Lattice towers and masts
BS 8100-3:1999 pdf free.Lattice towers and masts一 Part 3: Code of practice for strength assessment of mem bers of lattice towers and masts.
184.108.40.206 Horizonlal mnbrrs with pin a bracing Where the length of the horizontal edge members becomes large, it is nonnal to subdivide them as part of the plan bracing which provides a convenient basis to do this. (See 5.3.5.)
The system length of the horizontal members is taken between intersection points in the plan bracing for buckling transverse to the face of the structure, and between supports in plane for buckling in the plane of the frame.
Care is needed in the choice of the vv or rectangular axes for single angle members. The vv axis should be used unless suitable restraint by bracing is provided in one plane at or about the mid-point of the system length. In this case buckling should be checked about the vv axis over the intermediate length and about the appropriate rectangular axis over the full length between restraints on that axis.
220.127.116.11 Horizontal mesa brrs without plan bracing For small wi(lths of towers and for masts, plan bracing may sonietinies be omitted. (See also 5.5.lcl) for reduction factor KiI
The rectangular radius of gyration should be used for buckling transverse to the frame over system length Lh (see Figure 7). In addition for single angle members, the radius of gyration about the vv axis should be used over L [see Figure 7a)J unless restraint by secondary bracing at intervals along the length is provided in which case the system length is Lhl (see Figure 7b)J. however, buckling about the rectangular axis will be critical except in the case of an unequal angle.
Additional allowance should be made for the bending stresses induced in the edge members by loads transverse to the frame, e.g. wind which can produce significant point loads.
18.104.22.168 Cranked K braeinq
For large tower widths, a crank or bend may be introduced into the main diagonals (see Figure 8). This has the effect of re(lncmg the length and size of the redundant members but produces high stresses in the members meeting at the l)end and necessitates fully triangulated transverse support at the joint. Diagonals and horizontals should be designed as for K bracing, with the system lengths for diagonals based on their lengths to the knee joint.
22.214.171.124 Portal frime
A horizontal member is sometimes introduced at the bend to turn the panel into a portal frame (see Figure 9). The main disadvantage of this is the lack of articulation Present in t.he K brace. This system is sensitive to foundation settlement or movement and special consideration should be given to this possibility.
This example also shows special secondary bracing which is less sensitive to loads resulting from such movement.
5.3.5 Plan bracing
Phm bracings are required at bend lines of the leg, to provide lateral restraint to long horizontal meml)ers and to (list fll)LIte local loads from ancillanes, etc. Plan bracing is also required to maintain the shape
of square towers, particularly where K bracing is LlSed, afl(l 10 distribute eccentric loads. The non-tnangulated systems shown in Figure 6 do not satisfy the Iirst requirement.
Where the plan is not fully triangulated, additional allowance should be made for the bending stresses induced in the edge nwmbers by loads, e.g. wind. trans-verse to the frame whereby the main diagonal can impose a point load arising from the summation of the distributed loading on the various face menthers.
Attention should be given to vertical bending due to the self weight of plan bracing. Support from hip bracing cmi he u.se(I to reduce long spans-. The design should be detailed to eliminate any face slope effect which promotes downward displacement of inner plan brace members.
5.3.6 Multiple lattice bracing
Multiple lattice bracing members should be designed under the applied load on the mininutni system L11 for the slenderness ratio A = Ld The angle bracing members of a multiple lattice configuration should he checked on a system length from leg to leg rith the appropriate radius of gyration r,,, or Isee Figure 4e)) such that the overall effective slenderness ratio is- limited to 350.
For the stability of the panel r/r should be greater than 1.50, where r is the radius of gyration about (he axis parallel to the plane of the lattice.
5.3.7 Tie 8ystems
Great care is needed with the use of tie systems particularly if used on large masts or towers, on masts or towers with sloping legs or in colDunetion with secondary members. Each diagonal member of a pair of cross bracing members [see Figure 401 should be capable of carrying the full bracing shear load in tension. [)etailing to give an initial tension within the bracing and to provide mutual support at the central cross will be required to minimize deflection. It will also be necessary to pay special attention to possible fatigue problems.
The horizontal members should be (lesigned as horizontal strut members with or without plan bracing as appropriate for compression along their whole length.
NOTE 1 Teiiion syteiio an- ery sersitâ,c to tnethod or erection ud to iiiudiflcatkni or relative mo’erncnts, Spvtbl attention ahould be paid to poe4stbk’ excr e deflectiona and fatigac To dnsUrt their eftectivLllcaa nleaaurea ahould be taken to eflaure that theac niatter an’ can-fully conaidered,
NOTE 2 A iiuodinunt effvctI,e 4cndenica ratio of 350 1oi tenalon flIL’IflbCl5 ha good practice.
5.3.8 Compound members
126.96.36.199 Double angles
Compound members consisting of a pair of identical angles back to back (forming a T), separated by a small distance and connected at intervals by spacers and stitch bolts, may be used for bracing. They should be checked for buckling about both rectangular axes as follows. (See also 5.5.1.)
In order to keep the effect of this interaction to a minmmm, the spacing l)etween stitch l)OltS ShOtIl(I be limited to give a maximum value A1 of 90. Wheti oniy stitch bolts and packs are used, composite section properties should be based on either:
1) the actual space between the individual angle
2) a space taken as 1.5 times the thickness of one
of the angle members,
whichever is the smaller. If batten plates are used, composite section properties may always be used based on the actual space between the individual angle members.
188.8.131.52 Other coin pound in em beis
For other compound menther configurations and the sizing of battens and stitch bolts or welds, refer to 5.2.2.
5.4 Secondary members
Typical secondary bracing arrangements are shown in Figures 4 to 9.
NOTE A inaxiniuni effective slenderness ratio of 240 for secondary bracing members is good practice. The use of high slenderness can lead to the possibility of individual members vibrating, and can make theni vulnerable to dunage due to bending from local loads.
In order to design secondary members it is necessary to apply a hypothetical force acting transverse to the leg member (or other chord if not a leg) being stabilized at the node point of the attachment of the secondary member. This force varies with the slenderness of the leg member being stabilized and is expressed as a percentage of the leg load, F’ for ;which the following two checks should be carried out.BS 8100-3 pdf free download.Lattice towers and masts