Beaver+Stadium+North+Stands+-+Construction+Defects+and+Remediation

toc Beaver Stadium, home of Penn State Nittany Lion football, has a long history at Penn State dating back to 1909 when it was originally know as "New Beaver Field". From its original location next to Rec Hall, to its present location at the east end of campus, Beaver Stadium, or Beaver Field, has always been a key part of the University. The stadium has undergone several expansion renovations throughout its history, but the 1991 north-end expansion left people questioning the safety of the stadium. During construction in the summer of 1991 structural cracks were discovered in several of the new concrete columns. These cracks were in close proximity to the cantilevered beams supporting the ramp to the new upper deck. These cracks led to a thorough investigation of the stadium's new structure and required an effective yet efficient solution due to the time constraint to have the north-end open for the football season home opener on September 7.

Key Words
Beaver Stadium, Stadium, Penn State, Structural Cracks, Exterior Post-Tensioning, Cantilever, Concrete

History
1887 - On November 19, the first football game at Penn State was played on Old Main Lawn.

1893 - Football games were played on Beaver Field which was located behind present day Osmond Laboratory.

1909 - Football games were played in New Beaver Field which was located next to Rec Hall (Figure 1). The stadium was made up of wooden bleachers which were eventually replaced by steel (Figure 2). The seating capacity at the time was approximately 14,700.

1949 - North end zone was enclosed creating a horseshoe and the east and west stands were expanded (Figure 3). This increased the seating capacity to approximately 28,000. 1959 - New Beaver Field was dismantled into 700 pieces (Figure 4), moved to it's present location near the Bryce Jordan Center, and renamed "Beaver Stadium". The seating was expanded to approximately 43,990 making it the largest all steel stadium in the US (Figure 5).

1969 - 2,060 seats were added.

1972 - 9,194 seats were added.

1976 - 4,960 seats were added including permanent south end zone seating.

1978 - The stadium was jacked up, raising the original bleachers 12.6 feet (Figure 6), and 16,000 seats supported by a precast concrete structure where added to the stadium. The existing running track was eliminated, and the south end was closed off creating a bowl. 1981 - 7,131 seats were added increasing seating to approximately 83,300 (Figure 7). 1991 - 10,033 seats were added when the upper deck of the north end zone was added to the stadium (Figure 8). 2001 - Club seating, luxury boxes, and additional seating were added to the stadium bringing the total seating to 107,282 (Figure 9).

The Problem
In June of 1991 structural cracking was discovered during routine testing in the final construction phase of the north-end expansion. ("Cracks Found", 1991, p. 3B) The expansion included a ramp providing access to the new north end upper deck (Figure 10). This ramp was to be supported by cantilevered beams projecting 23 feet from (12) 6'x6' columns. Structural cracks up to 2 feet in length were found on 8 out of 12 columns (Figure 11) and were located near the connection between the columns and cantilevered beams (Figure 12).



Failure Theories
The cause of the structural cracking was never "officially" determined. The first reason the cause was never "officially" determined was that once the case was settled, many of the settlements included gag-orders. The reason for this was the desire to keep the problem relatively quiet to limit public concern. The second reason was that there was a sense of urgency to fix the problem rather than spend time determining the cause. Although no official cause was determined, there are two different failure theories concerning the structural cracking of the north-end expansion.

//Note: Information for this section taken from personal interviews.//


 * __Theory 1: Increased Required Curing Time due to Cold Temperatures and Early Removal of Formwork__**

In order to keep construction on track for completion in time for the 1991 football season, the concrete was poured early in the year, beginning in January. As the cast-in-place concrete was completed the form-work was removed, but as the forms were removed the concrete began to deflect. The construction team used temporary shoring to prop up the cantilevers until the concrete no longer deflected. This was done on all of the columns and cantilevers with the exception of one. This column and cantilever didn't deflect when the forms were removed, but did contain the most severe of the structural cracks found later in June. This led to the belief that the formwork was removed to early.

Typically, concrete strength is based on a 28-day curing time. After 28-days, under moderate temperature conditions, the concrete reaches its expected compressive strength. Under cold weather conditions the 28-day strength of the concrete is significantly decreased. ACI 306R-10 defines cold weather as "when the air temperature has fallen to, or is expected to fall below 40 degrees F". For example, under temperature conditions of 40 degrees F the 28-day strength of concrete is decreased by over 15%. Thus, to reach the expected compressive strength of concrete there is an increased curing time under lower temperature conditions. Along with increased curing time, the contractor also needs to prevent the concrete from freezing during early stages; make sure the concrete meets required strength for safe form work removal and maintain curing conditions for proper strength development (ACI 306R-10).

Due to the cast-in-place concrete being poured under cold weather conditions, the concrete may not have reached the expected concrete strength at the time the formwork was removed. The decrease in strength may have been significant enough to cause the structure to deflect under its own weight which would have been a source of the structural cracks.


 * __ Theory 2: Inadequate Reinforcing Hook Length/ Hook Placement __**

When the structural cracks were inspected, it was determined that they extended out around the reinforcing hooks and down the column. If the development length of the hooks was inadequate to tie the cantilever back to the column, the concrete wouldn't have been able to hold up under the tensile forces. ACI 318-11 Section 12.5 specifies the required development length of standard hooks in tension. The design of the reinforcement was examined for the columns and cantilevers, and it was determined that the design met the code requirements.

Although the development length met code, many believed this was not adequate to tie the cantilevers back to the columns. Several of the workers involved in the construction of the cast-in-place concrete said they felt the placement of the hooks was inadequate based on their experience in the construction industry. They felt it would have been a better idea to run the hooks to the back of the column and tie in the reinforcement there. When these concerns were expressed they were disregarded based on the fact that the hooks met code requirements.

If the hooks would have been tied back into the column, the cracking would have had to cross over the hooks, and the cracking and deflections may have been less severe or prevented.

**Solution**
Fixing the structural cracking was more complicated than just designing a solution that worked. A big concern was that there was a limited amount of time between when the cracks were discovered and the beginning of the football season. If the cracks were not fixed, and if the fans didn't feel that it was safe, approximately 10,000 seats would be left empty.

In order to contain the cracking until a permanent solution could be developed, two temporary solutions were utilized. The first was the placement of temporary supports under the cantilevers to carry the full dead load of both the ramp and the cantilevers. The second was the use of stirrups that were placed around the tip of the cantilevers and tied back up through the ramp to the steel structure. (Berkey, 1991) Figure 13 shows both the temporary support used under the cantilever along with the stirrups, while Figure 14 shows the tie-back to the upper steel structure. These solutions allowed a majority of the weight of the cantilevers and the ramp to be carried by the temporary supports and steel structure in order to decrease the stresses causing the structural cracking.



After careful consideration, a specific type of prestressed system was chosen as the solution.This system consisted of external post-tensioning which was effective in restoring the safety of the stadium and was efficient enough to be completed before the first home game.

Prestressed concrete systems involve the use of high strength cables and strands/bars that run through the member and are anchored at its ends. These cables are then stressed using a hydraulic jack. These stresses counter act the external loads. Figure 15 below is a graphic representing these stresses. The cables are most effective when placed near the tension side of the member because they induce compressive forces. Once loaded, the member experiences little to no tensile forces since the tensile forces and compressive forces counteract each.



Post-tensioned concrete systems are a type of prestressed system in which the cables are stressed after the concrete has cured. External post-tensioning is similar to interior post-tensioning with the exception that the cables run on the exterior of the member. According to STRUCTURE Magazine, external post-tensioning systems are "usually engaged in load sharing immediately after installation, and can provide strength increase and instantaneously improve the service performance such as by reducing tensile stresses (or cracking) and deflection. Strengthening with PT is particularly effective and economical for long-span beams and cantilevers members, and has been employed with great success to increase the bending and shear resistance and correct excessive deflections." (Alkhardaji, Thomas, 2003, p. 8-10) This gives merit to the choice to use external post-tensioning as the solution to the structural cracking at Beaver Stadium.

When applying the system to the cantilevers at Beaver Stadium, four steel rods were run along each side of the member (Figure 16), and 5,000 pound anchor plates were placed on the tip of the cantilevers and the back of the columns (Figure 17 and 18). (Schumacher, 1991) These plates were designed to distribute post-tensioned forces uniformly over the outer end of the cantilever, causing the cantilever to be pushed back into the column with a slight uplift. The f orce in rods varied due to different loads and different stress requirements for each cantilever. The cantilevers with the highest stress levels were post-tensioned with rods stressed to produce a force of 159 kips per rod after losses. When the post-tensioning force was applied, it was applied simultaneously to rods on each side of the cantilever. Special hydraulic jacks were used to allow tightening of the nuts on the rods at the same time force was being applied to the rods.



All 12 cantilevers were post-tensioned for two reasons (Figure 20). The first reason was that 8 of the 12 columns had exhibited severe cracking, and it was decided that all 12 should be post-tensioned in order to ensure the safety of the structure. The second reason was to create a uniform look and help the solution to "blend in" with the original design. As for the existing cracks, they were filled with a high-pressure grout. This would help to strengthen them and seal out moister. (Kaplan, "Rods to fix cracks" 1991, p. 1A)



Once the design solution was completed, the stadium was tested for strength and vibrations. The ramp and concrete cantilever supports were loaded with 28, 7,000 pound, concrete road barriers to test the strength of the structure (Figure 20). As the ramp was loaded, measurements were made to determine how much the cantilevers deflected once loaded, and whether or not they returned to their original position once unloaded 24 hours later. (Kaplan, "Stadium passes" 1991) The load tests were not required by code since the design calculations showed that the new design was strong enough, but the university wanted to take the extra step to ensure people that the stadium was safe. (Kaplan, "Ramps given safety OK" 1991) The structure was also observed during the first football game to determine how much movement occurred. It was designed as a flexible structure, but precautions were taken to make sure the flexibility was not excessive. Measurements were completed using surveyor's instruments, and the observations showed the structure actually moved less than anticipated. (Kaplan, "Penn State fans test" 1991)



**Conclusions and Lessons to be Learned**
Many students, alumni, and fans attend the football games at Beaver Stadium with little to no knowledge of the structural problems that make up part of the stadiums long history at Penn State. Although many people say "ignorance is bliss," ignorance often causes history to repeat itself. Another significant failure involving concrete cantilevers and a post-tensioning solution is Frank Lloyd Wright's historical Fallingwater. Fallingwater was built long before the north-end expansion, but the problem was similar enough to show that structural failures rarely occur only once. The true cause of the Beaver Stadium failure is unknown. It may have been the early removal of the formwork. It may have been the length/placement of the reinforcing hooks. It may also have been the combination of the two. No one will ever know for sure, but there are still a few lessons to be learned:

1. Faster is not always better. The formwork may or may not have been removed early, but often companies profit from being able to get the job done faster. Structural failures cost time, money, and many times the lives of innocent people. Engineers and construction workers both have an obligation to make sure that the structure is completed with as little error as possible, not to get it done as fast as possible.

2. When working with concrete, it is critical that proper testing of the concrete strength need to be completed. The samples need to be collected on site, and need to be kept under similar external conditions. This will allow the contractor to accurately determine the strength of the concrete and the length of curing time required. Concrete strength can vary depending on a number of things and is much harder to predict than other materials such as steel.

3. Just because the design meets code doesn't mean it is adequate. The reinforcement may or may not have been inadequate, but the fact remains that several people felt that it was. Many times engineers let code overrule personal intuition. Engineers, and construction workers, develop judgment as they gain experience. This judgement helps point out design flaws. If the person building the structure feels that there is a problem it is the engineers responsibility to verify the design, and this verification needs to be more than "because the code says so".