St. Stephen's Church
The Design
Precast concrete curved beams in the shape of huge elephant tusks have been used by the Boulder Architect, Charles Haertling, in the design of the roof structure of the first unit of a complex for St. Stephen's Lutheran Church in Northglenn, a suburb of Denver Colorado. The four concrete girders, essentially rectangular in section with 5 1/2" chamfer at the top and 7 1/2" chamfer at the bottom, varied in depth from 8'-0" near the center point to 2'-6" at the overhangs.
The roof structure, with a 155 foot diagonal span, is supported upon four concrete pyramids, diamond in shape and approximately 4' x 6' at the base and 24" x 15" at the top. These are placed at the points of the diamond in plan. then later post-tensioned. They are anchored to the columns by means of two 1 1/2" high tensile steel bolts set vertically in the column and threaded through the beams. The longitudinal ribs span 106' between supports with a 29' overhang, and weigh 54 tons, while the transverse ribs, weighing 44 tons and spanning 75' between supports overhang 12 feet.
The shorter beams were designed to carry those spanning the longer diagonal so that no center support would be necessary. The horizontal thrust caused by beam curvature and lateral loads was transferred into the perimeter columns through the 1 1/2" rods, and then into caissons. The large base areas of these columns were readily able to resist the moments due to these horizontal forces.
Two 20 wire 1/4" diameter tendons were set on the center line of the cast- in-place beams. These were stressed while the beams were on the ground so that no unsightly cracking would occur during handling. Five 20 wire 1/4" diameter tendons were anchored at the center line of the shorter beams near the point of support, then draped toward the bottom of the beam over the remainder of its length. For the longer ribs, four 12 wire 1/4" diameter tendons were anchored 18 feet from the outer ends and were placed in the top of the beam over the column to counteract stresses due to the 26' cantilever, then were draped along the bottom of the girder over the midsectlon. The negative moment induced by the reaction of the shorter beams required that the cables be positioned in the top of the girder at the intersection point.
Cables were suspended from the top of the beams in a diamond pattern forming a catenary support for a concrete roof deck. The number of 1/4' diameter wires per tendon varied from three near the center to six at the perimeter of the deck. 5/8" diameter reinforcing bars were placed perpendicular to the catenary cables. A 2 1/2" thick stone aggregate concrete formed the roof deck. The catenary cables were stressed to 3000 psi applying a minimum compression to the deck. A 1" insulation and plastic layer was applied over the concrete deck.
Construction Sequence
The four concrete beams supporting the roof structure were precast on the job site. When the concrete had reached a compressive strength of 4,000 psi, the two tendons along the center line of the beams were fully stressed from one end. This allowed one sixty-ton crane and one forty ton crane to pick up and move them into position without distress to the beam. The interior end of each beam rested on a common support made up from timber sections tied together with steel beams. The concrete girders were prevented from moving or rotating laterally by steel frames extending vertically from the timber cribbing. At the exterior columns the beams were carefully lowered over two high tensile steel rods anchored in the column. The beams were notched so that they sat firmly and evenly on the top of the columns. Lateral resistance was provided at this end by the two rods.
The small opening remaining at the intersection of the beams was filled with concrete so that there were two continuous intersecting beams with indented anchorages from where stressing of the beam tendons was carried out.
After the strength of this keystone concrete had reached 4,000 psi, the post tensioning operation began. Two cables in each beam were stressed, beginning at one longitudinal, then moving to an adjacent transverse member, and so on around the four beams. The remaining cables on each beam were stressed in the same sequence. After all stressing was completed, the tendons were grouted.
Calculations had shown that there would be negligible longitudinal shortening of the girders when the catenary cables were stressed so that once the girder tendons had been stressed there would be little further shortening. At this time, the high tensile steel rods were lightly stressed to 100 kips. The central support was dismantled leaving the area completely free from obstructions.
The roof catenary cables were draped between the beams and adjusted to pre-determined elevations. Each cable anchorage consisted of 1/2" plate bent to the top of beam profile with two 4" lengths of 4 channel 7.25 welded to this plate so that their common axis was parallel to the direction of the cable. The ends of the cables in adjacent quadrants were anchored at their common beam so that the complete stressing operation could be done from one of the two continuous beams. The reinforcing bars were welded to short plates embedded in the top of the beam, then tied to the cables at their intersection points.
Expanded metal lath was attached to the underside of the cables. No other formwork or support system was necessary. The concrete deck was placed in approximately 3'-0" strips concentrically around the roof starting from the outside edge. Once the deck concrete pour had been completed and reached a strength of 3,000 psi, the cables were stressed from one end using small hydraulic jacks. To minimize torque on the beams, a maximum of three cables were stressed in any one quadrant followed by the stressing of cables in the adjacent quadrant, and so on around the deck.
Additional photos from the project are available below:
Contact Joel Haertling to order the video or book.
