— 8R — 
To properly apply this table one must distinguish between a probable or ordi- 
nary combination and an improbable or extraordinary combination of stresses ; anti 
must recognize which stresses matj and which stresses must exist together. There 
are two distinct conditions, first when the bridge is empty, and second when it is 
wholly or partially covered by the moving load. 
Distinction must be made between the stresses in certain members of deck 
bridges and tlxose in the corrcspouding members of through or pony truss bridges. 
Again any particular member may receive its greatest stress when it belongs to 
the windward or to the leeward truss. Keeping these facts in view let us analyze the 
table. 
First when the bridge is empty we are concerned with the horizontal lines 
numbered 2, 4, C, 8, aud 12 ； and, when it is loaded, with the lines 1, 2, 8, 5, 7, 9, 
10 or 11 and 12. 
Commencing with the top chord, first when the bridge is empty, wc have 2 •’ 
4,* 6* and 8° acting but the latter is a combination of 2* and G° so need not be 
considered. We see that for both windward and leeward chords there are both C 
and T or — C acting simultaneously, for 4° and 6* cannot act independently. On 
this account and because the effect of 1° and 8* is so great we conclude that the top 
chord stresses when the undge is empty need not be considered. 
When the bridge is loaded 2°, 8 # , 5*, 7° and 9° act in through or pony truss 
bridges and for deck bridges 11。 may also act. In deck bridges 5* has a much greater 
effect than iu through or pony truss bridges, for it then includes the wind pressure 
upon the train. 
As before 9* may be omitted as it is a combination of 2° and 7*: this leaves for 
through and pony truss bridges 1°, 2* and 8* as the ordinary loading and 1°, 2°, B # , 
5* and 7° as the extraordinary loading. In tlie latter it will be noticed that for both 
windwtii tl äud lööwtird cliords W6 liave botli C uud T or— C acting situiiltdiiöously j 
for 5* and V must act together. On this account and because of the great effect of 
1° and 8° compared with 5°, it is evident that l c , 2° and 8° are the loading to be 
considered. 
In general as will have been noticed in reading chapter YI, extraordinarv loads 
need not be considered unless their effects exceed those of ordinary loads by about 
fifty per cent. 
In deck bridges it may be possible, though it is not probable, that the combined 
effect of l° f 2 e , 8®, ö° and V may be bo much greater than that of 1°, 2° and 8° that 
the method of proportioning given in chapter YI will Lave to be adopted. If there 
be curvature in the track 11° will always exist with both 1°, 2° and 8°, and with 1°, 
2*, 8°, 5° ami 7°. Its effect may bo considered as an increment of that of 1° aucl 8° 
in proportioning by the method of cliapt. VI, but it is to be noticed that it need 
increase the sectional area of the chord on the concave side of the track only. In 
11。 ifc is seen that the effect may be either C or T, but C must be taken, as the wind 
may act in either direction and consequently both with and against the centrifugal 
force. 
