This page is very much under construction, and will be greatly expanded upon and organised in the future. For the short-term, it is intended only to serve as a reference to users of the beta.
Self-supporting monopoles may be treated as a vertical cantilever beam subjected to horizontal wind pressures and the self-weight. It may be analysed by beam theory for the purpose of determining the resulting longitudinal stresses. The only error introduced in the analysis of the stack shell by beam theory, as opposed to shell analysis, is the assumption that plane sections remain plane such that the stresses are proportional to the distances from the neutral axis. However, the error introduced by this assumption is small and conservative.
[Troitsky M.S.1990, ‘Tubular Steel Structures – Theory and Design’, p. 5-9]
The minimum length of any telescopic (i.e. slip type) field splices for all structures shall be 1.5 times the inside diameter of the exposed end of the female section.
[American Association of State Highway and Transportation Officials 2013, ‘Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals’, Section 5.14.9]
SHIELDING MULTIPLIER (Ms)
The shielding multiplier (Ms) allows for a reduction of the effective wind speed when there are structures upwind of the structure under consideration, of equal or greater height than the height of the wind load being calculated. In the case of a chimney, the latter is a general height ‘z’. In all other cases it is the average roof height of the structure, ‘h’.
[Australasian Wind Engineering Society 2002, ‘Wind Loading Handbook for Australia and New Zealand’, p. 37]
DRAG FACTOR FOR CIRCULAR/18-SIDED/20-SIDED MONOPOLES
Projections such as ladders and pipes from the side of the circular cylinder will promote flow separation for wind directions for which the project is at, or about, 90° from the windward generator. For those wind directions a drag coefficient of 1.2 should be used in the vicinity of the projection, irrespective of the value obtained for the “clean” cylinder. This value should be adopted along the axis of the cylinder for a length equal to one cylinder diameter, either side of the projection.
[Australasian Wind Engineering Society 2002, ‘Wind Loading Handbook for Australia and New Zealand’, p. 67]
Load heights are defined by their relative height above the monopole base level (AMBL), which is often (but not always) the ground level. If the structure is on a rooftop, specify a value for the “Override Monopole Base Level AGL” option in the Settings menu under the “Wind” tab. If the structure sits on a plinth, specify it in the “Structure” tab under “Design” > “Foundation”.
The equivalent static shear pressure for galloping only needs to be applied to the surface area of sign panels, traffic signal heads, and backplates.
[American Association of State Highway and Transportation Officials 2013, ‘Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals’, Section 220.127.116.11]
This pressure range shall be applied along any 3.7 m length to create the maximum stress range, excluding any portion of the structure not located directly above a traffic lane. The equivalent static truck pressure range may be reduced for locations where vehicle speeds are less than 30 m/s.
The magnitude of applied pressure range may be varied depending on the height of the horizontal support and the attachments above the traffic lane. Full pressure shall be applied for heights up to and including 6.0 m, and then the pressure may be linearly reduced for heights above 6.0 m to a value of zero at 10.0 m.
The given truck-induced gust loading should be excluded unless required by the Owner for the fatigue design of overhead traffic signal structures. Many traffic signal structures are installed on roadways with negligible truck traffic. In addition, the typical response of traffic signal structures from truck-induced gusts is significantly overestimated by the design pressures prescribed in this article (NCHRP Report 469). This has been confirmed in a recent study (Albert et al., 2007) involving full scale field tests where strains were monitored on cantilevered traffic signal structures. Over 400 truck events were recorded covering a variety of truck types and vehicle speeds; only 18 trucks produced even a detectable effect on the cantilevered traffic signal structures and the strains were very small relative to those associated with the other fatigue design pressures.
Due to the complexity of defining the truck-induced gust simply, CHECKPOLE does not include truck-induced gust in the fatigue analysis. Should this be required, the analysis should be carried out separately.
[American Association of State Highway and Transportation Officials 2013, ‘Standard Specifications for Structural Supports for Highway Signs, Luminaires and Traffic Signals’, Section 18.104.22.168]
CHECKPOLE utilises the Rayleigh method to calculate the first mode natural frequency of the monopole, which may lead to slight variations in results from other analysis packages. From a practical standpoint, such a small discrepancy can be ignored as the natural frequency calculation is extremely sensitive to even small changes in geometry, stiffness, and applied masses.
For convenience, input flexibility, and to minimise analysis time, CHECKPOLE defines nodal locations based on the geometry of the monopole only and not the location of the loads. Therefore, the exact deflection caused by area/point/linear masses can only be known if said mass happens to be defined at one of these nodes, which is generally unlikely. Therefore, a close approximation utilising the axial force envelope is used (including area/point/linear masses) at each defined node which has a known deflection and centroid.
Furthermore, as the stiffness of the structure above the top of the monopole (including the height of any defined vertical extensions) is not defined within CHECKPOLE, the accuracy of the calculated natural frequency will be reduced as the height of an area/point mass above the top of the monopole increases. CHECKPOLE will warn users if the height of an area/point mass is located more than 5% of the total monopole height above the top of the monopole.
CHECKPOLE assumes an elastic stress distribution with all compression and moment forces being transferred directly into the anchor bolts. Grout is ignored for bolt forces due to the construction sequence for monopoles and the almost universal utilisation of levelling nuts.
[American Society of Civil Engineers 2012, ‘ASCE/SEI 48-11 Design of Transmission Pole Structures’, pp. 71]
It has been found that chimneys of circular cross sectio n oscillate more strongly across wind than along wind. This regular fluctuating side force, known as “Von Karman vortex shedding”, will produce strong oscillations at a velocity which gives resonance with the natural frequency of the structure.
The effect is resisted by high damping and may be prevented by helical strakes or other devices attached to a circular chimney. If the tendency is strong, it is not effectively withstood by an increase in strength alone. Fatigue damage may result if oscillations occur regularly even if stresses are lower than design values.
In a natural wind, the regular vortex shedding may be interfered with by fluctuation of the wind, so that the build-up of amplitude is not continuous as in a wind tunnel, and it may be more effectively resisted by mass and stiffness. There are different views as to how the vortex shedding should be allowed for in practice, but it is clear that low damping, low mass and large flexibility will increase the probability and amount of the oscillation of chimneys. In many cases the behaviour cannot be predicted with certainty. It is possible to calculate from information available that some chimneys are likely to oscillate excessively, and others are not. In some cases the oscillation will be limited to safe amplitudes if there is a margin of strength.
In the present state of knowledge it may be accepted that, even in the worst cases, a chimney can be made safe by applying guys or strakes at any time after construction, if experience shows them to be required, and provided that the chimney is made strong enough in the first place to withstand the additional load from guys or strakes applied later.
[British Standards Institute 1999, ‘BS 4076:1989 Specification for steel chimneys’, Appendix B)
The character of the vortex shedding forces for circular cylinders depends on the Reynolds number. While shedding tends to be organized at sub-critical (Re < 3 x 105) and trans-critical (Re > 3 x 106) Reynolds numbers, it tends to be irregular in the critical range (3 x 105 ≤ Re ≤ 3 x 106) unless the structural motion is sufficiently large to organise the fluctuating flow around the body. This phenomenon, referred to as “lock in”, becomes important for lightly damped members.
Vortex shedding tends to occur with steady continuous winds at a critical velocity. The velocity need not be very high, but it has been found that significant vibration does not occur unless the velocity is greater than 5 m/s. The periodic frequency of the vortex shedding can lock in on the natural frequency of the pole, resulting in very large alternating forces acting transverse to the wind flow direction. Occasionally, the vibration is so severe that fatigue cracks will appear.
Although vortex shedding can “lock in” and continue as the velocity increases or decreases slightly, if the velocity changes by more than 20 percent, the vortex shedding will stop. Gusty variable winds, such as might occur in a severe storm typically will not cause vortex shedding. In fact, if the wind velocity is greater than 15 m/s, the wind is generally too turbulent for vortex shedding to occur. In summary, the winds that are dangerous for vortex shedding are steady winds in the velocity range 5 to 15 m/s.
[Giosan, I, ‘Vortex Shedding Induced Loads on Free Standing Structures’, p. 6]
The Strouhal number (St) is a dimensionless number relating vortex shedding frequency (ncr) to wind speed (Vcr) and cross-sectional width (d).
For circular cross-sections, the Strouhal number varies with flow velocity and therefore the Reynolds number (Re). Experimental investigation has shown that the Strouhal number changes between sub-critical, critical, and trans-critical ranges, though a constant value of 0.2 can be assumed for practical application to the analysis of structures.
CHECKPOLE assumes the following values depending on the cross-sectional shape:
Why are my design actions for the G + F + SLS Wind load case larger than the 1.20G + F + ULS Wind load case?
If the monopole natural frequency is less than 1.0 and the ultimate limit state (ULS VR) and serviceability limit state (SLS VR) regional wind speeds are similar/equal, it is possible for the serviceability wind case to give higher values than the ultimate wind case. This is due to the application of Cdyn and the difference in structural damping ratios specified in AS/NZS 1170.2 (Section 6.2.2). This can be avoided by using an appropriate (i.e. lower) serviceability limit state regional wind speed.
Why is the natural frequency calculated by CHECKPOLE different to that calculated by my structural analysis software?
CHECKPOLE uses the Rayleigh method to calculate the first mode natural frequency, which has been recommended by the British Standards since the 60s. The Rayleigh method allows CHECKPOLE to take into account the mass of lap joints, non-uniformity in mass resulting from tapers, and additional masses above the top of the pole, something which a conventional stiffness formulation makes more difficult. Essentially, it breaks the pole into a series of subsegments of discrete masses and centroids at relevant heights.
If the natural frequency for a library pole calculated by CHECKPOLE is lower than you’ve calculated in a program such as Spacegass, that makes perfect sense: CHECKPOLE is taking into account the mass of material at the laps and also any openings along its length which will drastically reduce stiffness, neither of which are taken into consideration by Spacegass.
It’s important not to simply focus on the natural frequency number without putting it into the context of its use within the code. Monopole design is by no means an exact science and our codes are little more than a means of fitting curves to a very erratic series of data points. All formulations for natural frequency (including stiffness formulations) are technically approximations, which are made more accurate by increasing the number of node points. Ultimately, a 5% difference in natural frequency corresponds to a very minimal difference in Cdyn, which is governed more by pole geometry, terrain category and design wind speeds.
Norton AntiVirus warns that CHECKPOLE is behaving suspiciously and tries to remove the software.
For your security, we run a full virus scan on all of our packages prior to releasing updates. To overcome this problem, you will need to add CHECKPOLE to the Exclusion list by following these steps:
Step 1: Click the “Settings” link in the Norton AntiVirus window to navigate to Settings. The Computer tab is selected by default.
Step 2: Click the “AntiVirus and SONAR Exclusions” link and then click the “Configure” link next to “Items to Exclude from Scans.” The Scan Exclusions window opens.
Step 3: Click the “Add” button, then the “Browse” button. Select the file you want to exclude from scans and click “OK” twice to add the file to the Exclusions list. You can select an entire folder, and all its subfolders, if you want to exclude a group of files.
Step 4: Click “Apply” and then “OK” to apply and save the new settings.
Step 5: Click the “Configure” link next to “Items to Exclude from Auto-Protect, SONAR and Download Intelligence Detection.”
Step 6: Use the same method to add the file or folder to the Auto-Protect, SONAR and Download Intelligence Detection Exclusions list.
Step 7: Click “OK” to return to the main Norton AntiVirus window.