Wednesday, 22 July 2009

Leaded Glass Reinforcements

I received a query recently about this subject. As the correspondence may be useful to a more general audience, I present an edited version here. (all the personal chat has been taken out!)

“I was wondering if I would be able to ask you a question regarding re-enforcements. In regards to the hollow lead with the bar running through or using Reforce with the brass molded through it. Which is better?”

The lead covered steel is stronger. It has the disadvantage that if it gets moisture into it, it will corrode. Steel expands when it corrodes. This leads to progressive destruction of the surrounding glass.

Brass is weaker, but does not have the same degree of expansion when corroding.

Steel is cheaper than brass.

These factors have to be taken into account when deciding on which to use. So I don’t have a definitive response for your situation.

Really any time you need to reinforce a panel, it is because it is too large to reliably support itself. Often this is because it is too tall or too wide. Big windows have always been built in sections, with each stacked upon top of the lower ones. There are saddle bars or ferramenta added to the window opening to strengthen the window.

It is important to note that in compression glass is much stronger than steel. It is when the glass is in tension that it is weaker. So what the reinforcement is doing is resisting any lateral movement. It is not holding the glass up. The glass can do that very well on its own. The glass is subject to lateral movement from wind pressures mostly. But in some situations as in doors, it is subject to inertial movements - the door closes and sometimes slams. In other installations there is vibration – such as sidelights. The re-enforcement is to counteract or reduce this movement.

In general, if the panel needs reinforcement, it is too large as a single panel, and needs to be built in several panels. Some people hate to have the line of the panel joints, but the eye generally ignores those straight lines (unless they are out of true horizontal or vertical).

Some questions you need to ask yourself about reinforcement are:
Do you really need to reinforce?
Must it be within the panel?
Can you use external support?
Why would two hinges be better than one?

Remember the reason for not having a hinge is because the glass is the strongest element in a leaded or copper foiled window. Therefore a window with complicated lines will be a stronger window as the glass interlocks. If you look at many older windows you will see a number of hinges, and the windows are still there. I attach an image of a stair window that has been in place for just under 100 years. It has a multiplicity of hinges. I am not saying don't concern yourself about hinges, but keep a sense of proportion.

Nowadays, I keep all my reinforcements to the surface of the panels, not inside. Also if you want to join panels in a large window, it is not essential that the join be horizontal or vertical. It could be in a wave, sinuous curve or in a stepped fashion. Your imagination is probably the limit here, not the material.

The enquirer then sent pictures with further information.

“These are the latest 3 panels I’ve made for my bungalow out the back. They measure 1100mm high by 500 wide approx. I’ve made them all with different reinforcement applications. I was told they would not need any but still wanted to strength them up.

“The middle you can see I broke the hinge line with two pieces of re-force and on the other two I’ve gone all the way through to the outer border. All other lead lines do not go more than 2 pieces of glass before they are crossed by another piece of lead to break up that hinge thing.

“Due to the size/design I’d appreciate your thoughts on what I’ve done being correct/overkill?”

The two outer panels are supported appropriately. I believe the right one is adequately reinforced, and the left is over reinforced, but there will be no harm. The middle one is not adequately reinforced. The broken horizontal reinforcement transfers the stresses to the middle. The vertical one also transfers the stresses to the middle, only a little higher.

For reinforcement to work, it needs to transfer the stresses to the sides/tops of the panel where they will be captured by the framing. Thus the reinforcement needs to be a continuous line. The strongest reinforcement will be across the shortest dimension of the opening.

The weaving of the lead lines described by you as “lead lines do not go more than 2 pieces of glass before they are crossed by another piece of lead to break up that hinge thing” is exactly the right thing to do in these panels.

In the case you are illustrating, there should be no problems for several generations at minimum and possibly for a century.

"I’ve also included one other piece I’ve designed and cut which is going to be installed in an internal wall inside a home. Its 800/800mm approx and due to it being kept out of the weather was wondering about what type of re-enforcement structure would suit?"

As this will be an internal panel, I suggest that the best reinforcement would be a toughened/tempered sheet of 4mm float glass installed behind the panel. This will provide support in case someone leans against it. Yes, there is a diagonal hinge at the trunk, but the strongest reinforcing for this would simply be a horizontal bar behind the panel, which would look ugly and I don't think you want anyway.