Flat Arches – Approximate angle between skewback and horizontal For a flat (jack) arch, what approximate angle is commonly kept between the skewback surface and the horizontal to align with thrust and facilitate construction?

Introduction / Context:
In jack (flat) arches, the voussoirs are arranged nearly flat, but the thrust must still be safely transferred into the abutments. The skewback angle is chosen so bedding is normal to expected compressive thrust lines and so that brick cutting and laying are practical.

Given Data / Assumptions:

• Flat arch spanning an opening in masonry.
• Conventional empirical detailing for small spans.
• Need for adequate thrust transfer into abutment.

Concept / Approach:

Although the intrados is nearly straight, the arch still develops horizontal thrust. An inclined skewback avoids sliding and directs compression into the support. Traditional practice keeps the skewback at about 60° to the horizontal for small flat arches, balancing construction convenience and force alignment.

Step-by-Step Solution:

Verification / Alternative check:

Detailing guides for jack arches cite angles near 60°; field practice often uses templates or marks derived from span and rise assumptions.

Why Other Options Are Wrong:

0° gives a horizontal seat with poor thrust transfer; 30° is flatter than recommended; 90° is vertical, impractical for bedding; 120° is meaningless in this context.

Common Pitfalls:

Confusing skewback angle with arch rise; ignoring that even flat arches generate thrust requiring tie rods or adequate abutments.

Final Answer:

60°