Question in relation to a FIXED ARCH: Q. What do you think would happen to the h

Question

Question in relation to a FIXED ARCH:
Q. What do you think would happen to the horizontal reaction and fixing moment if the supports were to move outward in proportion to the horizontal reaction? If you are unsure, carefully load the arch at the crown and carefully move the Right Hand Support outward by 2-3mm. (Please read discussion below)
Discussion: I have mapped the data and it shows that both the Horizontal Reaction and Fixing Moment would increase. However, when this is done with the fixed arch equipment at the lab the fixing moment increases and the horizontal reaction decreases. Could someone explain the logic behind this?

Explanation / Answer

As it is a kind of conceptual question, I will try to make it understand in easy language.

To understand this, simply understand that the arches are the curved beams which minimize the bending moment, so that the transverse load is converted into the axial thrust. When we use a parabolic arch, it carries almost zero bending stresses, because the transverse loads get converted into the axial loads due to the arch action.

Now when you move the supports outwards, you vary the shape of the arch towards making it a beam. The beams carry mostly the bending stresses, and negligible axial loads(horizontal reaction). Therefore by moving the supports outwards the arch behaves more of like a beam and therefore the fixed bending moment increase but the horizontal reaction decrease.

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