5 Basic Structural Elements - Sticks Strings and Plates PDF

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5 – Basic Structural Elements: Sticks, Strings and Plates Essentially everything that we design in structures can be thought of as three fairly simple geometric forms, which are:   

Sticks Strings Plates

Each of these can serve multiple purposes depending on the orientation of the structural member and how the loads the members must carry are placed on the member. These three things make up all of our structural elements. If you understand the uses for and the limitations of each of these three types of members, you can design any structure imaginable. Sticks Nature has a lot of uses for sticks. They can be trees, columnar basalt and leg bones for example. Some examples are shown below in Figures 1A to 1C.

Figure 1A Tree as a beam

Figure 1B Columnar basalt as columns

Figure 1C Fossil tree branch as a stick

Whenever you see a column or a beam in a building, they can really be thought of as just “sticks”. Under normal gravity loads, columns experience compressive loads. Even if the column is a self-supporting flagpole, it experiences axial compression due to its own self-weight (Figure 2A). If a column is working as a part of a system in a building, there will likely be at least a pair of columns (or walls) and a beam spanning between the two columns/walls (Figure 2B). In this system (which is just a simple “post and beam type of construction” when columns are used), the beam transmits its loads to the columns through bending (flexure) under gravity loading conditions and these beam loads turn down when they get to the column in axial compression as shown below. Both the beams and columns are considered sticks.

Figure 2A Flagpole

Figure 2B Post and Beam System

The columns can experience axial tension if there is uplift on the column. This condition occurs when lateral loads are induced into the system and the post and beam system tries to overturn through uplifting (Fig 2C and 2D).

Figure 2C Uplift due to lateral loading

Figure 2D Overturning Kobe EQ 1995

Since a column can experience both compression and tension, we need to design it for both conditions to create stability. In axial compression, it only needs a foundation large enough to prevent it from sinking into the ground below the building. For uplift and possible overturning, we need to design the column with a foundation to resist overturning and a special connection from the column to the top of the footing called a hold-down mechanism. Columns that are in axial compression tend to get shorter and fatter as they are compressed although it is difficult to see these effects, however, the effects can be measured with sensitive instruments. If a string is compressed and it is too thin, it will undergo buckling which is the tendency to bend or bow. Therefore, we need columns with sufficient girth to withstand buckling as a result of their service loads (loads expected to be imposed during the life of the building). The tendency for buckling is called the columns “slenderness ratio” or SR. We will examine the mathematics of column SR’s later in the quarter. We can see that we can push on a string (compression), pull on the string (tension) and bend the string (flexure). There are other things that we can do to the string which include shear, torque, etc. We will discuss these other actions later. Strings Strings are generally quite thin as a ratio to their length. This means that we can only pull on “true” strings. True strings cannot carry compressive loads because of their tendency to buckle. True strings are also incapable of resisting shear loads or bending loads. They can resist torsional loads to some limited extent. As mentioned above, columns can become default strings under the right loading conditions. Typically, strings are not columns and can only best resist tension loads. A thin steel string is effectively a cable. The diagonal bracing found in the IDC is effectively strings even though they are solid steel. Their girth is very small when compared to their lengths. Generally, when we think of strings, we think of stranded or woven steel wire, we don’t think of eithe r columns or the sticks. Strings are common in tensile structure such as Foster’s Renault Building and as used in suspension type bridges. For some reason, most people think of spider webs when they think of cable type structures. Therefore, we’ll start with photos of spider structures in Figure 3A and then look at structures in Figures 3B to 3E.

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Figure 3A Do you see the monkey face on the spider’s abdomen and the mountain lion on it’s posterior? I have very strange and arty spiders in my garden. If you can’t see them, blow the photo up.

Figure 3B Cable Supported Walkway

Figure 3C Cable Supported Roof

Strings can be used to suspend structural elements as shown in the photos above or they can be used to brace very lightweight structures against lateral loads. Up to the 1980’s and even a little in the 1990’s, they were used as tension bracing elements on some more substantial structures. The diagonal bracing in the walls and roof of the IDC is an example of this type of bracing. It is almost impossible to use this type of bracing on most modern structures in California due to the high seismic loads imposed. Today, the code requires much more substantial tension elements to resist lateral loads when they are used as diagonal bracing (Figure 3D and 3E).

Figure 3D Diagonal Bracing, Sacramento

Figure 3E Typical Diagonal Bracing works in both tension and compression.

The reason for the larger members is due to observed failures during earthquakes. If the members are too thin, when compression loads are imposed, the bracing members will ten to buckle. Then, when the force directions are reversed during the earthquake, the inertial forces are sometimes so large that the thin diagonal members fail in tension at their connections placing the safety of the structure in jeopardy.

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The LA Museum of Science and Industry has a tension string feature that is quite interesting dynamically. If you are ever there, you might want to try a little experiment. The center steel structure that connects the I-Max Theater to the Museum has tension sticks that extend from the center structural ring to the main circular walkway (Figure 4A and 4B). Go outside to one of the cantilevered walkways within the round steel structure from the museum or the theater (the museum entry fee is less expensive). Grab one of the tension sticks shown in Figure 5B. The string has a “long period” so it is fairly easy to wiggle it up and down. When you get it going, the entire structure will shake violently if you continue to excite it. It isn’t dangerous, but it makes a lot of noise and freaks everybody out … including the security officers. Don’t do it too many times or they will arrest you. The structural engineer was Tom Sabol, SE who also happens to be an architect (Cal Poly SLO graduate). Sabol is a very well known and respected engineer. He has never admitted that he did this on purpose, but I’m glad that he did it nonetheless as the shaking experience is delightful.

Figure 4A CA Science Center

Figure 4B Tension Sticks supporting the walkway

The one thing that we can’t do with strings is make real “tensegrity structures”. These forms are more sculpture than structure. Some might want to point to the Tank Street Bridge (aka Kurilpa Bridge [translated to place for water rats]) that is intended for Brisbane, Australia as an example of a tensegrity structur e. It isn’t a true tensegrity structure. The bridge is fairly normal in its structural elements and the tensegrity element is simply normal compression and tension elements sitting on top of the bridge. For a structure to be a tensegrity structure, no element can be placed in bending by definition. The walking platform will be in bending on this bridge. Plates Plates are the most amazing and most versatile of the three basic structural shapes. They can work in compression, bending, tension, shear as well as torsion. They can be solid or hollow and can be rolled or folded into a variety of structural shapes. They can be very thick or very thin (fabric thin). Structurally, plates are simply amazing. Nature recognizes the versatility of plates and does a variety of interesting things with them. They can be wing covers on beetles (coleoptera), the webs on a duck’s feet, fancy seashells, dinosaur eggs, skulls, etc. some of which are illustrated below.

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Figure 5A Beetle shells

Figure 5B Wing covers made into an ornament

Figure 5D Real duck feet

Figure 5C My wife with beetle wing cover earrings

Figures 5E & F Man’s idea of duck feet – not all make sense

Figure 5G Murex seashell is a complex plate with sticks to create overturning overturning moment resistance

Figure 5H Hadrosaurid (dinosaur) egg

Figure 5I Oreodont (pig relative) skull

Figure 5J Smilodon skull is a very complex rolled and folded plate structure with sticks for front teeth

In the figures below, we have some of the typical aqrchitectural-structural uses for plates.

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Figure 6A Plate as a slab on grade

Figure 6B Plate as a pad footing

Figure 6C Plate as a wall

Figure 6D Plate as a diaphragm

Figure 6E Plate as a shear wall

Figure 6F Gusset plate

Figure 6G Column base plate

Figure 6H Structural steel hot rolled shapes are basically comprised of plates pressed out of a solid

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Figure 6I Plates can be formed into HSS shapes

Figure 6J Plates can be warped

Figure 6K Plates can be folded

Figure 6L Plates can be thin fabric

There are a wide variety of uses for plates in architectural design. Below in figures 7A & B are photos of a deformed plate that we rolled into a curved shell for the Riverside Transit Agency (RTA).

Figures 7A & B RTA Administration by Ruhnau McGavin Ruhnau Associates that utilized a plate rolled into a shell structure at the entry. Figure 8 below combines all three basic structural shapes into a single photograph. It is a piece of sculpture (façade) on the side of a parking garage in Santa Monica (Santa Monica Place) by Ball-Nogeus Studio. Can you identify each structural shape and the forces on each shape? Only consider the sculpture, not the building.

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Figure 8A Parking structure in Santa Monica (Santa Monica Place) with sculpture by Ball-Nogeus Studio

Figure 8B/C/D Detail photos (Photos 8A–D by Ball-Nogeus Studio)

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