weight to the support walls and is, in effect, no longer cantilevering from the previous tile line. This prevents any unnecessary loading on the weak PoP joints and avoids cracking.
Step 4:
Laying layer 2 in a herringbone pattern at 45 deg to the previous tile layer.
This ensures maximum breaking of joints between layers. PoP used as mortar in this case for fast setting as this is a quick experiment to test form and stability, not strength of the arch.
Layer 2 tiles are laid at 45 deg to Layer 2 breaking joints between the two layers.
Layer 3 tiles are laid horizontally to break the joints with the 45 deg layer 2.
Note how the first tile line of the first layer is only partially supported on the centering and the remaining arch cantilevers from it. The arch is probably in compression and is resting on the two end walls.
We will get confirmation once the centering is removed and the arch stands stable.
Step 5:
Removing centering:
If the arch stands without the centering support, we know that it is in compression and is stable in self weight.
The arch stands successfully without any centering support.
It is therefore stable in form and geometry and can take its self weight.
Since we planned to demolish the arch anyway to start our final experiment, we decided to test how much extra load it could take. We used bricks of 3kg each, placing them from both ends uniformly.
The deflection in the arch and the distance between the support walls was measured at regular intervals to check the stability of the arch till its breaking point.
OBSERVATIONS
1. The support walls swayed considerably when the arch failed at breaking load.
2. No change in distance between support walls was measured before breaking point.
3. Arch first fails at 1/3rd span on one side
Breakdown at 36 bricks
thickness of arch = 45 mm
Rise = 447 mm
Deflection @ breakdown = 4 mm = 0.9%
Self weight of arch (tiles + mortar) = 100 kg
Weight of bricks = 110 kg
TOTAL LOAD @ BREAKDOWN = 210 kg or 175 kg/m2
Load Testing the Arch to Failure
Failure of the Wall
DELIBERATIONS:
1. What failed first?
a. Did the support walls fail in bearing thrust hence resulting in arch failure?
b. Did the arch itself fail first due to the load causing the support walls to sway?
2. Possible reasons for support wall failure:
a. Support walls too thin to take the (horizontal) thrust load
b. No provision for any member to resist lateral loading (we were only depending on the dead weight of the wall to support the arch)
c. Support walls have no foundations, they can be easily lifted
d. No tension ties
3. Arch failure could result from
a. Span/ rise ratio not sufficient for 45mm thickness for this load. The line of thrust exceeds the thickness of the arch
b. Dynamic thrust exerted by the arch after its failure causing the support walls to give way and leading to the failure of the arch.
c. Poor material strength - our joints are in weak PoP mortar which is very brittle and has not much strength in compression
Building a prototype of our final design and checking for load bearing capacity of the vault while developing systems for construction.
Our first set of trial and errors, understanding working with PoP and its challenges. Learning how to build a vault in air using guides and no shuttering.
Specifications of materials used in the experiment with quantities and approximate costs to extrapolate the economics of such construction.