Dustin J. Wakefield · 2004-03-29 · Dustin J. Wakefield Structural Option William Penn Senior...

Dustin J. Wakefield Structural Option William Penn Senior High School – York, PA Primary Faculty Consultant: M. Kevin Parfitt

PSU Architectural Engineering Spring 2004 Senior Thesis

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Structural Solution In order to make this project more economical from both a cost AND time perspective, it is absolutely crucial to examine alternate lateral systems for the building. As shown in the previous section, utilizing the steel moment frame can be a costly method, involving extensive labor and highly specialized connections. Therefore, the proposed structural

solution consists of implementing diagonal steel bracing, as shown in these three illustrations. These figures are still-shots taken from the four-dimensional rendering which will be discussed at a later time. These images provide an overall view of the locations for the proposed diagonal bracing in the building. As discussed in the problem statement, the designers of the William Penn Senior High School were faced with real design issues that made the braced frame system unfeasible at the time. Architectural limitations such as freedom in locating partitions in the building prevented the structural engineer from choosing the most economical method for resisting lateral loads. Therefore, the final decision to use steel moment frames was not necessarily cost driven, but was, however, the most flexible system to use from a wall opening and partition standpoint. However, as shown here, choosing to consider diagonal bracing as an alternative for the lateral system of this building has the potential to save extraordinary amounts of time and money. Before any design comparisons can be undertaken, however, one of the most important and fundamental steps is to first determine the applicable loads that will be imposed upon the structure.



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Applicable Loads FLOOR DEAD LOADS

• Metal decking 2 psf • Concrete slab 51 psf • Additional concrete due to structural deflections and

placing the concrete surface level 5 psf • Structural steel 10 psf • MEP equipment 5 psf • Ceiling 1 psf • Partitions 20 psf • Misc. (floor finishes, fireproofing, etc.) 6 psf

Total 100 psf FLOOR LIVE LOADS It was conservatively determined that a uniform load of 100 psf shall be used for all floor live loads except in the cases where the code requires a larger load. This exception is mainly the library, requiring a live load of 150 psf. Classrooms, offices, etc. are typically in the neighborhood of 40 to 70 psf, so 100 psf is a slight over estimate of the actual design loads used for the building. ROOF DEAD LOADS

• Modified bitumen roofing system 5 psf • K-series joists 3 psf • Roof deck 2 psf • Insulation materials 2 psf • Ceiling 1 psf • MEP equipment 8 psf • Misc. (fireproofing, finishes, etc.) 4 psf

Total 25 psf ROOF LIVE LOADS A roof live load of 30 psf was used over the entire exposed surface. ASCE 7-98 also specifies that the ground snow load for York, PA is 30 psf, so this value was justly chosen. To simplify this procedure, it was determined that effects due to snow drifting were excluded. If the gravity system were to be more carefully analyzed, then drifting



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and sliding snow considerations would have to be examined in more detail. However, its effect on the overall lateral system was assumed to be negligible. WIND LOADS As previously mentioned, wind loads were determined based on the provisions of ASCE 7-98. The relative stiffness of each of the steel moment frames was calculated automatically by RAM, and the wind loads were distributed accordingly. The advantage of using RAM in this way is that wind loads can quickly be applied to the structure in both directions. In fact, to take torsional effects into consideration, RAM also applies the wind load in an unbalanced condition on each face of the building to impose an artificial torsion on the building. Again, to perform these complex calculations by hand would be very time consuming and would require a very simplified model to approximate the solution. SEISMIC LOADS Seismic loads are also calculated using the provisions of ASCE 7-98. It is necessary to incorporate a mass dead load for each of the levels in the RAM modeling phase for the program to calculate the weight of the building. This is typically the same value as the rest of the surface dead loads which are listed above. Based on the building weight and the other factors which the user must input, RAM will apply a story force at each level and will, as in the case of wind, apply them in both directions. The loads imposed on the building are combined based on the specified combinations from AISC’s LRFD Manual of Steel Construction, 3rd Edition. When using RAM, the designer must specify which design method (ASD, LRFD, etc.) will be used to determine the size of the members. Once this is determined, the program will apply the load combinations corresponding to that particular method. For instance, LRFD uses the following loading configurations:

1.4D 1.2D + 1.6L + 0.5(Lr or S or R) 1.2D + 1.6(Lr or S or R) + (0.5L or 0.8W) 1.2D + 1.6W + 0.5L + 0.5(Lr or S or R) 1.2D ± 1.0E + 0.5L + 0.2S 0.9D ± (1.6W or 1.0E)



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Cost and Time Savings It was previously discussed that smaller moment connections cost approximately $125 to detail, while larger ones can soar to $500 or greater. There are obviously many different sizes of moment connections contained in the William Penn Senior High School, so it is considered to be a conservative estimate to average the cost of all moment connections in the building at $375 per connection. With this value in mind, the next step in determining the cost savings from utilizing a braced frame system is to quantify the number of actual moment connections in the building. Table 1 below is a summary of the number of connections at each level of the building.

This table shows that there are 957 moment connections throughout the entire building. Multiplying this number by the average cost of $375 that was discussed above, it is estimated that $358,875 will be spent on erecting and detailing all of the moment connections. Conservatively speaking, approximately one-third of this cost would be spent if all of these moment connections were completely replaced by simple shear connections:

$358,875 x 1/3 = $119,625 (new cost of connection detailing) SAVINGS = $358,875 - $119,625 = $239,250

Simply by switching to diagonal bracing and eliminating all of the moment connections, an estimated savings of $239,250 is expected in the construction costs of steel detailing. Of course, there are many other factors that affect the cost of a connection, such as shop fabrication, unforeseen fabrication inaccuracies, etc. However, this still serves as a good basis for determining an approximate value for monetary savings. From a time perspective, it was previously noted that a moment connection takes about twice as long to erect and detail as a simple shear connection. Originally, the structural steel construction activities were scheduled to last from the beginning of September, 2003 to the beginning of April, 2004. Upon successful replacement of all moment connections with diagonal bracing and shear connections, it is estimated that the steel

Table 1: This table is a summary of all of the moment connections at each floor of the William Penn Senior High School. This data will be used as a basis for determining how much money can be saved by eliminating the moment connections and replacing them with simple shear connections.



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activities could be finished by late December, 2003. Of course, as with cost issues, there are many other factors that determine the pace at which certain construction tasks are completed. Some items take longer than others and may be completely unrelated to the type of connections being used. Weather is always an unpredictable factor when it comes to construction projects as well. Again, however, this is still a good way to approximate the amount of time saved by choosing an alternate lateral resisting system such as diagonal steel bracing.



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Other Considerations When Using Diagonal Bracing Even though switching to a braced frame system appears to be quite simple on the surface, there are numerous other considerations that arise and must be examined. In other words, nothing is absolutely free, and everything comes with a price. The first thing to consider when switching from a system with a very uniform distribution of lateral resisting elements to one with frames in very distinct locations is uplift forces at the base of the columns supporting diagonal braces. As seen in this member forces report from the output generated in the RAM Frame module, the negative values in the axial force category represent uplift forces on the column, which must be accounted for in design. In this example, which happens to be a column in one of the frames along the northern-most column row, the values range from 14.32 kips to 76.73 kips. In order to account for this phenomenon, it is important to understand how caissons are typically reinforced. Generally, if a caisson is designed simply to transfer load into adequate soil below, then the reinforcing steel is inserted down into the caisson a fraction of its overall depth. In the case of the William Penn Senior High School, the designers chose to reinforce the caissons to a depth of ten feet, or the depth of the caisson, whichever was less. When examining the structural drawings, it can be seen that some of the caissons reach a depth of up to 34 feet below grade, depending on the quality of the soil beneath it. Therefore, some of the caissons, relatively speaking, only receive reinforcement throughout a small portion of their depth. However, when uplift forces are introduced, and the caisson becomes responsible for providing resistance to being lifted out of the ground, a new set of criteria must be considered when designing the reinforcement. The first thing to be specified when

Table 2: This table is a Member Forces summary generated in the RAM Frame module for a sample column along the northern most column row of the William Penn Senior High School. The negative axial force values indicate uplift forces which must be accounted for.



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designing a caisson to resist uplift forces is that the reinforcing cage must span the full depth of the member, ensuring that the tension load is fully transferred through the caisson and into the ground below. Keeping in mind that concrete is inherently weak in tension, this specification makes perfect sense now that the foundation member is put in a situation where it will be resisting tensile forces. Figure 3 is a representative detail of the caissons used in this design, produced by the structural engineer of record. In addition to running the reinforcing the whole way to the bottom of the caisson, additional considerations may be necessary, depending on the magnitude of the uplift force imposed on the member. The first thing to examine is whether or not the caisson can carry the load based solely on its own weight. These caissons range from 30 inches to 54 inches in diameter, yielding some fairly heavy foundation members. After checking to see if the caisson can carry the uplift force by its own weight, the designer may need to consider looking at the surface friction or “skin friction” and its impact on the resistance to uplift force. Since the caissons were not originally designed with this type of friction in mind, a more thorough investigation would be necessary to determine if this resistance is feasible and whether or not the soil has the appropriate properties to utilize friction. The report provided by the geotechnical engineer is a good place to locate the soil characteristics on the site.

Figure 3: Typical detail of a caisson used in the William Penn Senior High School foundation system. The blue arrow indicated that the reinforcing steel must be extended to the full depth of the member when considering uplift forces.



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Another very important consideration when attempting to incorporate diagonal bracing members is being aware of the actual implications of installing the bracing. Typically, one of the most difficult aspects of this design is trying to coordinate the diagonal

members with the architectural features of the building, such as window openings, doors, partitions, etc. The William Penn Senior High School is bordered on two sides by existing structure and open to the environment on the other two. The two sides exposed to the environment contain a large amount of windows, making it difficult to place diagonal bracing in these locations. Therefore, determining a strategic layout of bracing can be quite trying. In addition to these concerns, the location of the column center line with respect to any masonry wall features or other

architectural components plays an important part in how bracing members can be coordinated with the rest of the design. As seen in figure 4, the

column centerline along the left side of the structure coincides almost exactly with a masonry wall, introducing some complications when trying to incorporate the bracing members. One solution to this problem may be to install the bracing members “tight” against the masonry wall, meaning to actually cut out a space for the bracing member in the wall. This solution, although sometimes practiced, can result in excessive block cutting and the need for numerous wall ties on the bracing members to ensure lateral stability. This can become very labor intensive. One might also consider trying to offset the wall and/or column center line. This can be a tricky alternative, as it produces additional architectural considerations. This solution also exposes the diagonal bracing members, causing the need for additional fireproofing. This may cause more trouble than is desired, and therefore may not be the best solution to the problem.

Figure 4: This figure shows the interference between the column center line (location for the new diagonal members) and the existing masonry wall, shown with the cross-hatched pattern.

Figure 5: This figure shows the suggested configuration for the incorporation of diagonal steel bracing. The bracing members are to be placed between the two parallel CMU walls.



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The most practical solution to this problem would be to form two parallel CMU walls as seen in Figure 5 which are centered about the column center line. This would not present the need to move the column center line, and at the same time, it would provide the necessary fireproofing for the steel bracing members. This technique was also used in other locations within the building, for instance around the stair towers in the south corners. Therefore, the construction workers should already be familiar with the process. Although braced frames are very rigid by nature, story deflection is still crucial to control in situations involving expansion joints connecting existing structures to new designs.

The expansion joint used in this case is 3 inches wide, typically allowing a maximum drift of half this distance, or 1.5 inches. The illustration shown here represents all of the newly added braced frames in the William Penn Senior High School, and their corresponding deflected shapes under lateral loading (without the rest of the structure, for clarity). One of the major advantages to using braced frames is that the drift resistance is not

solely based on the stiffness of the individual beams and columns in the frame. The entire load is now transferred into the diagonal members, providing a very rigid system with small deflections. A moment frame can become very flexible by nature, and hence result in extremely large columns and beams to acquire enough stiffness to resist the drift. Although story displacement is a crucial parameter to consider when looking at allowable drift against an existing structure, some stories will most likely be more important than others. In this case, the roof level is the critical level, as this is the highest point where the two structures are physically connected via the expansion joint. As seen in Table 3 produced in the RAM Frame module, the roof level (level 3) receives its maximum displacement under seismic loading S2, or seismic loading in the x-direction. This loading condition yields a story displacement of 0.411 inches. However, the seismic provisions of ASCE 7 require that a

Table 3: This table is a summary of the story displacements under all of the applied loading configurations. The maximum displacement at the roof level occurs under seismic loading in the x-direction and yields a value of 0.411 inches.



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deflection amplification factor of 3.5 be applied to all seismic drift values. With this in mind, the actual displacement that must be accounted for is 1.44 inches, which is still less than half of the three inch expansion joint width. Therefore, all deflection criteria have been met according to the specifications of ASCE 7. Another major consideration to examine with this new lateral system design is uniformity in the specified diagonal bracing members. As in any area of structural engineering, it is known that keeping the shapes as uniform as possible can help to save money in the project and reduce the risk of members being placed in the wrong location. Therefore, based on the output generated in the RAM Frame module, it was determined that all of the bracing members can fall into one of four size groups, which are listed here in Table 4.

The same attempt was made to provide uniformity to the rest of the braced frame elements as well, as seen here in Table 5. Since the beams and columns are no longer needed to resist the lateral loads with their own stiffnesses, the sizes are now much smaller than before the lateral resisting system was changed to diagonal bracing. In fact, to provide enough stiffness to control drift, the original design called for extremely heavy members, some over 300 pounds per linear foot. The largest column in the lateral system now is a W14x132, where the largest beam is a W27x84. This has the potential to save a fair amount of money in raw steel costs alone.

Table 4: This table is a summary of the bracing members chosen for the design of the new braced frame system. The design was reduced to these four shapes alone to increase uniformity in the design and reduce the risk of misplacement of members in the field.

Table 5: This table is a summary of the beams and columns used in the braced frame system of the William Penn Senior High School. An attempt was made to limit the variety of shapes used in the frames to be more uniform and efficient.



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The last major thing to look at when using diagonal braced frames is the horizontal distribution of the lateral loads to the individual frames. In the original system, a typical non-composite steel roof deck was specified. This was acceptable because the moment frames were much more closely spaced together than the braced frames in this proposed design. Therefore, the load was easily transferred through the deck, as even the lightest gage deck is capable of some diaphragm action. However, since the frames are now in much more distinct locations, a stronger deck with more diaphragm action must be used in order to transfer the loads to the frames. Before sizing a steel deck, the first thing to determine is the total story force at the roof level. Seen here in Table 6 are the applied story forces generated by RAM Frame. The roof level (level 3) experiences 46.98 k of force in the x-direction, and 53.55 k of force in the y-direction. Simply dividing these numbers by the corresponding width of the building in that direction will yield an approximate shear force distribution at this level.

The chart on the following page is an excerpt from the Vulcraft Steel Deck Manual. Based on the approximated distributed force value and the span of the deck, an appropriate steel deck can be specified which will assure that the load is transferred to the individual braced frames. Specific to this project, it was determined that a 1.5” 20 gage 36/3 steel deck with 2 sidelap fasteners per span at a 6’-0” span will suffice with a capacity of 233 plf. The maximum approximated distributed load at the roof level was calculated to be 208 plf. Note that this manual uses a 2.75 factor of safety for design.

Table 6: This table is a summary of the applied story forces on the building. These worst case loads can be used to approximate the distributed load at each individual level, and then determine the necessary steel deck to adequately transfer the lateral loads to the braced frames.



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Dustin J. Wakefield · 2004-03-29 · Dustin J. Wakefield Structural Option William Penn Senior...

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