Historic Churches 2017

BCD SPECIAL REPORT ON

HISTORIC CHURCHES

TH ANNUAL EDITION

be mounted at key locations. Typical

locations are on the bell frame at pivot

level, on the foundation beams and on

the tower walls in the bell chamber.

Another combination may be to locate

the accelerometers at different positions

up the tower to measure the tower shape

as it moves. This, in conjunction with

measuring the natural frequency of the

tower, will help to predict the benefits

of changing the level of the bell frame.

Plots A–C (above) show the movement

for various combinations of excitation at

different locations within the same tower.

When monitoring the movement,

there are a number of methods of

excitation. To measure the natural

frequency of the tower, a single pulse

input while measuring the decay

frequency would be optimum. It is

not possible to achieve that with a full

circle swing of a bell due to the complex

combination of forces which are applied

as the bell swings. A better method is

to have a heavy sandbag swinging on a

rope and impacting on a wall. However,

in reality, even when ringing a peal, it is

possible to pick out the natural frequency

from the plots (Plot D).

Determining the maximum potential

movement of the tower can be difficult

due to the wide range of peal sequences.

I have used a combination of the bob

minor peal (when bells are rung in

sequence) and ‘firing’ (when all the bells

are rung simultaneously) in one of the

orthogonal directions. Firing will produce

a large jerk (see Plots A–C).

Where there are cracks of specific

concern, it is possible to measure the

movement in each side of the crack

simultaneously, with the difference

between the two plotlines representing

the movement at the crack.

There is a limit to the accuracy of

the movement recorded. The electronic

integration method introduces background

‘noise’ in the signal. As a result, movements

of less than about 0.2mm will be lost in the

background noise.

One clear lesson of the monitoring is

that human perception is very sensitive

and actual movement is usually much less

than people expect.

BELL FRAME ALTERATIONS

Stiffening up the bell frame or foundation

beams to reduce movement of the tower

can have unpredictable effects. The

slackness in the system means that the

energy transfer from the bells to the tower

is delayed, with energy temporarily stored

in the frames and beams as they flex, and

then released at slightly different times.

This can change the frequency of the

forces imposed on the tower and it can

also increase them.

Imagine placing a football against

a wall and kicking it onto the wall. The

impact on the foot, and hence the wall

will be tolerable. Now replace the ball

with a hollow metal ball of the same

mass and kick. Broken toes could be the

result, indicating a much higher impact

force. The energy absorption of the

frames and foundation beams is similar.

It is possible that stiffening of the frames

and foundation beams could increase

the movement of the tower, either due

to the sharper impact pulse or because

the modified input frequency is nearer

to the natural frequency of the tower.

Unfortunately, monitoring of the tower

will not help with this dilemma.

Changing the level of the bell frame

can also have unpredictable effects if

it means that the force is applied at a

position nearer to the point of maximum

movement of the tower. Some towers

will sway as a simple cantilever from

the ground. Others, possibly with stone

spires with a higher centre of mass, may

develop a ‘standing node’ approximately

two thirds of the height, about which the

structure oscillates. If the force is applied

near the point one third of the height,

excitation can occur. Hence the need to

monitor the shape of the displacement up

the tower. Harry Windsor and others have

attempted to analyse what happens but it

seems to me that too many changes take

place when a bell frame is altered to allow

isolation of specific parameters.

Changing the configuration of the

bells in the frame can also change the

forces on the tower. One of the reasons

bell ringers want to have their bell frames

modified is to improve the arrangement

of the hanging bell ropes. Ideally, they

should form a circle so that each ringer

can see all the other ringers. This often

means changing the direction of swing

of some of the heavier bells. The tenor

might have been swinging east-west for

200 years and will now swing north-south

and be close to the west wall of the tower.

The north-south forces in the west wall

could increase significantly.

TOWER WEAKNESSES, CRACKS

AND REMEDIAL MEASURES

Generally, where vertical cracks have

developed in the centre of each wall of a

tower, these do not get significantly worse

after formation. However, if they do ‘work’

during ringing, tiny particles of debris

will work their way down the cracks,

slowly enlarging them. Cracks can also be

worsened if changes made to a bell frame

alter the forces induced in the tower walls.

For this reason, when it is proposed to

alter a bell frame in a way that will change

the forces induced in the tower, it is

recommended that the cracks are tied.

Historically, cracks were tied with

This plot shows the movement of the south wall at bell chamber level and the

clock chamber 4m below. It can be noted that the impulse momentarily distorts

the wall of the bell chamber in the east-west axis (yellow) while the wall below

(green) moves much more closely with the natural frequency of the tower.

With a peal (with bells swung in both axes), the frames move much more than

the wall in both X (red) and Y (green) axes. The yellow and blue plots give a

clear indication of the natural frequency of the tower in each direction.

PLOT C: Bells ‘firing’ east-west simultaneously, with accelerometers on

south wall of bell chamber and south wall of clock chamber

PLOT D: Full peal, with accelerometers on bell frame and

south wall of bell chamber