With a total surface area of only two square kilometres, the Frankfurt bridges do not influence the climate of the 250 square kilometre city of Frankfurt on a mesoscale, but only on a microscale

Accordingly, the Frankfurt bridges indirectly do a lot of good for the city climate locally: by reducing road traffic, watering green spaces and trees throughout the inner city area, and as an alternative to the usual sealing of open spaces as a result of the increasing demand for housing

In order to identify the neuralgic points at which the bridges in Frankfurt pass, the Frankfurt Climate Plan Atlas and the Rhine-Main Air Quality Control Plan were used as a basis

Thermal load and pollutant load are highest in the inner city - (1) Thermal load

The climate plan atlas shows which urban areas tend to overheat in summer, making it too hot and/or too oppressive for people to live there. Excerpt from the Frankfurt Climate Atlas: The darker the red, the more overheated the area.

Thermal load and pollutant load are highest in the inner city - (2) Pollutant load

The nitrogen measurement plan along Frankfurt's major traffic axes is a good indicator of the overall traffic-related pollutant load in the respective urban areas.

The map section from the nitrogen oxide analysis of the LRP Frankfurt shows in red areas with the highest pollution.

If one compares the neuralgic areas on both plans, there is an exemplary location on Mainzer Landstraße that is critical in both plans

Content of the urban climatic analysis: How will the thermal and wind dynamic conditions along Mainzer Landstraße towards Platz der Republik change due to the construction of the Frankfurt bridges?

The effect of the bridge body, including the buildings as well as the new greenery (on and next to the bridges) has been investigated with regard to thermal load and wind comfort. The following questions were taken as a basis:

What is the benefit in terms of thermal conditions of a bridge compared to the current situation?

How do the parameters PET (physiologically equivalent temperature) and wind speed change in the street space or in the area of the nearest buildings?

Which fluid dynamic effects and modifications of the wind field are caused by the bridge?

How far does the thermal and wind dynamic influence of the bridge extend laterally into the development?

When delineating the study area for the effects of the bridge construction, the adjacent areas must also be taken into account

Two simulation runs were made for each of the two model-based analyses

The basis for the simulation was a detailed model of the bridge section and the road with the existing buildings

Thermal analysis: The thermal situation was simulated under the meteorological conditions of a hot summer day preceded by a tropical night

In this weather scenario, daytime highs exceed 30° C, while skies are cloudless and overhead winds are light. At night, temperatures do not drop below 20° C.

In temperate latitudes, such hot days are particularly frequent in July and August. Outdoor swimming pools and ice cream parlours are very popular - and shady areas are increasingly sought out.

In order to illustrate how people feel thermally under such weather conditions - without or with the Frankfurt bridges - the "physiologically equivalent temperature" (PET) is used

The effects that the Frankfurt bridges have on the characteristics of the PET can be seen in principle from a direct comparison of the model results of the zero and plan cases

The comparison shows: Without the shady bridge, the PET rises up to 37 degrees on a hot summer day - with the bridge, however, it is only 27 to 29 degrees

The situation is shown at 3 p.m. (summer time), when the highest daytime temperatures prevail and the thermal load tends to be greatest. The height level 2 m above ground describes the conditions in the lower level of Mainzer Landstraße; the level 10 m above ground corresponds to a height of 2 m above the bridge deck.

In the area close to the ground, the temperature deviations can be seen very easily: Without the shady bridge structure, the PET in the Mainzer Landstraße area reaches peak values of 35 or even 37 °C almost everywhere, while the temperatures there are often only between 27 and 29 °C with the bridge corpus in place.

When comparing the PET on the bridge plate, however, the differences are not immediately obvious. This requires a different form of representation...

The PET differences between the actual and the planned state can best be shown by means of so-called "difference maps"

If you look at the PET in the cross-section of Mainzer Landstraße, the lowering of the PET under the bridge caused by shadows becomes visibly apparent. The extent to which it becomes more bearable there in summer is enormous

The Frankfurt bridges cause a slight modification of the thermally induced wind field

If the superordinate flow field has only low wind speeds, a thermal wind system forms in the city in summer due to temperature contrasts and the resulting pressure differences. Due to the dense building structure of Frankfurt's inner city, however, this is already very weak in the area of Mainzer Landstraße in its current state. At the hottest time of day (3 p.m.), the wind in the street area is rarely stronger than 0.8 m/s at an altitude of 2 or 10 m above ground - a gentle breeze that corresponds to wind force 1 on the 13-step Beaufort scale and is barely perceptible to people. In the inner courtyards of the peripheral buildings as well as in the cross streets leading into the city, there is practically no wind at all.

In the plan condition, the situation does not look fundamentally different both in the lower level and on the Frankfurt bridges. It is true that the wind speeds in the course of Mainzer Landstraße show a tendency towards a reduction in ventilation compared to the actual case - which is not surprising in view of the fact that the Frankfurt bridges and the additional buildings at Platz der Republik act as flow obstacles. However, it is decisive that both the absolute velocities and the reduction amounts (generally less than 0.5 m/s) are on such a low level that in no case a noticeable or physiologically relevant deterioration of the air exchange can be assumed. Of course, this is even more true for the downstream streets in the vicinity of Mainzer Landstraße, where the thermal-dynamic conditions are even less influenced.

The change in wind speed due to the construction of the bridges is so small in the study area that it is hardly perceptible to humans

Interim conclusion: From a thermal point of view, the Frankfurt bridges represent an enrichment for Frankfurt's urban climate

From a thermal point of view, the Frankfurt bridges do not lead to a significant deterioration of the current situation. On the contrary, the shadows cast by the bridges, their discreet structure and their greenery create a climatic enhancement of Mainzer Landstraße that can be transferred to all other neuralgic stretches of road.

For thermally induced wind systems, the Frankfurt bridges also represent only a minor flow obstacle, so that even in sunny weather no perceptible impairment of air exchange is to be expected.

In hot summers, Frankfurt's bridges thus create pleasantly cool comfort zones in which shading effects both underneath and on top of the bridges effectively counteract the thermal stress on citizens.

Dynamic analysis: The dynamic situation was simulated under the meteorological conditions of a cloudy day with stronger overlying winds

The flow dynamic effects of the Frankfurt bridges vary depending on the wind direction

The flow-dynamic influences of the Frankfurt bridges can best be highlighted by setting the simulated wind speeds of the plan case in relation to those of the zero case. A value of 1.5 thus implies an increase in velocity of 50 %, whereas a value of 0.75 implies a decrease of 25 %, with very low changes in wind velocity of less than 0.2 m/s being excluded from the outset.

The presented ratio values are - under the given premise of dynamic wind conditions - not bound to certain wind speeds, but are subject - like the corresponding processes in nature - to the so-called Reynolds independence: If one forms the ratio between two states, then this is dependent on the most different wind speeds.

speed levels can be applied. Only when the overriding wind field is so weak that it can hardly be detected in the road space, the thermal effects gain the upper hand over the flow pattern.

In addition, the simulated flow patterns of the actual and planned case can also be compared in the cross-sectional view. This shows whether and to what extent the occurring turbulences are modified by the Frankfurt bridges. The Mainzer Landstraße southwest of Francois-Mitterand-Platz and the Platz der Republik were selected as cross-sections for this purpose, as these are particularly representative bridge sections.

Modification of the flow dynamics for actual and planned case (I) With northeasterly inflow

In the vertical cross-sectional view, it becomes clear that the influence on the flow dynamics remains limited to the area of Mainzer Landstraße: In the street northwest of Mainzer Landstraße as well as in the adjacent block development to the southeast, vortex systems are formed in the actual condition, which generate an upward vertical movement in the street canyon of Mainzer Landstraße. This flow pattern is also largely preserved in the plan case with the bridge structure, since the lateral deflection of the wind below the bridge is not propagated into the airspace above.

The cross-sections also reveal that the effects of the bridge construction on the local surroundings are limited: Thus, the flow pattern in the actual state is characterized by a clearly pronounced spiral vortex in the northwestern area of the intersection, which also arises almost unchanged in the plan case. The flow pattern is only modified in the vicinity of the bridge structure with the buildings standing on it, where a rotating vortex is formed due to displacement effects southeast of the building ensemble.

Modification of the flow dynamics for actual and planned case (II) With inflow from the southeast

Within the street canyon of Mainzer Landstraße, the Frankfurt bridges have no influence on the circulation patterns with a given inflow from the southeast, as can be seen from the cross-sectional view: The ideal-typical vortex, which occurs between the towering buildings in the actual state, also occurs without restriction in the plan case. This is due to the column-based elevation of the Frankfurt bridges, which means that the wind is hardly resisted and can flow under the building. Changes in the ventilation conditions are therefore not to be expected.

At Republic Square, on the other hand, a major modification of the wind dynamic conditions takes place with crossflow: While the flow in the actual state advances almost undisturbed into the open crossing area and only changes into a frontal vortex at the northwestern edge due to the impact on a high building complex, the flow field is to a certain extent "disturbed" by the buildings on the bridge in the planned state. Minor turbulence occurs between the buildings and a wake vortex in the lee of the building complex leads to a flow reversal below the bridge level. This can result in reduced aeration in the crossing area.

Modification of the flow dynamics for actual and planned case (II) With inflow from the southwest

If one compares the vertical sections of the actual state and the planned state with an inflow from the southwest parallel to the road, then differences in the flow pattern are only recognizable for the area of Mainzer Landstraße: Due to the obstacle effect emanating from the bridge structure, lower velocities occur above the bridge in the planned case and higher velocities in the lower area. Below the bridge level, a reduced aeration effect is therefore not to be assumed.

The situation at the Platz der Republik is more complicated: There, the upwind building complex in the northwest leads to the formation of a complex vortex system, which, however, is almost unchanged in the actual and planned state. In both cases, a crossflow is induced towards the southeast, which is in turn locally modified by the bridge structure. It follows that the velocity level below and above the bridge level is locally reduced to a small extent.

Interim conclusion: The Frankfurt bridges can be considered unproblematic for ventilation and the air-hygienic situation

In accordance with expectations, the Frankfurt bridges as flow obstacles lead to local modifications of the wind field, which is pre-dominated by the high buildings on Mainzer Landstraße. Small-scale, highly complex effects occur in the dynamic flow pattern, with zones of reduced and zones of increased wind speed balancing each other out.

Under the Frankfurt bridges, wind acceleration may occur in some places, resulting in better ventilation of the street space. However, there is a tendency for many flow patterns that already occur in the current state of Mainzer Landstraße not to be significantly affected, which is due to the columnar character and associated wind permeability of the Frankfurt bridges.

In any case, the dynamic effects of the bridges are always limited to the immediate vicinity, which is why no far-reaching effects can be assumed.

Since Mainzer Landstrasse is considered a "worst-case" example of the Frankfurt bridges' route due to its exceptionally high and dense perimeter development, all other - less critical - route sections should at best exhibit similar flow patterns and not cause any deterioration in the dynamic effects.

If, contrary to expectations, the urban climatic investigation of other critical points in the bridge route should reveal negative effects in advance, there is also the possibility of providing the bridge corpus with air holes protected by railings in order to improve ventilation.

In addition to the thermal and dynamic analyses under consideration, investigations of the immission load are also common urban climatic pre-planning analyses - which could be omitted here due to the long planning horizon.

The "Frankfurt Bridges" construction project is expected to require a planning phase of 5 years and a construction period of 10 years. In this time horizon, a drastic change in the immission landscape can be expected due to the change in vehicle engines in Germany. Since vehicle emissions, while not 100%, account for a significant portion of the pollutant load on the roads, it is not possible at this point in time to make valid planning analyses for the effects of the Frankfurt bridges in the future on the current data basis.

However, should the introduction of e-cars and hydrogen vehicles proceed at the planned rate, such a significant improvement in the air in our cities can be expected that the bridge structure should have extremely little impact by today's standards in such a "clean" environment.

Immission levels in Hessen have been falling for years - By the time the Frankfurt bridges are completed - in 2037 at the earliest - the situation is likely to ease further

Immission levels from inner-city traffic have fallen continuously in recent decades. In Hesse, for example, as in the rest of Germany, emissions are already below the EU limits. This is due, among other things, to improvements in the emission values of internal combustion engines, which have to meet increasingly stringent requirements. However, a greater reduction can only be expected when internal combustion engines disappear from cities altogether.

Electric cars are being touted as the emission-free alternative of the future, but even they cannot completely escape responsibility for particulate pollution: The reason for this is the general brake abrasion of vehicles - and e-cars are about 300 kg heavier than conventional gasoline engines due to their large batteries, according to studies, so more abrasion is generated by tire rubber and brake pads when braking. However, this problem is being counteracted by increased lightweight construction, recuperation technology and also research into multi-disc brakes, so that by 2035 the trend is again expected to be offset to some extent.

Pollution levels in Frankfurt have also shown a downward trend in recent years

The fact that the EU limit values for the annual mean values of PM10 and PM2.5 have not been exceeded for many years is an indication that Frankfurt's city air has also become increasingly clean in recent years. At the same time, however, the WHO guideline values, which have only been met in isolated cases and will be tightened again in 2020, make it clear that there is still a great need for action to ensure healthy air quality in Frankfurt.

For each city, however, it must be examined separately whether the concept of the Frankfurt bridges also produces a positive - or at least neutral - effect on the urban climate there

Many cities suffer from housing shortages, water shortages for their green spaces, and space shortages for their very ambitious renewable energy expansion goals.

The concept of the Frankfurt bridges could help here - but in order to be able to make definitive statements about its urban climatic compatibility, a separate microclimatic simulation of the potential effects caused by the bridge structures is indispensable for every other city.

This is because not only the building structure of a city, but also many other factors such as the regional climate, the location of the city in the relief, etc. are of central importance for the assessment of such an infrastructural construction project.