Stability of Structures in regard to Wind-Pressure. 63 



To resist this action, girders of any size are braced 

 laterally. These braces usually consist of a series of tension- 

 rods crossing diagonally between the girders for the full 

 length of the bridge, and fastened to the lower flanges. In 

 very large structures the girders are sometimes braced both 

 top and bottom, and this is theoretically the proper way of 

 •dealing with the question — that is, by providing two 

 horizontal webs to the main girders, which act as the booms. 

 Where such a system cannot be adopted, a simple method of 

 steadying and laterally strengthening long spans, would be 

 by two tension-rods arranged as shown by red lines on 

 accompanying diagram. No. 1, thus forming a lateral truss 

 of a very common type. Any sagging in the rods would 

 always assist in keeping a strain upon them. Methods 

 somewhat similar to this have been adopted for suspension 

 bridges, which_, of course, are peculiarly liable to lateral 

 oscillation. 



Before leaving this part of the subject it might be well to 

 allude to the care which should be exercised in the design 

 of the attachments between the bracing and columns or 

 girders. 



This might appear to some an unnecessary piece of advice. 

 It might also be termed a mere truism to mention that the 

 strength of a chain is measured by its weakest link. 



But the necessity of recollecting these things is most 

 apparent. 



Every day structures are put up in which material and 

 ■consequently money are sacrificed to obtain a strength 

 which is purely imaginary. It is not a very uncommon 

 thing to find that want of attention to some small detail, 

 results in the destruction of the whole structure. The great 

 Tay Bridge was destroyed mainly from the weakness of the 

 cast-iron lugs. It is the old case, in fact, of the spoiling of 

 the ship for the want of a ha'porth o' tar. 



The following is an instructive example, taken from a 

 large iron railway-bridge. It consists of one of the 

 fastenings of the wind bracing to the flange of the girder. 

 It may be strong enough, but it shows material wrongly 

 placed, and want of attention to detail. It is illustrated by 

 diagram No. 2, and the strengths of the various parts are 

 given below. It will be observed that by simply reversing 

 the gusset-plates end for end, as shown in the diagram by 

 red lines, the strength of the weakest part of the system 

 would be increased just twofold. The tensile and shearing 



