WEEK THREE
Topic: Construction of Arches: Semi-elliptical, Segmental, Semi-circular.
Content:
(i) Meaning of arch.
(ii) Typical structure of an arch.
(iii)Types of arch and construction e.gSemi-elliptical, Segmental, Semi-circular.
Meaning of an arch
An arch could be defined as a curved structure that is supported by straight sides sometimes created in wall openings such as doors or windows of a building. In buildings, arches act as lintel because they have the ability to support the weight of blocks and other materials placed upon them.
Typical structure of an arch
A typical structure of a semi-circular arch consist in addition to the crown which is the highest point of the arch the intrados, extrados, rise, clear span, keystone, inpost,voussoir, abutment and run. See diagram below for a typical structure of a semi-circular arch.
THE STRUCTURE OF A SEMI-CIRCULAR ARCH
Evaluations
1. What is an arch?
2. Draw a typical structure of a semi-circular arch.
Types of arch and construction e.gSemi-elliptical, Segmental, Semi-circular.
1. Semi-elliptical:
Method of construction: To construct an elliptical arch when given the major axis(span) AB and the minor axis.
R is half the given minor axis.
Draw a line AB equal in length to the given major axis.
Locate the mid-point O on line AB. Draw a perpendicular OC at O equal in length to half the given minor axis. Note: Minor axis is by formulae half of the Major axis.
With C as centre and radius equal to half the major axis, draw an arc to locate the positions of the focal points F and F1.
With F1 as centre and radius F1F, draw an arc. Then with F as centre and radius equal to twice F1B, draw another to intersect the previous one at point Q. Repeat the same procedure to get the remaining points. Join the points together with a French curve to obtain the required arch.
2. Segmental: As the name implies, this type of an arch has the shape of the segment of a circle.
Method of construction: To construct a segmental arch when given the span AB and rise R.
Draw a line AB equal in length to the given span.
Draw the bisector of line AB.
Mark off the rise R on the bisector to get point C.
Join AC and BC.
Draw the bisector of lines AC and BC respectively to get point O.
With O as centre and radius OB or OA, draw the required segmental arch.
3. Semi-circular: This type of arch is semi-circular in shape; hence the name.
Method of construction: To construct a semi-circular arch when given the span AB.
Draw a line AB equal to the length of the given span.
Locate the mid-point O.
With O as centre and radius OB or OA, draw a semi-circle.
Evaluations
1. Sketch the diagram of a semi-elliptical arch.
2. Construct a segmental arch whose span and rise are respectively 100 and 35.
Reading assignment
1. Technical drawing by J.N Green page 133.
2. Use www.google.com to search for types of arches.
Weekend Assignment
Objective
The figure below shows a semi-circular arch. Use it to answer questions 1 to 3.
1. The part labelledX is called the A. springing line. B. span. C. base line. D. crown.
2. The distance labelledY is the A. depth. B. rise. C. thickness. D. span.
3. The part labelledZ is the A. crown. B. springing line. C. intrados. D. extrado.
4 Keystone is not a characteristics of the following except stair. B. arch. Roof.Foundation.
5. The inner part of a semi-circular arch is called A. span. B. crown. C. intrados. D. run.
Theory
1. Sketch a segmental arch.
2. Draw a detailed structure of a semi-circular arch.