Type of towers – stiff, soft or soft soft?

In the last month I spent a lot of time discussing about “soft soft” towers.

But what does it exactly means?

Steel tower for wind turbine are classified as stiff, soft, or soft soft based on the relative natural  frequencies of tower, rotor and blades.

You obviously want to avoid that your tower is excited by dynamic loads and start resonant oscillations.

The primary sources of dynamic loads on the tower are the rotational speed of the rotor (usually indicated with P) and the blade passing in front of the tower. The blade passing speed will obviously be 3P. I think that it’s worth mentioning that rotational frequency loads will arise only when the blades are unbalanced.

We call “stiff” (or “stiff stiff”) a tower whose fundamental  natural frequency is higher than that of the blade passing frequency. This is a very good thing (the tower is unaffected by  the rotor) but a bigger mass is needed – therefore the cost can be very high. Additionally, a stiff tower tends to radiate less sound.

“soft” is a tower whose fundamental frequency is lower than the blade passing frequency, but above rotor frequency.

“soft soft” is a tower whose natural frequency is below BOTH rotor frequency and blade passing frequency.

“stiff stiff” design is not usual.

Currently, towers in  the market are either “soft stiff” or “soft soft”.

Soft towers are usually lighter (= cheaper) but require more dynamic analysis.

Concrete towers for onshore wind farms: an overview

Concrete towers are still an uncommon solution nowadays, basically they are still more expensive than they steel counterparts.

But the trend we see in the market is toward highest tower, of 100 meters and above, while current 2MW-3MW turbines normally have an hub height around 80 meters: this can lead to situation where an hybrid solution (concrete base plus steel top) or even a full concrete solution is finally economically profitable.

The reason for this increase in tower size is the need to increase the productivity (the wind speed increase exponentially with the eight) and to overcome surface friction, although it must be noted that almost every country has laws to limit the height of the tip of the rotor (on average, around 100 meters).

The biggest problem of tapered steel towers is that they maximum dimension in onshore wind farms is limited by transport issues: normally the biggest diameter allowed to circulate on public highways is below 5 meters, due to the free height of existing bridges. Tallest towers need bigger diameters, so there is a legal/technical limitation to the use of steel.

Several turbine manufacturers (for instance Enercon, GE and Nordex) have full concrete or hybrid towers in their catalogue, normally with eight above 100 meters. Often different sections are considered for the purpose of strength and stiffness design, fabrication and erection:

  • Base zone, made of thick walled precast concrete segments or in situ concrete. Here the thickness can be around 40-50 cm.
  • Middle zone: here the wall thickness is determined by concrete cover to reinforcement rather than by the necessary strength and stiffness, so a saving in material is possible.
  • Upper zone: here the wall thickness will be around 10 cm only. It normally includes a steel section of about 2 meters to connect the yaw ring and the nacelle.

Various configuration, techniques and details have been proposed, all of them normally considering the use of vertical prestressing. It is normal to see solution with concrete rings divided in 2, 3 or even 4 segments, assembled and joined together in situ with 2 type of joints (vertical and horizontal) using mortar, fishplates or other technical solutions.

The weight of the components is really high: can be 50+ tonnes (in some solution even more), so there can be cases where it is more difficult to lift a tower section than the nacelle.

Several documents with conceptual design of concrete wind tower are freely available on the web.

Check for instance:


Concrete Towers for Onshore and Offshore Wind Farms




Wind turbine tower

Towers are an important element in the structure of a WTG, not only for structural reasons (they transmit the loads from the nacelle to the foundation) but also for economical reasons: they highest the tower, the biggest the amount of energy produced.

For instance, with an increase of 20 meters from the standard 80 meters height an additional energy production around 5% can be obtained. In the future higher towers are expected, also because in mature market the most productive locations have already been used.

The average height of tower installed in Europe is around 80 meters. Modern towers have a lift inside, a ladder and several intermediate platforms. Another item that can be hosted in the tower, normally at the bottom, is the MW transformer.

Towers, above all standard steel towers, aren’t a high technology product and there are several companies with the expertise and capabilities to do them. For this reason they are often outsourced.

Towers can be made of several material following different design concepts:

  • Lattice
  • Steel (tubular or segmented)
  • Concrete
  • Hybrid

Lattice tower were common in the past when turbine where smaller (lass than a MW), but are seldom used today. Their biggest problems are a notable visual impact, and higher construction and maintenance costs. They have several advantages: they use less materials (about 50% of a standard steel tower with the same stiffness) and they produce less shadow.

Tubular steel towers are the most widely used solution. They normally have a conical shape and a diameter varying from approximately 4.5 meters at the base to 2 meters at the top, divided in 3 or 4 sections assembled at the wind farm (they are bolted together). The length of a section can vary from 20 to 30 meters. Basically they are manufactured with steel sheets cut, rolled and welded.

The new steel towers of more than 100 meters have a base section diameter over 5 meters: this can be a problem, because in many countries the maximum transportable size by road is less than 4.9 meters.

Siemens is currently working with Andresen Towers to a longitudinally, on site bolted steel-shell tower.

Concrete towers are a solution in countries were steel price is unusually high (for instance in Brazil, where steel production is almost a monopoly). They are made of several smaller precast pieces assembled on site. This solution allows an easier transportation due to the smaller dimension of the components and a good control of the quality of the materials. They biggest problem is the weight (unless they are designed in a biggest number of pieces, they can weight more than the nacelle)

Hybrid towers are another solution used by several manufacturer to reduce the exposition to the steel price volatility, the main drawbacks is that they are quite complicated to assembly, so they have higher installation costs.

Other solutions are available (for instance guy-wired pole tower), but they are used only in very small turbines.