Building a Red Brick Archway

[Clarej5]

Hey Guys,

I recently tried to build a red brick arch but it unfortunately it collapsed.

On Day 1 - I Built of two 1.8 meters height pillars, one tied to the gardens outer wall, the other is free standing, I also constructed a wooden former etc (pillars are both small, each layer has only two bricks).
On Day 2 - I built the arch over the former, however when building i noticed the free standing pillar was bending, I kept a close eye but after a couple of days it collapsed entirely...

I would just like to pick your brains about where I went wrong....
-Firstly the arch was not a proper Semi-Circle, it was more of a flattened semi circle, and would this have affected weight distributions in such a way that the arch was pressing outward on the pillars more than downwards....
- Secondly Its seems like a very silly mistake but am I correct in assuming my biggest issue was building the pillars and arch so close together.
- Thirdly any other thoughts but be much appreciated before I attempt to rebuild.

Regards


James

 

[jeds]

All arches impose lateral force which will push the supports over. Flatter the arch the more the force. The only option is to make the supports strong enough to counteract the overturning force. That can be done by making them heavy enough or by the shape of the support. Think of flying buttresses on the bigger churches.

 

[^woody^]

The pier could be the problem!

What size bricks are you using, and does the one side need to be a slender pier or can it be a bit bigger or longer?

 

[stuart45]

A rough rule of thumb for arches. Semi circular arches need abutments at least half the size of the span. Segmental need to be even bigger. Gothic can be a bit less.
Garden arches often fail because of this and also there is no weight over the top of them.

 

[Space cat]

Semi circular arches need abutments at least half the size of the span. Segmental need to be even bigger

(My italics.) This isn't very scientific but it should do as a rule of thumb:

You have a segmental arch. Continue the line down to form the complete semicircle then build your piers half the width of that.

 

[tony1851]

Continue the line down to form the complete semicircle then build your piers half the width of that.

Space cat; if you don't mind me asking as a matter of interest, where does this come from?

 

[ABCwarrior]

A 215mm sq pillar 1.8m high...and then a day later having not only gravity but lateral forces to deal with. Apart from the mortar probably needing longer to set properly the footprint of the pillar is tiny for the height. As others have suggested some sloping buttressing would be better if done well or if you did a brick and a half (300mm) sq pillar you could have put in a decent deep(600mm min) foundation and set in some decently thick rebar up to the desired 1.8m and built the brick around it.

 

[Space cat]

where does this come from?

It's a logical extension of the rule of thumb for semicircular arches. Imagine the segmental arch being part of a complete semicircle with its ends now inside the piers. Like all rules of thumb, it has its limits. A particularly large radius segment might be part of a semicircle with its ends below ground. Would the rule still hold?

To do the job properly you must first calculate the side thrust of the arch from its shape and the weight above it. This is not a trivial exercise. Next, you multiply this side thrust by the height of its ends above the ground. The result is a turning force (aka moment or torque) which is trying to tip the piers about their lower outside edges. The weight of the piers multiplied by half their width is another turning force which, if it's big enough, will keep them upright.

There are therefore two ways to stop an arch from pushing its piers over: make them wider or make them heavier. Maybe you've noticed the ornate stonework on the tops of buttresses along the sides of churches and thought it was just there for decoration. Not necessarily; it might also be the extra weight needed to stop the roof arches from pushing the buttresses out sideways.