7/30/2019 27588.guydes

1/3

GUYDESDesigning a guy wire supported stack can be an extremely challenging problem. This is due to the complexity that

is introduced by the use of non-linear cable elements for the support system. A non-linear analysis of this

magnitude is impractical to perform by hand, and the few software packages that are available for non-linear cable

elements are very expensive. Even when a good software package is available, the time required to model all of the

different scenarios is significant. With conventional structures it is often readily apparent which scenarios wouldcontrol the design. However, in the case of a guyed stack it is very difficult to anticipate the worst case scenario. In

fact, on most guyed stacks different aspects of the design are controlled by different scenarios. Consequently, many

scenarios must be evaluated in a guyed stack design.

With GUYDES a designer can quickly model the system, while accounting for all of the design scenarios that must

be considered. As with any software package, GUYDES is not a substitute for good sound judgement made by an

experienced designer. However, GUYDES was generated specifically for guyed stack design, and so it providesmany checks and balances which will warn the designer if

common mistakes are being made. One of the most powerful

feature of GUYDES is its ability to summarize the analysis in a

useful manner to the user. Up to 36 different scenarios may beevaluated, and GUYDES summarizes all of this information so

that it can be evaluated in a manner of minutes.

Geometry:

GUYDES is configured for up to six (6) sets of guy wires. Each

set consists of three (3) guy wires evenly spaced 120 degrees

apart. This configuration represents the most commonly foundgeometry of guy wire supported stacks, and it has proven to be

the most economical. The program can also accommodate up to

two different sets of deadmen, each set consisting of three (3)

evenly spaced 120 degrees apart.

Non-Linear Cables

The cables that are used to support a guyed stack are non-linear elements. This non-linearity is a result of the axial

stiffness of the cable increasing as the load in the cable increases, which is demonstrated in the figure below. Whenthe load in the cable is low, the cable begins to sag, and the axial stiffness of the cable is relatively low. As the load

in the cable increases to approximately 20% of the cable

breaking strength, the stiffness begins to remain relatively

constant. This is an important relationship to understand inguy wire supported stacks.

Cables setting guyed transmission towers are often set at a

pretension of 20% to 40% of the cable breaking strength.

At a pretension level this high the cable can essentially bemodeled as a linear spring, which can simplify the

analysis. However, guyed stacks are typically set at

pretensions of only 6% to 12% of the breaking strength.

This is due to the large thermal growth that stacks canexperience due to the hot gases flowing inside the stack. This stack thermal expansion is resisted by the guy wires.

As the stack experiences thermal growth, large forces can be generated on the stack, cables, and foundations.

It is very important this non-linear effect be considered in the design of guy wire supported stacks. Not considering

this effect can lead to the following problems:

Under estimation of lateral stack deflections

Understatement of stresses in the stack

Overstatement of loads on the cables

Cable

Cable

Stiffne

Non-Linear

20% of Br.

Up to 6 Guy Sets

(only 3 shown)

Up to 2 Deadmen

7/30/2019 27588.guydes

2/3

GUYDES Solver:

GUYDES generates a global stiffness matrix based upon the geometry and material properties that are entered. The

load is then applied to the structure in increments. The user selects the number of increments in which the load will

be applied. After each load increment, the stiffness of the cables are recalculated. The net effect is then summarized

based upon each of these incremental loads, and the results are totaled. The user has the option of selecting as fewas 3 load increments, and as many as 100 load increments. The higher the number, the higher the accuracy of thesolution, but the execution time is increased.

Load Directions:

Since three (3) cables spaced 120 degrees apart are not

symmetrical, the guyed stack system does not behave the

same way for all load directions. Three (3) critical wind

directions can be considered to represent the worst case

loads. Unfortunately, there is typically not one controlling

load direction that is the worst case for all elements of the

design. For example, often one direction will provide the

greatest stresses in the stack, while another load directionmay provide the greatest cable loads. All three (3) loaddirections must be considered in order to perform acomprehensive analysis.

Stack Conditions:

The stack must be checked in the hot, cold, corroded, and uncorroded conditions.

Hot - In the hot condition the stack experiences thermal expansion, which often contributes

to the maximum load in the cable. The hot condition also dictates the maximum vertical load

on the foundations. In the hot condition the yield stress of the stack material is also at a

minimum.

Cold In the cold condition, the lateral deflection is often the greatest because of the absenceof the thermally induced tension in the cables. Because the deflection is often greatest in the

cold condition, the stresses in the stack MAY also control in the cold position.

Corroded In this case the stack wall is at a minimum, and so the stresses may becontrolled by the corroded condition.

UnCorroded In the uncorroded condition the stiffness of the stack is the greatest,

which may contribute to more load being resisted by the stack.

The following combinations will be checked by Guydes:

Hot and Corroded

Hot and UnCorroded

Cold and Corroded

Cold and UnCorroded

Wind & Seismic:

Guydes calculates Wind loads per ASCE 7-95 or ASCE 7-98, and Seismic loads are

calculated per UBC 97. Since the natural frequency is required for both sets ofcalculations, the program automatically calculates the natural frequency for each loaddirection that is selected. The gust factor is automatically calculated for wind loads, since

guyed stacks are typically considered to be flexible structures by ASCE 7.

Direction # 1

Direction # Direction # 3

HotCold

Thermal Growth

Corrosion

Allowance

7/30/2019 27588.guydes

3/3

Output Summary:One of the most difficult aspects of guyed stack designs, is the laborious task of analyzing and summarizing all ofthe different design scenarios that must be considered. As many as 36 different scenarios (3 Load Types x 3 LoadDirections x 4 Stack Conditions = 36 Scenarios) may be considered.

Load Types Load Directions Stack ConditionsNatural Frequency* Direction # 1 30 Deg to Cable Hot & Corroded

Wind Direction # 2 Bisecting 2 Cables Hot & UnCorrodedSeismic Direction # 3 Along a cable Cold & Corroded

Cold & UnCorroded

Natural Frequency required to calculate Wind loads per ASCE 7-98 or 95

GUYDES summarizes all of this information so that it can be quickly analyzed. Worst Case Summaries are madefor the following results:

Deflections

Stack Stresses Cable Loads

Foundation Loads

Natural Frequencies

Attachments:An attachment is defined as any object that adds significant weight and/or windarea to the stack. There are many types of attachments that can be placed on astack. GUYDES has templates available to accommodate the most commontypes of attachments:

1. Ladder(s)2. Platform(s)3. Piping4. Refractory5. General Attachment

The General Attachment option allows the user to enter nearly any other typeof internal or external attachment that may be required.

GUYDES vs. FEA:GUYDES offers an affordable option to FEA, when it comes to designing guyed stacks. When using an FEA tool, itis important to understand if the tool truly supports non-linear cable elements. For example, STAAD-III has a cableelement, but it is not a non-linear element.

The advantage that GUYDES can offer is twofold. First, the time required to assemble the model is easily 50% lessthan that to build a finite element model. This is because GUYDES is set up specifically for Guyed stacks, and sothe input is streamlined. The second savings is in the evaluation of the results. GUYDES provides the user withPass or Fail criteria, using industry accepted design practices.

Example:

Assume a consulting engineer charges her services at a rate of $55 /hr. Assuming that a this Engineer is designing a250 OAH Guy Wire Supported Flare Stack.

Estimated Hrs Required using FEA Tool 40 hrs x $55/hr = $2,200Estimated Hrs Required using GUYDES 20 hrs x $55/hr = $1,100

Based upon this example, in the first design the Engineer recovered more than double the amount paid forGUYDES. In addition, because GUYDES is designed specifically for guyed stacks the potential for oversightand/or error is significantly diminished.

Attachments