Moser Tower Structural Assessment
Editor’s Note / The following information was provided to the media during a press conference on June 1, 2017, in the Naperville Municipal Center.
This report provides a comprehensive assessment of the current condition of the Moser Tower’s structural elements. The assessment focuses on the interior and exterior portions of the tower which were analyzed by visual inspections performed by experienced staff. The interior inspection of the structure was performed by Engineering Resource Associates, Inc. (ERA) and the exterior inspection was performed by Collins Engineers, Inc. (Collins) as sub-consultants to ERA.
The original architectural design drawings prepared by the Charles Vincent George Design Group in 1999 were referenced to guide the interior and exterior inspections and to inform the inspections regarding structural connections encased in concrete or otherwise hidden from view. The City of Naperville Reserve Study prepared by Joseph C. Renn, PE was also instructive regarding the Moser Tower’s history.
The Moser Tower’s structure is composed of individual precast, prestressed concrete sections that have been post-tensioned together to form four major vertical pillars. The four pillars have been laterally braced together with seven structural steel rings intermittently spaced over the tower height. A review of the precast concrete shop drawings would help identify how these connections were made. The precast concrete shop drawings were not available for us to reference; therefore, any such changes are unknown at this time.
Appendices A and B each include a comprehensive summary of interior and exterior visual observations along with corresponding photos. Appendix C includes the original architect’s drawings prepared by the Charles Vincent George Design Group.
Moser Tower was constructed in 1999 and 2000 by the Millennium Carillon Foundation. The Tower was originally intended to be an enclosed building to a height of 72′ 9″ above ground level. Construction was terminated due to a lack of adequate funds. At that time, the main structure had been erected. Site grading, electrical service, lighting, fire alarm system, and carillon cabin access were incomplete. Over the next four years, several concrete pieces broke loose and fell from the structure. Several repair attempts were made with epoxies, sealants and other materials with little success.
In 2005, the City retained a consultant to investigate the cause of the spalling concrete. The report identified that grout expansion within pockets of the precast concrete elements of the structure was the primary contributing factor leading to freeze-thaw damage and further spalling. The full report outlining the details of this investigation may be found in Appendix E. The last documented occurrence of concrete pieces falling from the structure was in 2005.
In 2005, the City of Naperville assumed responsibility for construction of the project bringing the structure to completion. In doing so, the portion of the Tower below the 72′ 9″ level was converted to an exposed structure, associated mechanical systems were deleted, and site work was simplified. A significant consequence of that decision was that completed structural steel to precast concrete connections, which were originally intended to be protected from the weather, were now exposed. All of the steel connections now exposed to weather are, therefore, more susceptible to corrosion. This corrosion is particularly serious where moisture trapped between steel and concrete leads to plating of the steel, associated volume expansion of the embedded steel, and cracking of the concrete where the connection is responsible for structural lateral stability. Another consequence of converting the lower portion of the Tower to an exposed structure was that the elevator was no longer protected by an enclosure which may shorten the length of its useful service life.
It is now evident that weather intrusion has caused premature corrosion and deterioration of the structure. Improper storage and handling of many of the precast concrete panels between the first and second phases of construction caused extensive cracking and chipping. These deficiencies were repaired with mortar and patches which are now showing signs of deterioration and failure. Further, the current degree of surface erosion in some of the precast concrete panels appears to be an indicator of poor quality concrete. Photographs of these observed deficiencies are included in Appendix D.
In our professional opinion, the primary structural risks are:
- Deteriorated concrete patches and cracked concrete failures leading to falling concrete without notice.
- Reduction in lateral structural stability due to structural steel connection corrosion between the precast pillars, fins, and the seven structural steel rings.
- Structural damage due to steel stair, steel platform, steel railing, and reinforcing bar corrosion from weather intrusion and leakage.
Based upon our investigations and observations of the structure, in order to repair deficiencies and to prevent further deterioration which threatens the structural integrity of the Moser Tower, we recommend continued close monitoring and addressing of the following primary issues.
- Structural steel corrosion
- Post-tensioning anchorage exposure
- Precast concrete cracking
- Precast concrete surface delamination
- Precast concrete mortar joint deterioration
- Sealant deterioration
- Plaza leakage
It is anticipated that addressing these issues will require a combination of maintenance, rehabilitation and replacement of certain elements. Structural details showing the type and extent of potential improvements are included in Appendix F.
Alternatives with Advantages & Disadvantages
Several alternatives to repair and rehabilitate the structure were evaluated. The following is a summary of each alternative and their estimated costs. Preliminary engineer’s opinions of probable construction costs are provided in Appendix G. Costs outlined in these estimates are conceptual in nature, provided in present worth dollars and shall be refined during the design phase.
Alternative 1A–Repair Existing Structure (Single Phase) $2,785,000
Alternative 1B–Repair Existing Structure (Multiple Phases) $3,058,000
Alternative 1 includes repair of identified defects as described in the report to address structural steel and concrete issues as either a single phase or multiple phase project. The primary advantage of this alternative is that it addresses issues that have led to continued, accelerated deterioration of the structure. The primary disadvantage is that it is significantly more expensive than continued maintenance and parts of the structure that were intended to be enclosed will continue to be exposed to the elements.
Alternative 2A–Enclose and Repair Structure (Single Phase) $3,547,000
Alternative 2B–Enclose and Repair Structure (Multiple Phases) $3,750,000
Alternative 2 includes repairs described in Alternative 1 plus enclosure of the lower portion of the structure as it was originally designed as either a single phase or multiple phase project. The primary advantage of this alternative is that it addresses identified structural issues and protects the lower portion of the structure from continued exposure to the elements. The primary disadvantage is that it is the most expensive alternative.
Alternative 3–Structure Maintenance & Decommission Structure $1,576,000
This alternative involves simple maintenance measures to keep the structure in operation including plaza membrane repairs; concrete panel surface and crack repairs; mortar joint repairs; structural steel cleaning, reinforcing and painting; anchorage flashing repairs and replacements; and sealant replacements. The primary advantage of structure maintenance only is that it delays much more significant expenditures to repair identified defects or to enclose the structure as originally designed. The primary disadvantage is that the structure will continue to deteriorate at defect and exposed locations and it does not address potentially imminent concrete spalling issues.
At some point, maintenance may become too expensive and the structure may need to be decommissioned; therefore, this alternative also includes the dismantling and removal of the entire structure and restoration of the site to a grassed condition. This would involve a one time expenditure to decommission the structure and would end the use of the Moser Tower and the Millennium Carillon.
The purpose of this investigation and report is to substantially define the Moser Tower’s current structural condition and to develop recommendations for potential repairs to deter further deterioration and extend the structure’s useful lifetime.
In 1999, the Moser Tower and Millennium Carillon was conceived and adopted by the local community as a civic symbol to mark the entry into the 21st millennium. The initial concept was that the 160-feet tall tower would house a 72-bell carillon, a small exhibition center, and represent a public amenity to complement the adjacent Riverwalk and parks.
The tower comprises a reinforced concrete foundation which supports a composite architectural precast concrete facade with an integral structural frame. The tower was planned to be enclosed with glass up to the 72′ 9″ level, and the interior atrium so formed to be mechanically heated, air conditioned, and ventilated. The entire enclosed area would be protected with an automatic fire sprinkler system and fire alarm system.
Construction on the tower commenced in 1999 and was terminated in 2001 due to lack of funds. The structural work completed included concrete foundations, the steel tower structure, the precast concrete facade, structural steel for the elevator shaft and proposed roof at 72′ 9″ above ground level. Major rough site grading was completed plus an access road but the majority of sidewalks, steps, retaining walls, lighting, a water feature, etc. were not completed. The carillon cabin was enclosed and temporary heating and air conditioning installed. Carillon bells, clavier and controls were installed and operational.
The final construction cost for Phase I construction was approximately $3,820,000. This included architectural, engineering and construction administration costs. In 2004, the City developed a completion plan for the tower that would result in substantially less cost to complete construction and result in less operational costs. The concept included:
- Converting the building to an exposed structure by eliminating the glass panels between ground level and 72′ 9″ above ground level.
- Deletion of the mechanical, heating, air conditioning, ventilation, and fire protection sprinkler systems required for an enclosed structure.
- Simplifying the site work and concrete retaining walls and eliminating the water feature at the base of the tower.
In 2005, prior to completing Phase II work as outlined above, the City retained a consultant to investigate the cause of the spalling concrete. The resulting report identified sulfate attack in grout pockets as the contributing factor. The sulfate attack caused the grout to expand and the expansive forces exerted on the precast concrete surrounding the grout pockets caused initial cracking in the concrete. Water was then allowed to enter the cracks which led to freeze-thaw damage and further spalling. Recommendations were made in the report to remove all the grout from these pockets at locations where spalling occurred, remove unsound spalled concrete and develop a repair plan to replace the spalled concrete and deteriorated grout. In general, all unsound grout and concrete were removed; however, grout was not removed at all locations. Grout work only occurred where concrete was spalling. Grout removal at all locations would have required substantial structural review and likely temporary support well beyond the scope or budget of the repair project and, therefore, was not completed.
Phase II work was completed in 2007 at a cost of approximately $3,300,000. This cost included all architectural, engineering and construction administration costs.
Level Descriptions & Components
As shown in Figure 1, the Moser Tower consists of above and below grade levels. Each level consists of a combination of concrete and steel components. Structural observations detected and documented during the interior and exterior inspections are catalogued by level and are included in Appendices A and B.
The primary structure is composed of four precast, post-tensioned, concrete and steel columns. The post-tensioning was designed to place the components under high compression stresses in order to provide resistance to wind and seismic bending stresses. Post-tensioning cables are anchored at each end of the columns and fins. Cable anchorage is critical to structural stability. The four post-tensioned columns are laterally braced with seven polygon shaped, high-strength, steel tube compression rings. The seven compression rings are critical to structural stability. Based upon our inspections, water intrusion and onset corrosion is evident at many of the compression rings. The connections between the four post-tensioned columns and the seven polygonal compression rings are key stability components.
The locations of these features are also shown in Figure 1 below. Figures 2 and 3 include cross-sectional plan views of Moser Tower at each level. Figure 4 shows how columns were inspected and documented from the interior of the structure.
Figure 1: Elevation View Looking West
This drawing is taken from the Charles Vincent George Design Group drawings.
Figure 2: Typical Plan View for Basement Level
Figure 3: Typical Plan View for Entry Level 1 to Cabin Level
Figure 4: Typical Plan View of Concrete Columns
Interior & Exterior Inspection Methods
Structural engineering inspections were visually performed from the ground, interior stair and platform structures, and by rappelling down the exterior of the structure. Structural elements evaluated include precast concrete columns and fins, steel support structures, and steel and concrete surfaces of the stairs and observation decks.
The interior inspections of Moser Tower were completed by ERA between July and September, 2015. Interior facing elements, stairs and platforms were inspected individually within each level of the structure as shown in Figure 1. Visual observations were documented and inventoried including photographs of accessible elements. This information is provided in Appendix A.
The exterior inspection was completed by Collings by rappelling down the tower on August 25, 2015. This work was overseen by ERA to coordinate inspection and documentation procedures. Exterior facing concrete and steel elements were also inspected at each level as shown in Figure 1. Concrete was sounded with an inspection hammer to determine areas of delamination and to identify areas of apparent deterioration and/or distress. This information is provided in Appendix B.
Observations & Analysis
Observations were visually performed from the ground, the interior stairs, platform structures, and by rappelling down ropes along the exterior. Identified defects and damages generally include structural steel corrosion, post-tensioning anchorage exposure, precast concrete cracking, precast concrete surface delamination, precast concrete mortar joint deterioration, sealant deterioration and plaza leakage. Information collected and included in Appendices A and B are summarized below.
Editor’s Note: The information provided in this report by Engineering Resource Associates, Inc. (ERA) and its sub-consultant Collins Engineers, Inc., as well as feedback from attending the press conference led by Director of TED Bill Novack and Riverwalk Commission Chairman Geoff Roehll and subsequent public meetings of the Riverwalk Commission bring to the forefront the difficult decisions facing the future of this amenity along the Naperville Riverwalk.
Every week that the observation deck of Moser Tower has been open, this publication has highly recommended for visitors to climb the tower to observe the magnificent view of this Tree City USA and way beyond Naperville in the distance.
Thanks for reading… Be informed…
RELATED PN POSTs / Notice in Feb. 2015 that the Moser Tower Assessment would begin
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