Ballast Flashcards
One way structural Systems are…
systems where the load is transmitted through structural member in one direction at a time.
Wood is primarily used in what type of structural system?
one way structural systems
Typical sizes for wood joists and typical spacing between them. Maximum Span?
sizes: 2x6, 2x8, 2x10, 2x12 spacing: 12in, 16in, 24in maximum span: 20-25 feet
The met common use of solid wood beams is with …
plank and beam framing in which members of a 4 in or 6 in nominal width span between girders or bearing walls at spacings of 4, 6, 8 ft. Wood decking, either solid or laminated, is used to span between the beams, with the underside of the decking being the finished ceiling. The maximum span for beams is 10 to 20 ft.
What is glulaminated construction
aka glulam. a popular method of wood construction in which structural members are made up of individual pieces of lumber 3/4 to 1 1/2 inches thick and gulled together in the factory Standard widths: 3 1/8, 5 1/8, 6 3/4, 8 3/4 typical spans: 15 - 60 ft
What are the advantages of glulams?
longer spans and appearance. glulams are usually left exposed to express structure in a building. they can also be made tapered beams, tapered and curved beams, and various arches.
What are I shaped joists?
Joists comprised of a top and bottom chord of solid or laminated construction separated by a plywood or OSB web.
Advantages of an I shaped joist
allows longer spans, increase utilization of forest products while minimizing the problems of defects and limited strength in solid wood members
What is a box beam?
beam fabricated with plywood panels and glued and nailed to solid wood members, usually 2x4 framing -used infrequently because often not made properly and made on site -used in locations where the depth of the member is not critical and where other types of manufactures beams cannot be brought to site
stressed skin panels?
constructed of play wood and nailed and glues to 2x4 framing -used for floors and walls
What is a ductile material?
material that can tolerate some deformation and return to its original shape -material that will bend before breaking
Describe beam ad girder steel construction systems?
large remember span between vertical supports, and smaller beams are framed into them girder span the shorter distances while the beams span the longer distances -typical spans are from 25 - 40 ft with beams being spaced 8 - 10 ft -topped with steel decking and lured concrete
Describe open web steel joist systems?
-span between beams or bearing walls -can span up to 60 ft, long span joists go 96 ft, and deep png span joists go 144 ft. -joist depth range from 8 to 30 in in increments of 2 inches -topped with steel decking and lured concrete -used in low rise construction where overall depth of the floor/ceiling systems is not critical
Two Primary types of structural concrete systems are..?
Cast in place: -require framework and generally take longer to build -can conform to a wide variety of shapes -usually uses mild steel reinforcing, but could use postentioning steel. Precast: -formed in a plant under controlled conditions -quality is better controlled and structure it built quickly -usually prestressed
tilt up panels
structural concrete that is precast on site -limited to wall panels or moderate size
lift slab construction
procedure in which floor slabs of a multistory building are cast one on top the the next on the ground around the columns and them jacked into place and attached to the columns
two types of cast-in-place concrete
one-way systems: the slabe and beams are designed to transfer loads in one direction only -beam and girder; spans rage 15-30ft -concrete joist system; spans range 20 - 30 ft; good for light to medium loads two way systems: -flat plate, simplest; slab is designed and reinforced to span in both directions directly into columns; light loads and short spans;good when floor-to-floor height must be kept to a min or an uncluttered underfloor is desired -flat slab; columns have truncated pyramids or cones to to resist punching shear -waffle slab; heavy loads and longer spans; formed from prefabbed metal or fiberglass forms; often exposed with lighting integrated into coffers
precast structural memebers
come in a variety of shapes; members are connected in the field to welding plates that are cast into the member at the plant
prestressed
-hight strength steel cables are stretched in the precasting forms before the concrete is poured -after the concrete attains a certain minimum strength, the cables are released and transfer compressive stresses tot he concrete
single tee and double tee
-precast concrete beams used as floor or roof decking - topped with a 2-3 in concrete pour to give uniform finish
camber
-upward curvature of a structural member caused by prestressing forces of steel cables in the concrete - live and dead loads reduce camber after the structural member is installed
post tensioned concrete
-post tensioning steel (aka tendons) is stressed after concrete is cured -made up of high strength wire, seven-wire strands, or solid bars -post tensioned structural systems are useful where high strength is required and where it may be too difficult to transport precast members to job site
3 types of masonry bearing wall construction
single wythe, double wythe, cavity
advantages of unit masonry walls
compressive strength, design flexibility, attractive appearance, resistance to weathering, sound insulation, ideal for passive solar applications
In masonry walls the thickness of the wall determines three important properties, which are
1)the slenderness ratio: ration of the wall’s unsupported height to its thickness and is an indication of the ability of the wall to resist buckling when a compressive lead is applied from above 2) flexural strength: ability to resist lateral forces such as from wind 3) fire resistance: depends on material and thickness.
what is composite construction
-any structural system consisting of two or more materials designed to act together to resist loads -employed to utilize the best characteristics of each of the individual materials
coefficient of expansion
material property that causes material to shrink and expand due to temperature changes low coefficient materials: masonry how coefficient materials: wood
trusses are…
-structure comprised of different members from a number of triangles -can be used in any direction to span large distances -can be field fabricated or assembled in factory -limited in its ability to be transported from factory to job site
Masonry Joints:
- Horizontal joint reinforcement:
- 16”o.c.
- Must be reinforced horizontally at regular intervals to
- Strengthens wall & control shrinkage cracks
- Ties multi wythe walls together
- Provides way to anchor veneer facing to structural backup wall or substrate
Arch Supports
Hinged or fixed supports
• Hinged arch is primarily subjected to compressive forces
Funicular shape:
hape of an arch to resist a given set of loads only in compression
- Found by suspending the anticipated loads from flexible cable upside down (Gaudi)
- For hinged arch supporting uniform load across its span is a parabola
- However: this never happens. No arch is subjected to just one set of loads. There are
always combinations of compression and some bending stresses
Supports of a hinged arch:
Vertical reactions & horizontal actions or thrust
- Loads on arch tend to force it to spread out thrust must be resisted
- Tie rods: hold two lower portions together
- Foundations: specific to prevent thrust
- For a given span thrust is inversely proportional to the rise/height of the arch
- If rise is reduced by one half, the thrust doubles
- Shape of arch selected for aesthetic appeal not always ideal shape for loading
- Typical arch spans:
- Wood: 50’ – 240’
- Concrete: 20’ – 320’
- Steel: 50’ – 500’
- Although arches may have fixed supports, they are usually top hinged to allow it to remain flexible and avoid developing high bending stresses under live loading and loading due to temperature changes and settlement
Three hinged arch:
additional hinged connection at apex which makes structure statically determinate
- Two-hinged or fixed arches are statically indeterminate
Rigid Fames
In contrast to a simple post-and-beam system, a rigid frame is constructed so vertical and horizontal
members work as a single structural unit
• Efficient structure b/c three members resist vertical and lateral loads together
• Beam: restrained by columns and becomes more rigid to vertical bending forces
• Columns: resist lateral forces b/c tied together by beam
what structural system is this?
simple post and beam
what structural system is this?
rigid frame with fixed connection at column base
stiffer than hinged frame thrust is greater
what structural system is this?
rigid frame with pin connection at coumn base
Gabled frame:
when a horizontal beam is not required, often in one story construction
- This shape decreases bending stresses in two inclined members and increases compression making configuration more efficient
- Because these rigid frames develop a high moment at connections between horizontal and vertical members, amount of material is increased
Space Frames
Very stiff system consisting of trusses in two directions rigidly connected at intersections
• To and bottom chords directly above and below one another
• Bays created by intersection of two sets of trusses then form square or rectangles
• Triangulated space frame: more common type where bottom chord is offset from top by half a
bay and ea is connected with inclined web members
• Advantages:
- Span: 350’
- Span-to-depth ratio: 20:1 to 30:1
- Lightweight & extremely stiff
- Repetitive nature of connectors and struts for easy fabrication & erection
Folded Plates
• Structure where loads are carried in two directions
- First: load carried in transverse direction thru each plate supported by adjacent plates
- Second: longitudinal direction with each plate acting as a girder spanning between vertical
supports
• Since plates act as beams between supports, there are compressive stresses above the neutral axis
and tensile stresses below
• Reinforced concrete 3” – 6” thick
Thin-Shell Structure
Curved surface that resists loads thru tension, compression and shear in the plane of the shell only
• No moment or bending stresses in a thin shell
• Typically always reinforced concrete 3” – 6”
• Forms: domes, parabolas, saddle shaped hyperbolic paraboloid or barrel vaults
• Span: 40’ – 200’
- Hyperbolic paraboloid: 30’ – 160’
Stressed Skin Structures
Panels of sheathing attached on one or both sides of intermediate web members that panel acts as
a series of I-beams with sheathing being the flange and intermediate being the webs.
• Since two or pore pieces, connection between parts must transfer all the horizontal stress
developed
• Typically made of wood
Suspension Structures
• Typically bridges
• Building example: Federal Reserve Bank in Minneapolis
• Structures are similar to arches: loads must be resisted by vertical reactions and horizontal thrust
reactions
• Suspension structures only resist loads with tension, shape of cable used changes as load changes
• No bending stresses are possible
• With single concentrated load, cable assumes shape of two straight lines (not counting the intermediate sag due to the weight of cable)
• With two concentrated loads, the shape is three straight lines, and so on……
Suspension Structures Disadvantage
since only resist loads in tension, instability due to wind must be stabilized or stiffened with heavy infill material, with cables attached to ground or secondary grid of cables above primary set
Difference between arches and suspension structures
vertical reactions are up and the horizontal thrust reactions are outward
• Since sag tends to pull ends together, horizontal reaction is dependent on amount of sag in cable
- Shallow sags result in high horizontal reactions
- Deep sags result in lower horizontal reactions
What is this Showng?
Uniform horizontal load across suspension structure
What is this showing?
uniform load along cable length
Inflatable Structures
Only resist loads in tension and are held in place with constant air pressure that is greater than the
outside air pressure
• Simplest form: single membrane anchored continuously at ground level & inflated
• Double skin inflatable structure: created by inflation of a series of voids (sim. Air mattress)
- Need for airlock is eliminated
- Other example: one large air pocket supported on bottom by a cable suspension system and
with top supported by the air pressure
• Unstable in wind and cannot support concentrated loads
- Stabilized with a network of cables over top of membrane
Structural System Selection Criteria
FiSt CRIBS
Fire resistance
Style
Construction Limitations
Resistance to Loads
Integration with other Building Systems
Building Use and Function
Social and Cultural Influences
Structural Selaction Criteria: Resistance to Loads
- Primary consideration
- Anticipated loads calculated from known weights of materials and equipment and requirements of building codes
- Unanticipated loads such as changes in use, overloading by extra people or equipment, unusual snow load, ponding of water & degradation of structure itself
- Often, an unusual loading condition will be primary determinant of structural system
Structural Selection Criteria: Building Use and Function
• Occupancy type is one of the primary determinants of a system
- Parking garage vs other • Some needs not so apparent
- Example: Building height is limited & client wants to squeeze as many floors into a multistory building as possible
- Result may be concrete flat plate with closely spaced columns
- Example: Laboratory may need large spaces between floors to run mechanical service
- Result may be deep span, open web trusses but if were to house delicate motion sensitive equipment, then a rigid massive concrete structure
Dead loads
• Vertical loads due to weight of building and any permanent equipment
• Dead loads of structural elements cannot be readily determined b/c weight depends on size which in
turn depends on weight to be supported initially weight must be assumed to make a preliminary
calculation, then actual weight can be used for checking the calculation
• Easily calculated from published lists of material weights in reference sources
• IBC requires floors in office buildings and others with live loads of 80psf or less where partition
locations are subject to change be designed to support a minimum partition load of 20psf & is
considered part of the live load
Load Combinations
• When calculating load, all sources will not act at full values at once
- Snow load will not be present when full wind load exist b/c wind will blow snow off building
• IBC recognizes this and requires several combinations of loads be calculated to rind the most critical
case
• Basic load combinations per IBC Section 1605.2.1 using strength design or load and resistance
factor design are:
Water Loads
Load developed from water is equal to the unit weight of the fluid in pounds per cubic foot multiplied
by its depth
• Water weighs approx 62lbf/ft3
-force exterted on structures by water is called hydrostatic pressure
Wind loading:
pressures, direction and timing are constantly changing
• For purpose of calculation however wind is considered a static force
Factors:
- Velocity: pressure on building varies as the square o the velocity ppsf = .00256v2mph
- Height of wind above ground
- Surroundings: other buildings, trees and topography
- Size, shape and surface texture
Positive pressure on windward side
• Negative pressure (suction) on leeward side & roof
• Local areas where pressure is greater such as corners, overhangs & parapets
Soils are classified into four groups
Sands & gravels
- Clays
- Silts
- Organics
Core borings:
undisturbed samples of soil
- Recovered bore samples are lab tested
- Strength test for bearing capacity
- Resistance to lateral pressure
- Slope stability
- Number of borings taken is determined by
- Size of building
- Suspected subsurface geological conditions
- Local code
- Typically a minimum of four bores unless wide variation is detected
Boring log:
shows material, depth it was encountered, standard designation, moisture content, density
Standard penetration test (SPT):
measures density of granular soils and consistency of some clays
- 2” dia sampler is driven into the bottom of a bore hole by a 140lb hammer falling 30”
- Number of blows (N) to drive cylinder 12” is recorded
Test pits:
trench dug for visual inspection of strata and direct collection of undisturbed samples
- B/c open, practical limit on depth of 10’
• Location of each boring or test pit is shown on a plot plan and given and numbered corresponding
to the boring log
• Tests requested by the architect and paid for by owner & are NOT part of contract documents
Hydrostatic pressure:
Difficulties
- Additional loads
- Waterproofing more difficult b/c pressure forces water into cracks or imperfections
- Ways to minimize:
- Slope ground away from building minimum of 1/4” per foot
- Water from roofs and decks drained away from building with gutters and drainpipes, etc
- Below grade: use drain tile or open web matting
- Relieve pressure against floor slabs
- Layer of gravel below slab
- Use water proof membrane & drain tiles if pressure is significant
Soil Treatment Methods
Dr FiCoDeSu
Drainage
Fill
Compaction
Densification
Surcharging
Proctor test:
method of determining optimum moisture content of fill where fill samples
are tested in the lab to determine a standard for compaction
Specifications call for fill to be compacted between 90% to 100% of optimum Proctor
density
- Higher values for heavy loaded structures
Proctor test moisture content percentage:
2-4%
Wind loading:
pressures, direction and timing are constantly changing
• For purpose of calculation however wind is considered a static force
Factors:
Velocity: pressure on building varies as the square o the velocity ppsf = .00256v2mph
- Height of wind above ground
- Surroundings: other buildings, trees and topography
- Size, shape and surface texture
Describe Wind Loading Pressures around a building
Positive pressure on windward side
• Negative pressure (suction) on leeward side & roof
• Local areas where pressure is greater such as corners, overhangs & parapets
Dynamic structural analysis:
Tall buildings with complex shapes or unusual conditions a computer
model is used to study what forces are developed
- Most cases building codes allow a static analysis of the loads
Static analysis method:
total horizontal shear at base is calculated according to standard formula - Total lateral force is distributed to various floors
Dynamic load:
when load is applied suddenly or changes rapidly
- Dynamic load is treated as a static load multiplied by an impact factor
Impact load:
when force is only applied suddenly
- Automobiles in parking garage
- Elevators in shaft
- Helicopter
IBC specifies minimum requirements for these type of loads
Resonant load:
rhythmic application of a force to a structure with same fundamental period as
structure
Fundamental period:
time it takes structure to complete one full oscillation such as complete swing from side to side in a tall building or up-and-down bounce of a floor
- Usually small but slowly build over time as load repeatedly amplifies the motion of the structure
- Can effect entire structure as repeated gusts of wind on a tall building
- Common problem is vibrating machines where the floor has same period as the machines
vibrations
Tuned dynamic damper:
dampen effects of wind sway that is a very heavy mass attached to the sides of a building with springs of the same period as the building and when building oscillates in one direction the spring mounted mass moves in opposite direction counteracting the action of the wind
Can aide in the reduction of costly wind bracing normally required
Coefficient of expansion:
measured in inches per inch per degree Fahrenheit
- Low coefficient: wood
- High coefficient: plastics
If material is retained so it cannot move and is subjected to temperature change, a load is
introduced on the material
• Failing to account for temperature induced loads causes other failures
- Tight fitting glass breaking when metal frame contracts
- Masonry wall cracking when expansion joints are not provided
Statics:
branch of mechanics that deals with bodies in a state of equilibrium
Equilibrium:
said to exist when resultant of any number of forces acting on a body is zero
Fundamental principles of Equilibrium
- Sum of all vertical forces acting on a body must equal zero
- Sum of all horizontal forces acting on a body must equal zero
- Sum of all moments acting on a body must equal zero
Force or Loads:
any action applied to an object
• Internal structure must resist external load stresses with internal forces of their own equal in
magnitude and of opposite sign
Vector quantity:
direction and magnitude of force
Direction:
line with arrowhead
Principle of transmissibility:
orce acting anywhere along the line of action can be considered
equal or unchanged as long as the direction and magnitude do not change
Types of forces:
- Colinear forces: vectors lie along the same straight line
- Members subjected to collinear forces such as tension or compression are said to be
two-force members - Concurrent forces: lines of action meeting at a common point
- Nonconcurrent forces: lines of action do not pass through a common point
- Coplanar forces: lines of action all lie within same plane
Structural forces:
Any combination of colinear, concurrent, coplanar, and nonconcurrent forces
Stress:
internal resistance to an external force
- Tension
- Compression
- Shear
The formula for stress is expressed as
f = P/A
f= stress
P= force
A= area
Torsion:
type of shear where member is twisted
Bending:
combination of tension 7 compression
Combined loads:
example of when a column is resisting loads from above & lateral wind loads
What steel connection is more suited to resist rotation?
welded connection
Strain:
is the deformation of a material caused by external forces
Moment:
tendency of a force to cause rotation about a point and is the product of the force times
the perpendicular distance to the point about which it is acting and is measured in foot-pounds,
inch-pounds or kip-pounds
