ANALYSIS OF GUYED MAST

Structure:– A guyed mast is a tall thin vertical structure that depends on guy lines for stability. The mast itself has the compressive strength to support its own weight, but does not have the shear strength to stand unsupported, and requires guy lines. Guy lines are diagonal tensioned cables attached to the ground, usually spaced at equal angles about the structure’s base, to resist lateral forces such as wind loads and keep it upright.

Guyed masts are frequently used for radio antennas. Guyed masts are sometimes also used for meteorological measurements at certain heights above ground level. They support equipments for measurement of wind speed. Anemometers are placed in different direction at the top of the mast. The mast done by us are mainly for meteorological purposes.

Geometry :

Mast :-Masts are generally 100m to 140m tall. Mostly they are triangular in plan. Three vertical tubular members usually 450mm to 600mm apart, braced horizontally and vertically at regular intervals


Guys :- Guys  are prestressed  tensile elements. Each set of guys are connected to the masts at same levels on all the three vertical members and are anchored to the foundation. Several set of guys will be connected at different levels of the mast. Four to five guys connecting the vertical members will be anchored to same point. In plan the anchoring points will be in 1200.

Task:-  To carry out nonlinear analysis of the structure and investigate the behavior of tower under critical conditions and to provide design loads for

  1. Mast foundation
  2. Anchor for guys
  3. Mast members.

Critical conditions:

  1. Different Loads acting on guys and mast.
  2. Probability of one of the guy is snapped

Apart from the self-weight of the mast, guys and the equipments mounted, the main forces acting on the structure are snow loads, wind acting in different directions. Different combination of these loads will be acting on various elements of the structure.

Procedure adopted

The guyed mast was modeled using the software SAP2000 by Computers and structuresm as a 3D space frame. Mast elements were modeled as beam elements. Cable elements were used for modelling guys. Pretensioning loads were applied on guys.

Loads due to dead load, equipment load, snow load, wind in different directions acting at different levels were applied.

Nonlinear analysis is performed for different load combinations. Analysis was repeated assuming one of the guy was broken.

Behavior of the mast was investigated for the above cases. By changing the diameter of the guys, prestressing force etc., it was ensured that the deflection was not exceeding the limit.

On finalizing the optimum diameter and prestressing force in the guys, the model was analyzed again for all load combinations in both cases, i.e. for guys intact and one of the guy snapped off.

Based on these analysis, following design forces were computed.

  1. The forces acting on the mast supports
  2. The forces acting on guy anchors
  3. Design loads on mast elements 
  4. Diameter and pretensioning load of guys

 

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