The River Theater: An Interface of Geometries

June 15, 2015
Chicago Riverwalk 34

The Riv­er The­ater is per­haps the most excit­ing ele­ment of the Chica­go River­walk exten­sion. Its hum­ble pro­gram­mat­ic goal is to serve as a ver­ti­cal cir­cu­la­tion path. Yet it aspires to be a place of great dynamism — a river­side land­mark, a back­drop for fash­ion shoots, seat­ing for a con­cert, a work-out space, the scene of an action movie chase sequence! At oth­er times it may seem serene and per­son­al, as vis­i­tors relax on ter­raced seat­ing under the canopies of hon­ey locust trees. In fact, we first called the space the Riv­er The­ater” imag­in­ing spec­ta­tors qui­et­ly tak­ing in scenes of the riv­er itself — the mov­ing water chang­ing col­ors with the weath­er, kayak­ers pad­dling by, tour boats, and traf­fic pass­ing along the bridges above.

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Slope variations

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Pre-cast concrete modules

The Riv­er Theater’s form looks rather com­pli­cat­ed, but is gov­erned by a series of sim­ple rules. As I men­tioned, the space’s duty is to pro­vide a cir­cu­la­tion path, uni­ver­sal­ly acces­si­ble, con­nect­ing Upper Wack­er Dri­ve and the River­walk. In the greater plan of the River­walk, acces­si­ble entry points are pro­vid­ed every cou­ple of blocks. To the east, Wabash Plaza (between Wabash and State Streets) fea­tures a switch­back ramp. A future ramp will be pro­vid­ed west between Franklin and Lake Streets. Between these points the Riv­er The­ater is built around a sloped walk­way that grad­u­al­ly links side­walk to riv­er. Instead of a steep ramp, we felt a gen­tler walk­way was more appro­pri­ate for such a focal civic space. This means tech­ni­cal­ly that the slope must be 5% or less to avoid the place­ment of rail­ings and inter­mit­tent land­ings, those required ele­ments that char­ac­ter­ize the geom­e­try of ramps (between 5% and 8.3% in slope). Ver­ti­cal­ly, Wack­er Dri­ve and the River­walk path are sep­a­rat­ed by close to 20 feet, mean­ing a 5% walk­way con­nect­ing the two grows to 400 feet in length.

At first glance, this seems to leave lit­tle space for oth­er pro­gram­mat­ic ele­ments in our space, which mea­sures about 300 feet long and 40 feet wide. But, we were famil­iar with many instances where ver­ti­cal cir­cu­la­tion — stairs and ter­races — serve as flex­i­ble and pop­u­lar gath­er­ing spaces. The grassy over­looks of the near­by Wabash Plaza, a por­tion of the River­walk we designed that was com­plet­ed in 2005, are filled with tourists and lunch-break­ers when the weath­er is even remote­ly pleas­ant. Rome’s famous Span­ish Steps and those under the grand arch of La Défense in Paris are land­mark gath­er­ing spaces as much as they are func­tion­al stairs. With these exam­ples in mind we saw an oppor­tu­ni­ty to blend our grad­ual path with a ter­raced seat­ing area and a stair — some­thing that would smooth­ly con­nect the riv­er, to which access has become lim­it­ed, to the streetscape above. The result is a ver­sion of the form affec­tion­ate­ly referred to by its com­pound name, stramp,” although in our case it is nei­ther whol­ly a stair (the cen­ter wedge is ter­raced seat­ing) nor a ramp (it is by def­i­n­i­tion a sloped walkway).

Existing condition

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Land creation

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Slopped concrete slab

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Pre-cast concrete steps

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Prece­dent stramps” whose geom­e­try we inves­ti­gat­ed, and there were sur­pris­ing­ly many, dis­play a geo­met­ric rig­or that reveals itself in a clear and ele­gant way: as the path slopes up to a height equal to one stair ris­er, it jogs up-flight, shift­ing over the width of one stair tread. In this man­ner the stairs blend with the path. There are sub­tle vari­a­tions to this method, but the Riv­er The­ater devi­ates more dramatically. 

The rea­son is two-fold: first, we want­ed to pro­vide a com­fort­able seat­ing area for peo­ple to gath­er. This was achieved by design­ing dou­ble-sized stairs between the switch­back slopes. The sec­ond rea­son was a reac­tion to the restrained depth of the space. 

By a Con­gres­sion­al act, the design was allowed to project into the exist­ing nav­i­ga­ble chan­nel of the riv­er an addi­tion­al 25 feet from its pri­or extent. This gave us about 40 feet total in width. Unfor­tu­nate­ly, even build­ing a sim­ple stair would take up more than 31 feet of this, leav­ing lit­tle room for the stramp” path cut­ting through it, nor the river­front path that con­nects each space along the Riverwalk.

To address this spa­tial con­straint, the inner edges of the sloped path were off­set to com­press the shape while main­tain­ing a com­fort­able cir­cu­la­tion width. The oth­er device was to notch the dou­ble-size treads, cre­at­ing a pro­ject­ing fin­ger” allow­ing the treads to fit tighter to the path. In total, this approach used about 15% less width than the sim­pler stramp” prece­dents, and enabled a gen­er­ous 8 – 10 foot wide river­front path.

Tree installation

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Finished River Theater

Chicago Riverwalk 43

As the design advanced, we inves­ti­gat­ed how the stramp” might best be con­struct­ed. Because the form devel­ops in a repet­i­tive, mod­u­lar man­ner, it rep­re­sent­ed a good oppor­tu­ni­ty for pre­cast con­crete, which can be pro­duced rapid­ly and with a con­sis­tent qual­i­ty fin­ish, with­out the cost of stone. Stone steps are often con­struct­ed by dow­el­ing tread-shaped blocks into a sloped con­crete slab that sup­ports them. Work­ing with our engi­neer­ing con­sul­tants, we devel­oped the Riv­er The­ater in this same man­ner — a cast-in-place sloped slab, com­pli­cat­ed where the trans­verse slope of the path inter­sects, sup­ports mod­u­lar pre­cast blocks that make up the ter­races, stair treads, and path pavers.

Between the sloped slab and the marine struc­ture below (the land­fill and con­crete enabling us to build out into the riv­er), is a wedge-shaped void. With­in it are three con­tin­u­ous, lin­ear planters that sup­port the 17 hon­ey locust trees that punc­tu­ate the space above.

A water-har­vest­ing sys­tem col­lects the Riv­er Theater’s drained stormwa­ter in a struc­ture under the sloped slab. The stored water then sup­plies an irri­ga­tion sys­tem to sup­port the trees.

The Riv­er The­ater was con­ceived as a space where Chicago’s streetscape folds down to the riv­er in a smooth and dynam­ic way. We hope the geo­met­ric log­ic of the stramp” is leg­i­ble in its form, and that those who tra­verse it find that they enjoy deci­pher­ing its rules. Mean­while, the block-long The­ater offers itself as a unique set­ting for years to come, treat­ing down­town vis­i­tors to a per­haps unex­pect­ed place of both qui­et reflec­tion and dynam­ic excitement.

Morde­cai Scheck­ter, AIA, LEED AP

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