This section is from the "Architectural Iron And Steel, And Its Application In The Construction Of Buildings" book, by WM. H. Birkmire.. Also see Amazon: Architectural Iron And Steel, And Its Application In The Construction Of Buildings.
Truss No. 4 (Fig. 2) has horizontal top and bottom chords. One half of truss being drawn, the centre line of truss is at tie N.
To draw the strain diagram Fig. 2A, lay off the loads as heretofore, commencing with the joints nearest the support. Thus bd = joints 1 and 2, df = joints 3 and 4,fj = joints 5 and 6, and nj and nl each equal one half of loads at 7 and 8. One half of the total load on the truss being sustained by each support and commencing at one support we have the force bn, the stress in rafters ba and in tie na, closing the figure. At joint 1 we know ba and bd; draw dc and ac, closing the figure. At joint 2 we know na and ac; draw ce and ne. At joint 3 we know dc, ce and df; draw eg and fg. At joint 4 we know eg, ne; draw gh and nh. At joint 5 we know gh, fg and fj draw hi and ji. At joint 6 we know hi, nh; draw im and nm. At joint 7 we know im, ji and; draw lk and mk.
The centre rod has no strain excepting the holding up of lower chord from sagging.
The length of
= strain on
The length of
= strain on
(Fig. 1)Draw strain diagram Fig. 2 as before. Draw ab parallel to AB, ak to AK, cd to CD, ac to AC, cl to CL, ef to EF, le to LE, eg to EG. The dotted lines indicate the other half of truss. The strain scaled off will be:
The length of
= strain on
If the principal chord supports a ceiling, the weight is divided similarly to that on the rafters, supported by the rods CL and EG and carried to the points of support BD, DF and FH.
The sum of the ceiling and roof loads is then taken, proceeding with the strain diagram as before.
The weight of ceiling, in addition to that scaled from the strain diagram, is the strain on ties CL and EG. For example: the point at joint CLEFD supports 10,000 pounds of roof, the point at joint ACLK 5000 pounds of ceiling, which weight is carried to the first joint by rod CL. Then, line df on strain diagram being completed, the 5000 pounds is to be added to the scaled distance on cl, for the reason that by carrying 5000 pounds to point DF, CL is required to perform its portion of the complete truss, and the weight of ceiling is still to be supported directly by CL.
Iron trusses are far superior to wooden trusses, in that they may be built stronger and lighter, and are more durable. The sections used in the different members are to be considered when the truss is designed. The forms in common use are I beams, deck beams, channel bars, T and angle bars.
Flat bars are more suitable for ties. For struts it is necessary to use some form of section offering resistance to bending, such as T irons, two angles riveted back to back, or four angles in the shape of a star. The fewer the members in compression the more economical will be the truss. For rafters of trusses on large spans channel irons are used placed back to back, also with plates riveted top and bottom to make up any section required, the struts and ties being connected to the webs of channels by pins. (See chapter on "Rolled Iron Struts.")
For simple and economical trusses angle irons and plates are used, as shown by the details in plate of truss No. 5, Fig. 3 being the apex of roof, Fig. 4 the point between D and F, Fig. 5 the heel of truss, Fig. 6 the centre of bottom chord. The dotted line A represents the neutral axis of each member, or the line of the diagram of strains which should be the centre of the bearing of truss on the wall at B.
Wooden Purlins are connected and secured by knees to the main rafters, as shown at B, Fig. 3, the covering in this case being boarding and sla'e. For an iron or fire-proof covering purlins of angle iron at C, and T's or L's at rafters E, should be used. The angles of the main rafter G are separated by the plates at the connections. To keep these angles in a straight line, filling pieces as shown by the dotted line are placed between and riveted through hole F.
After the proper struts and ties have been calculated and arranged, the connections are the important part of the truss. It may have more than is necessary in its struts and ties; but if the connections have not the proper proportion of pins and rivets, the truss is proportionately weak. For shearing, bending and crushing strains on rivets and pins, see article on "Bolts and Rivets" and table of Rivets and Pins.
When two or more plates are to be riveted, they are held together by temporary bolts inserted through some of the holes.
The rivets, heated red hot, are then inserted into the holes up to the head and hammered by hand, or pressed as in machine riveting.
In a good joint, especially when newly riveted, the friction of the plates is very great.
In calculating the strength of the frame, the strength due to friction is not considered, as it cannot be relied on in cases where the frame is subjected to shocks and vibrations.
Note. - For
30 feet to
45 feet use truss
40 " "
60 " " "
60 " "
100 " " "
" 3. 4 and 5.