Determining Structural Systems

Question 1

What are 3 primary factors in determining the structural system:

Answer 1

1. Occupancy
2. Program
3. System’s resistance to applied loads

Question 2

What is the first consideration when determining the system’s resistance to applied loads?

Answer 2

First consider the type of span required:

1. Shorter spans are controlled by shear or moment forces = modify members by increasing their strength
2. Longer spans are controlled by deflection = modify members by increasing stiffness

Question 3

How do you decrease the size of a wood member?

Answer 3

Increase the grade.

Question 4

How do you decrease the size of a steel member?

Answer 4

Increase yield strength.

Question 5

How do you decrease the size of a concrete member?

Answer 5

Increase compressive strength.

Question 6

A span over ___ ft is considered a long span. What is one of the most important characteristics of a long span structure?

Answer 6

60 ft

Redundancy. Failure of one portion affects a much greater area and can even collapse the entire building.

Question 7

What are two categories of long span structures?

Answer 7

1. one-way system

2. Two-way system

Question 8

What is the difference between a one-way long span system and two-way long span system?

Answer 8

one-way system = linear members span in one direction and resist loads primary by bending

two-way system = distributes loads to supports in both directions and involve complex 3d methods of resisting loads

Question 9

What are steel girders?

What are their typical:
span
depth
depth to span ratio

What are 3 types of steel girders?

Answer 9

a type of one-way long span system also known as a wide flange

Span = 10’ to 72’
Depth = 4” to 3’
Depth to span ratio = 1/20

1. Rolled steel girder = steel is rolled into the ‘I’ beam shape
2. Steel plate girder = composed of sheet steel for the web and bars or angles for the flanges. Can achieve longer spans but are often 8ft + deep.
3. Tapered girder = plate girders that are tapered toward the middle of the span and used in roof framing

Question 10

Where would you use a steel plate girder over a rolled steel girder?

Answer 10

When you need to transfer the load of a column to two wider spaced columns to create a clear span below. Like for a hotel lobby that needs to transfer loads above down to a wide open space.

Question 11

What is a rigid frame?

What are the: 
typ span
typ depth
typ depth to span ratio 
of steel and glulam rigid frames?

Answer 11

A one way long span structural system made of steel or glu lam that has a sloped roof (“arch action”) with a rigid, moment-resisting connection between the columns and roof structure OR columns/roof are one continuous member.

Steel:
typ span = 30’ to 150 ‘
typ depth = 2’ to 5’
typ depth to span ratio = 1/20 - 1/30

Glulam:
typ span = 30’ to 120 ‘
typ depth = 1.5’ to 4’
typ depth to span ratio = 1/20 - 1/30

Question 12

How are loads transferred/resisted in rigid frame?

Where is moment greatest in a rigid frame?

Answer 12

The vertical and horizontal members resist loads primarily by flexure, and moments are transferred from beams to columns. Vertical loads are transferred to columns through compression and bending (flexure).

At the junction between the column and the sloped roof.
Structure is usually more built out here to resist these forces.

Question 13

Where a rigid frames often used?

Answer 13

Instances where a simple, rectangular open space is required:
facilities
warehouses
manufacturing plants

Question 14

What are 2 ways in which trusses are efficient in long span systems?

Answer 14

because of their primary reliance on compression and tension to resist forces, rather than bending, and their height strength to weight ratios.

Question 15

What is the typical: 
span?
depth?
depth to span ratio?
of a:
pitched wood truss? 
flat wood truss? 
pitched steel truss? 
flat steel truss?

Answer 15

pitched wood truss:
span = 40’ to 100’
depth = 7’ to 17’
depth to span ratio = 1/6

flat wood truss:
span = 40’ to 120’
depth = 4’ to 12’
depth to span ratio = 1/10

pitched steel truss:
span = 40’ to 150’
depth = 5’ to 20’
depth to span ratio = 1/6 to 1/8

flat steel truss:
span = 40’ to 300’
depth = 4’ to 30’
depth to span ratio = 1/10 to 1/12

Question 16

What are open-web steel joists?

What is their typical:
span?
depth?
depth to span ratio?

Answer 16

prefabricated truss members using hot-rolled or cold-rolled steel members

span = 40’ to 120’
depth = 4’ to 12’
depth to span ratio = 1/10

Question 17

What are the three major groups of open-web steel joists?

What are their span, depth, and depth to span ratio?

Answer 17

K-series = spans up to 60’ (not long span)

LH-series = long span span = 25’ to 96’
depth = 1.5’ to 4’
depth to span ratio = 1/20 to 1/24

DLH-series = deep long span
span = 90’ to 144’
depth = 5’ to 6’
depth to span ratio = 1/20 to 1/24

Question 18

For which applications are LH and DLH open web joists suitable?

Answer 18

LH = floor and roof decks
DLH = roof decks

Question 19

What is camber?

Answer 19

rise in beam to compensate for deflection

Question 20

What is a joist girder?

What are their
span
depth
depth to span ratio

Answer 20

primary structural members that support evenly spaced open-web joists.

span = 20’ to 100’
depth = 2’ to 10’
depth to span ratio = 1/10 to 1/12

Question 21

What is the standard designation of joist girders?

Answer 21

##G = inches depth (ex. 48G)
#N = number of joist spaces (ex. 8N)
#.#K = required design load at each panel point (ex. 8.8K)

Question 22

What is a Vierendeel Truss?

Answer 22

a structure composed of a series of rigid rectangular frames. not considered a true truss since there are not diagonal members, and it must resist bending as well as tension/compression .

Question 23

Are glu lams considered long span structures?

Answer 23

Not alone since they span up to 60 ft. But they can be composed into rigid frames or straight rectangular beams

Question 24

Precast used to span long distances must be either:

Answer 24

prestressed or post tensioned

Question 25

What are the 3 common precast concrete sections for long spans?
What are their respective span, depth, and depth to span ratio?

Answer 25

Single Tee
span = 20’ to 120’
depth = 1’ to 4’
span to depth ratio = 1/20 to 1/30

Double Tee
span = 20’ to 80’
depth = 1’ to 1.5’
span to depth ratio = 1/20 to 1/30

AASHTO girder = mostly used on highway bridges but can be used in buildings
max span = 120’

Question 26

What are 5 advantages of double tee precast concrete sections?

Answer 26

1. function as structure and decking
2. relatively inexpensive to produce
3. can be erected quickly
4. can be used as either horizontal or vertical members
5. space between webs can be used for mech and elec service runs

Question 27

Where can post-tensioning be used?

Answer 27

in beams, floor slabs, or other sections to allow for longer spans

Question 28

What is an arch?

Answer 28

A structure that resists imposed loads primarily by compression with some bending stresses involved.

Question 29

What shape does an arch take that is supporting a uniform load?

Answer 29

A parabola.

Question 30

What is an arch’s thrust?

Answer 30

Thrust = spread of the arch’s 2 connection points
Thrust = 1/Height

Question 31

What is the difference between hinged arch and a fixed arch?

Answer 31

Hinged arch = hinged at supports and can move under loads caused by temperatures, sol, settlement, wind without high bending stresses

Fixed arch = fixed at supports.

Question 32

What is the most efficient shape of two-way long span structures?

Answer 32

a square, such as waffle slab or flat plate

Question 33

What are 4 advantages of a two way span over a one way span? 2 disadvantages?

Answer 33

1. Redundancy: the failure of one joint will not cause failure of the whole system
2. More efficient with materials
3. Can carry heavier loads
4. Can span farther
5. Complicated to design and build
6. Long span two way systems are almost always limited to use in only roof assemblies

Question 34

What is a space frame? What are two types?

Answer 34

A two-way long span system that primarily resists loads in tension and compression. They also have redundancy.

1. Two-way truss space frame = simplest type, where top chords and bottom chords are stacked vertically
2. Offset grid space frame = most common type, where top chords and bottom chords are offset

Question 35

What is a domes?

What are two types?

Answer 35

A two-way long span system that is one of the most efficient structural systems because they resist completely in tension and compression; and sometimes shear. They are composed of meridian arches supported laterally by hoops. The top hoops are in compression and the bottom are in tension.

1. Schwedler dome = one of the most common types of domes in which intersections between arches and hoops are connected by diagonals
2. Geodesic domes = like space frames in the shape of a sphere. Very strong, lightweight, and can enclose the greatest volume with the least surface area. Can span up to 400 ft.

Question 36

What is a thin-shell structure? What are two types?

Answer 36

A two-way long span system that is a form-resistant structure whose strength is a result of their ability to support loads through tension, compression, and shear in the plane of a shell. Rigid and efficient, but labor-intensive to construct.

1. Singly curved (barrel vault): upper portion in compression, lower portion in tension
2. Doubly curved

Question 37

What is a type of singly curved (barrel vault) thin shell structure?

Answer 37

Lamella roof = barrel vault but with intersecting grids

Question 38

What are 2 types of doubly curved (barrel vault) thin shell structure?

Answer 38

1. Synclastic shells = curves on same side of the surface (ex. dome)
2. Anticlastic shells = curves on opposite sides of the surface (ex. Hyperbolic paraboloid)

Question 39

What are membrane structures?

Answer 39

A two-way long span system that is a class of form-resistant structure which can only resist loads in tension so need to be anchored between elements that can be placed in compression.

Question 40

What are air-supported (pneumatic) structures?

Answer 40

A two-way long span system that is a membrane help in place by air pressure, can only support loads through tension so need to be stabilized with cables over the top to stiffen.

Question 41

What are folded plates?

Answer 41

A two-way long span system that is composed of thin slabs bent to increase load-carrying capability. Stronger than simple flat plates because structure depth is as deep as the fold of the plate. Supports loads in compression (top) and tension (bottom of plates)

Question 42

What are suspension structures?

How do tensile force and sag relate?

Answer 42

A two-way long span system that can only resist loads in tension, assumes its funicular shape for any given set of loads. Needs to be balanced with compression members or connecting the cable to a foundation support.
Amount of tensile force is inverse to the sag.

Question 43

What is the most common method of preliminary sizing?

Answer 43

Calculate the depth-to-span ratio