Architectural Structures

Precast concrete

Precast concrete comes in a wide variety of shapes for both structural and architectural applications. Presented are structural systems and members: floor and roof members, columns, and walls. Though precast members may be of ordinary concrete, structural precast concrete is usually prestressed. The primary reinforcement of prestressed concrete is with tendons, yet normal rebars are often used as stirrups to resist shear. Rebars are also added for different loads during transportation and erection. Compared to site-cast concrete, precast concrete provides better quality control, repeated use of formwork, faster curing with steam, and concurrent operations while other site work proceeds. The advantages must offset the cost of transportation to a construction site. Precast concrete is similar to steel framing by allowing preparatory site work to be concurrent, yet it has the advantage to provide inherent fire resistance. Steel on the other hand, has lower dead weight, an advantage for seismic load that is proportional to dead weight. To reduce high costs of formwork the number of different precast members should also be reduced; yet this objective must be balanced by other considerations. For example, fewer parts may result in a monotone and uninspired design. Combining precast with site-cast concrete may satisfy economy as well as aesthetic objectives.

Precast framing allows many variations, both with and without site-cast concrete. A few typical examples are presented. They are possible with columns of several stories, limited primarily by transportation restrictions. The capacity of available cranes could also impose limitations. In such cases, columns should be spliced near mid-height between floors where bending moments from both gravity and lateral loads are zero.

1T-columns with deep spandrel beams support floor and roof slabs. Shear connections between adjacent beams combine them to moment frames to resist lateral as well as gravity loads

2Frames of split columns and deep spandrel beams support floor and roof slabs for gravity and lateral loads. Shear connections at adjacent split columns tie the frames together for unified action

3T-columns with normal spandrel beams support floor and roof rib slabs

Shear connections between adjacent beams combine them to moment frames to resist lateral as well as gravity loads

4Tree-columns with beam supports allow flexible expansion. Twin beams allow passage of services between them. Lateral load resistance must be provided by shear walls or other bracing

5Rib slab or double T’s supported on site-cast frame

6U-channels with intermittent skylights supported on site-cast frame

23-20 MATERIAL Concrete

24

Fabric and Cables

Material

Tent membranes have been around since ancient history, notably in nomadic societies. However, contemporary membrane structures have only evolved in the last forty years. Structural membranes may be of fabric or cable nets. Initial contemporary membrane structures consisted of

•Natural canvass for small spans

•Cable nets for large spans

Industrial fabric of sufficient strength and durability was not available prior to 1970.. Contemporary membrane structures usually consist of synthetic fabric with edge cables or other boundaries. Cables and fabric are briefly described.

Fabric for contemporary structures consists of synthetic fibers that are woven into bands and then coated or laminated with a protective film

Common fabrics include:

•Polyester fabric with PVC coating

•Glass fiber fabric with PTFE coating

•Glass fiber fabric with silicon coating

•Fine mesh fabric, laminated with PTFE film

Fabric properties are tabulated on the next page. Foils included are only for very short spans due to low tensile strength. Unfortunately the elastic modulus of fabric is no longer provided by fabric manufacturers, though it is required for design and manufacture of fabric structures. The elastic modulus of fabric is in the range of:

E = 2000 lb/in, 11492 kPa/m to E = 6000 lb/in, 34475 kPa/m

Cables may be single strands or multiple strand wire ropes as shown on following pages. Cables consist of steel wires, protected by one of the following corrosion resistance:

•Zinc coating (most common)

•Hot-dip galvanizing

•Stainless steel (expensive)

•Plastic coating (used at our cable nets at Expo64 Lausanne)

Depending on corrosion protection needs, zinc coating comes in four grades: type A, type B (double type A), type C (triple type A), type D (four times type A). Cables are usually prestressed during manufacture to in increase their stiffness.

Elastic modulus of cables:
E = 20,000 ksi, 137900 MPA	(wire rope)
E = 23,000 ksi, 158,585 MPa	(strand > 2.5 inch diameter)
E = 24,000 ksi, 165,480 MPa	(strand < 2.5 inch diameter)
24-1 MATERIAL Cable/Fabric

Fabric

Type	Makeup	Common use	Tensile strength
Coated fabric*	Polyester fabric	Permanent + mobile	40 to 200 kN/m
	PVC coating	Internal + external	228 to 1142 lb/in
Coated fabric*	Glass fiber fabric	Permanent	20 to 160 kn/m
	PTFE coating	Internal + external	114 to 914 lb/in
Coated fabric	Glass fiber fabric	Permanent	20 to 100 kN/m
	Silicone coating	Internal + external	114 to 571 lb/in
Laminated fabric*	Fine mesh fabric	Permanent	50 to 100 kN/m
	Laminated with	Internal + external	286 to 571 lb/in
	PTFE film
Foil	PVC foil	Permanent internal	6 to 40 kN/m
		Temporary external	34 to 228 lb/in
Foil*	Flouropolymer foil	Permanent	6 to 12 kN/m
	ETFE	Internal + external	34 to 69 lb/in
Coated or	PTFE fabric	Permanent + mobile	40 to 100 kN/m
uncoated fabric*	(good qualities	Internal + external	228 to 571 lb/in
	for sustainability)
Coated or	Flouropolymer	Permanent + mobile	8 to 20 kN/m
uncoated fabric*	fabric	Internal + external	46 to 114 lb/in

*Self-cleaning properties

SI-to-US unit conversion: 1 kPa/m = 5.71 lb/in

Fire rating			UV light resistance		Translucency	Durability
++ incombustible			++ very good
+		low flammability	+ good
0		none
+			+		0 to 25 %	15 to 20 years
++			++		4 to 22 %	> 25 years
++			++		10 to 20 %	> 20 years
++			++		35 to 55 %	> 25 years
0			+		Up to 90 %	15 to 20 years
						internally
++			++		Up to 96 %	> 25 years
++			++		15 to 40 %	> 25 years
++			++		Up to 90 %	> 25 years
Maximum fabric span*
	Tensile strength		Maximum span
		500 lb/in	60 ft
		1000 lb/in	120 ft

*Assuming:

Live load = 20 psf, 956 Pa (wind or snow) Safety factor = 4

Fabric span/sag ratio = 10

24-2 MATERIAL Cable/Fabric

Cables

Cables may be of two basic types and many variations thereof. The two basic types are strands and wire ropes.

Strands have a minimum of six wires twisted helically around a central wire. Strands have greater stiffness, but wire ropes are more flexible. To limit deformation, strands are usually used for cable stayed and suspension structures.

Wire ropes consist of six strands twisted helically around a central strand. They are used where flexibility is desired, such as for elevator cables.

Metallic area, the net area without air space between wires, defines the cable strength and stiffness. Relative to the gross cross section area, the metallic area is about: 70% for strands and 60% for wire ropes. To provide extra flexibility, some wire ropes have central cores of plastic or other fibers which further educes the metallic area.

1Strand (good stiffness, low flexibility) E = 22,000 to 24,000 ksi; 70% metallic

2Wire rope (good flexibility, low stiffness) E = 12,000 to 20,000 ksi; 60% metallic

Cable fittings

Cable fitting for strands and wire ropes may be of two basic types: adjustable and fixed. Adjustable fittings allow to adjust the length or to introduce prestress by shortening. The amount of adjustment varies from a few inch to about four feet

3Bridge Socket (adjustable)

4Open Socket (non-adjustable)

5Wedged Socket (adjustable)

6Anchor Stud (adjustable)

ASupport elements

BSocket / stud

CStrand or wire rope

24-3 MATERIAL Cable/Fabric

1Cable-to-cable connection with integral strand fitting

2Cable-to-cable connection with wire rope thimble

3Open socked connection, perpendicular

Trapezoidal gusset plate for synergy of form and reduced weld stress

4Open socked connection, angled

Sloping gusset plate for synergy of form and uniform weld stress distribution

24-5 MATERIAL Cable/Fabric

Mast / cable details

The mast detail demonstrates typical use of cable or strand sockets. A steel gusset plate usually provides the anchor for sockets. Equal angles A and B result in equal forces in strand and guy, respectively.

AMast / strand angle

BMast / guy angle

CStrand

DGuy

ESockets

FGusset plates

GBridge socket (to adjust prestress)

HFoundation gusset (at strand and mast)

IMast

24-6 MATERIAL Cable/Fabric

1

2

Production process

Fabric pattern

To assume surface curvature, fabric must be cut into patterns which usually involves the following steps:

•Develop a computer model of strips representing the fabric width plus seems

•Transform the computer model strips into a triangular grids

•Develop 3-D triangular grids into flat two-dimensional patterns

The steps are visualized ad follows:

1Computer model with fabric strips

2Computer model with triangular grid

2 Fabric pattern developed from triangular grid

Pattern cutting

Cutting of patterns can be done manually of automatic.

The manual method requires drawing the computer plot on the fabric

The automatic method directs a cutting laser or knife from the computer plot

Note:

For radial patterns as shown at left, cutting two patterns from one strip, juxtaposing the wide and narrow ends, minimizes fabric waste.

Pattern joining

Fabric patterns are joint together by one of three methods:

•Welding (most common)

•Sewing

•Gluing

Edge cables

Unless other boundaries are used, edge cables are added, either embedded in fabric sleeves or attached by means of lacing.

Fabric panels

For very large structures the fabric may consist of panels that are assembled in the field, usually by lacing. Laced joints are covered with fabric strips for waterproofing.

3

24-7 MATERIAL Cable/Fabric