— lo ^ 1 
BILL OF LUMBER- 
Shims ( Oak ) 
16 
1 " 
8 " 
2 V 
1568 
lOS yf •■餐 ••• •《春 ••畢 • 0 • 
138 
r 
8 r/ 
6, 
3864 
y } • • ••• • • * • • • * 
9 
r 
C> 
378 
it ff ••• ••• •• * •寧》 
2 斗 
r 
8 ,; 
i a ， 
1344 
Foot Planks ( Pine ) 
i6 
3 ひ 
12" 
iV 
ioo8 
Total Number of feet b. m._ ... 
… 
… 
… 
… 
8162 
The total weight of iron, less ü small amount belonging to the floor system pro- 
per projecting beyond the end pins, divided by 168 gives 1282 pounds as the weight 
per foot for the ironwork, agreeing very well with the amount found by interpolation 
from Table I. 
T】ie total weight of lumbor i.s 7154 x + 1008 x 2^- = 38,521 pounds, "which 
divided by 172 feet, the distance over which tlio lumber extends, gives 195 pounds 
as the weight per foot of the lumber. Tlio total weight per foot will therefore be 
1282 十 195 + 45 = 1522. 
From this must be subtracted something to allow for the weight of iron that rests 
directly upon the masonry, such as tlio heavy pedestals, ena lower lateral struts, bed 
plates, rollers, anchor bolts &c. , in all about 5800 pouuds weight or 32 pounds per 
lineal foot, making tlio proper dead load per lineal foot 1522 — 82 = 1490 pounds. 
The assumed dead load was 1430 pounds, making tlio difference 60 pounds per lineal 
foot, or just four per cent., which is within the allowable limit of error specified in 
Chapter YI. It will, therefore be unnecessary to make the calculations anew. 
It may appear to the ro«ader who has carefully followed out all the calculations 
in tliis chapter, that the designing of iron bridges, and estimating weights thereof» 
involve a great deal of work, and demand considerable time : but such is not neces- 
sarily tlie case ； for an expert could have made tins design in from three to fo ur 
hours, because liis experience would have told him tlio sizes of many of tlie details 
and the mimber ot rivets to employ. In this chapter everything lias been figured 
out carefully enough for making working-drawings, instead of merely an estimate of 
weight ; for the author considers that it is better to teach the beginner exact metlio^ 6 
iu the first place, and leave him to develop approximate ones as his practical experi- 
ence increases. 
