In a "semicircular" arch, the stress line runs mostly sideways, pushing the columns apart. This type of arch is commonly used in architecture and engineering to create strong and stable structures.
When an arch is too tall or wide, it can start to sag in the middle and eventually collapse. This is due to the weight of the structure causing the arch to bend downwards, unable to withstand the forces acting upon it.
To prevent this collapse, engineers use various techniques to reinforce the arch. One method is to use additional support structures such as buttresses or piers, which help distribute the weight of the arch more evenly and provide additional stability. Another technique is to reinforce the arch with materials such as steel or concrete, which can increase its load-bearing capacity and prevent sagging.
By understanding the principles behind arch design and the effects of load and stress, engineers can create structures that are both visually appealing and structurally sound. Properly designed arches can withstand the forces acting upon them and remain stable for long periods of time.
I hope this helps you. :)