General Structures Flashcards
Minimum Uniformly Distributed/Concentrated Live Loads (Table 1607.1)
- Gives the required floor live loads in psf (for uniform loads) or lbs (for concentrated loads) for different occupancy types or uses
- (e.g.: Heavy Manufacturing occupancy requires a uniform live load of 250 psf, OR a concentrated live load of 3,000 lbs)
• Except for roof uniform live loads, all other minimum uniformly distributed live
loads in table 1607.1 are permitted to be reduced
• May not be reduced for any public assembly occupancy with live
loads ≤ 100 psf
• May not be reduced for any member supporting 1 floor of a parking garage
• Floors must also accommodate concentrated loads
- If a concentrated load acting on any area that’s 2’-6” x 2’-6”, the stresses would be greater than the uniform load of the area and would therefore fail.
- Live loads for each floor of commercial or industrial buildings must be conspicuously posted
1 kip = ?lbs
1000 lbs
Axial Stress is …
that tends to change the length of a body.
Compressive stress is axial stress that tends to cause a body to become shorter along the direction of applied force.
Tensile stress is axial stress that tends to cause a body to become longer along the direction of applied force.
What kind of truss is shown?
Vierendeel truss
subject to bending moment in addition to axial forces
Truss: framework consisting of rafters, posts, and struts
What causes bending stresses in truss members
Trusses withous diagonals
closely space joists place loads between panel point
truss joints that provide restraint against rotation
during the design of a building, the deflection of a beam is calculated to be 0.90”. In order to limit the maximum deflection of the beam to 3/4 inch, how should the deign be changed
a. substitute a beam having a section modulus 20 percent greater
b. substitute a beam having a moment of intertia 20 percent greater
c. substitute a beam having a moment of interia 83 percent greater
d. substitute a beam having a yield point 20 percent greater
substitue a beam having a moment interia 20 percent greater
To find deflection of a beam
Deflection (∆) = 5 x weight in lbs (w) x length in feet x 12”4 (L4) / 384 x 12” modulus of Elasticity (E) x Moment of Inertia (I)” ∆ = 5wL4 / 384EI
To find shortening of a column or elongation of a horizontal member
Deflection (e) = Force (P) x Length (L) / Area of cross section (A) x Modulus of elasticity (E)”
Section Modulus:
is the ratio of a cross section’s second moment of area to the
distance of the extreme compressive fibre from the neutral axis
Deflection:
the displacement of a structural element under a load
Modulus of Elasticity:
how stiff a material is (through how it resists stress)
a material’s resistance to non permanent (or elastic)
deformation
Moment of Inertia:
measure of an object’s resistance to changes to its rotation.
the measure of bending stiffness of a section is called
In designing a cantilever wall, what factors are considered
Overturning moment
sliding force
soil pressure under the footing
bending moment in the stem
what’s this?
Gravity walls: resist forces by own weight only
- Non-reinforced concrete
Retaining walls fail as a whole by overturning or sliding.
• To prevent this, the friction between the footing and the surrounding soil/earth
pressure in front of the toe must be 1.5 the pressure that typically causes the wall
to slide.
What’s this?
Cantilever wall: resists forces by the weight of the structure & weight of the soil on the heel of base
slab
- Often with a key projecting from bottom to increase resistance to sliding
- Most common type & constructed of reinforced concrete
- Toe omitted if at property line or adjacent obstruction
- Economically limited to 20’ – 25’ tall
What’s this?
Counterfort wall: similar to cantilever but a counterfort placed at distances equal or a bit larger than 1/2 the height
- Counterforts: reinforced concrete webs act as diagonal braces
As a whole, how to retaining walls fail?
by overturning or sliding
To prevent overturning or sliding, the resisting moment or forces that resist sliding are generally considered sufficient if a safety factor of 1.5
Example: total dead load of wall + weight of earth backfill acting on footing of a cantilevered retaining wall should be at least 1.5x the overturning moment caused by earth pressure
To prevent sliding, friction between footing and surrounding soil and earth pressure in front off toe must be 1.5x the pressures tending to cause the wall to slide
- Individual components fail such as arm or stem breaks due to excessive movement
- To prevent individual components failure thickness, width and reinforcing of wall must
be designed to resist the moment and shear forces induced
To find the horizontal force on a retaining wall
TO find the pressure at any point along the height of a retaining wall
P=30h
P=Pressure
h=height of wall
The stress at which a ductile material continues to deform without an increase in load is called the …
yield point.
Strain is proportional to the amount of stress applied…but only up to a certain point, which depends on the type of material. that point is called the elastic limit.
Once the elastic limit is reached, the material which change length at a faster ratio than the applied force until it gets to the yield point.
The yield point is when the material continues to deform with little to no load applied. It’s the point of no return…because after that the material will rupture once it hits its ultimate strength.
The line of action
is parallel to and in line with the force.
If lines of action of several forces pass through a common point, forces are concurrent
• If the lines of action don’t pass through a common point, the forces are non- concurrent
• The point is called the center of moments or axis of rotation and the distance, called the moment arm or lever arm, is measured in a direction perpendicular to the line of action of the force
What is staticaally indeterminate
beams whose reactions CANNOT be found from the equations of equilibrium only, but require addditional equations are statically indeterminate
includes continuous and fixed end beams
What is statically determinate
beams whose reactions CAN be determined from the equations of equilibrium only
Simple beams, cantilever beams, and overhanging beams that rest on 2 supports
are statically determinate
What does redundancy in a structure refer to?
Having the ability to redistribute loads to other structural elements in case of overload or failure
Where is one most likely to find redunancy in structure
seismic deisgn
Reduncancy is often provided by secondary systems that can resist part of the lateral force if the primary system fails or is damaged
The description of a given force includes
its magnitude, direction, and point of application.
If the lines of action of several forces pass through a common point, the forces are called
concurrent.
If the lines of action do not pass through a common point, the forces are
non-concurrent.
It is sometimes convenient in the analysis of structure to replace one force with two or more other
forces that will produce the same effect on a body as the original force. These forces are called
–1–, and the procedure is called–2–.
1) components of the original force
2) resolving forces
The centroid of an area is equivalent to the —.
center of gravity of the area.
Total stress is the total internal force on a section and is measured in—.
pounds or kips.
Unit stress is the stress per unit of area of the section and is measured in —.
pounds per square inch, or
kips per square inch.
In every case, the unit stress (f) is equal to –1– divided by –2–
1) the load (P)
2) the cross-sectional area (A).
The ultimate strength of most steel used in building is
58,000 to 80,000psi
Concrete is compression
has an ultimate strength of about
3,000 to 6,000 psi,
Wood
has an ultimate strength of about
2,000 to 8,000 psi.
The maximum permissible unit stress is called
the working stress or allowable stress.
If a load acts through a body’s center of gravity, then
the body has no tendency to rotate, and, tends
to translate in the direction of the applied force.
Hook’s Law states …
that up to the elastic limit, unit stress is in direct proportion to unit strain.
A water tank, plus its contents, weighs 30 kips and is supported on the four-legged braced frame shown. The seismic load is equal to 0.30W and is assumed to act parallel to either axis of the frame. What is the total overturning moment on the frame?
Overturning moment is equal to the seismic load multiplied by the distance from that load to the base of the structure =
(0.30x30)kips x(12 ft. +12 ft. +4 ft) = 9 kips x 28 ft.
=
252 ft-kips
A wide flange floor beam in a building is required to support a new piece of equipment, which will overstress the beam in bending. It is therefore necessary to strengthen the beam. Access is only from below. Which of the methods shown would be most effective, assuming there is sufficient headroom?
D
As no info is given about loads, span, or beam size, the only fact we know for is is that we want to increase the beam’s flexural strength. TO accomplish this we must increase its section modulus. To increase the section modulus efficiently we must provide as much material as possible the maximum distance away from the neutral axis. Of the 4 choices D is most effective
Section Modulus (S) = Moment of Inertia / given constant (c)
A steel column supports a dead load of 120 kips and a live load of 150 kips. The allowable soil bearing value is 4,000 pounds per square foot. What is the smallest pad footing that may be used?
A. 5’6”x5’6”
B. 6’2”x6’2”
C.6’9”x6’9”
D. 8’3”x8’3”
D. 8’3”x8’3”
Remember the basic formula f=PA
P=total load on the footing
A= area of footing
f=(120,000 +150,000)/4,000#ft sq. = 67.5 sq ft
TO determine the side dimension we calculate the square root of 67.5 sq ft=8.22
What is the purpose of the footing shown?
to support two columns where one of the columns is too close to the property line to have a symmetrical footing
Combined Footing:
when 2+ columns are too close to each other or a property
line for separate footings, one footing is poured for them all
Strap/Cantilever Footing:
ike a combined footing, but columns are far apart
Spread Footing:
Most economical…$ method.
• Delivers load directly to soil over a large area
• Area of the footing = load/safe bearing capacity.
THe soil biring log for a building site shows that the upper 15 ft. of subsurface material is loose fill, below which is a thick layer of dense sand. Which of the following foundation systems might be apporpriate on this site
I. Footings placed after the fill is removed and recompacted
II. Footings extending through the fill into the dense sand
III. belled caissons bearing on the dense sand
IV. Piles extending through the fill into the dense sand
III. belled caissons bearing on the dense sand
IV. Piles extending through the fill into the dense sand
Because the upper soils consist of loose fill we must penetrate through the fil to bear on the dense sand below.
Removal and recompaction of fill is only economical up to a soil depth of how many feet
6 ft.
Levels of Soil:!
A=topsoil
B=minerals
C=parially weathered or fractures rock3
D=bedrock
Types of soil
Gravel: well drained and able to bear loads (+2 mm)
Silt: stable when dry, swells when frozen, do not use when wet (.002 - .05 mm)
Sand: well drained and can serve as foundation when graded (0.5 - 2 mm)
Clay: must be removed, too stiff when dry and too plastic when wet ( < .002 mm)
Site Class:
classification assigned to a site based on the types of soils present and
their engineering properties (A: hard rock, B: rock, C: dense soil, D: stiff soil, E: soft soil, E: varies, F: varies w/multiple characteristics)
Eccentric Load:
A load imposed on a structural member at some point other than the
centroid of the section
Strain:
the deformation of a material caused by external loads. Tensile loads stretch,
and compressive loads shorten.
federal reserve bank
john hancock building
kresge auditorium
lake point tower
A steel bar two inches in diameter and 20 feet long resists a tensile load of 50,000 pounds. What is the unit tensile stress in the bar?
Stress (f) = Total Force (P) / Area (A)
Stress (f)=50,000/(π1^2)
=15923 psi
REMEMBER axial stress is the same as both tension and compression!
Statically indeterminate beams include…
beams fixed at one end and simply supported at the other
continuous beams
fixed beams at both ends
What is the purpose of seismic isolation?
The structure is isolated form the ground by specially designed bearings and dampers that absorb earthquake forces.
most of the buildings displacement occurs at the isolators, which are located below the columns at the lowest level.
Seismic separation is…
…separating adjacent buildings to prevent pounding.
True or false. the lateral displacement of s seismically isolated building is less than that of a conventional building.
false
they are the same
Which tends to amplify earthquake motion. Soft ground or firm ground.
soft ground
If the period of the ground motion waves coincides with the natural period of the building, the acceleration of the building will
amplify, causing the building acceleration to be much greater than ground acceleration.
Two framing plans are shown. Compared to the girders in plan A, how much greater is the required section modulus of the girders in plan B?
4 times greater
The maximum moment of the girder in either plan equals wL^2/8 (see equation), where Moment(M)=uniformload(w)xlength(L)2 /8”.
A girders = 72w
B girders = 288w
Also rememner S=M/Fb
Section Modulus (S) = Moment (M) / Bending Stress (Fb)
Flat plate system:
• Basically a Two-Way slab with no supporting beams, only columns.
• Reinforced slab spans in both directions directly into columns at 25’ with 6” - 12”
thickness
• Typically used for light loads, short spans, when floor-floor height must be
minimized, and/or when simple under-side of slab appearance is required
• Has low shear capacity and low stiffness
Advantages and Disadvantages of a continuous beam over simple beams
Advantages:
- continous beams have less maximum positive bending moment
- continuous beams have less maximum deflection
Disadvantages
-continuous beams are subject to negative bending moments over its supports
In seismic design, what forces are usually accounted for?
Although ground motions in an earthquake are both horizontal and vertical , seismic design usually accounts for the horizontal forces only, but not vertical effects.
Which is more likely to withstand the effects of an earthwuake without failure, a special moment resisting frame or a shear wall
special moment resisting frame
The natural fundamental period of vibration of a building is the time it takes for it to …
…go through one complete back and forth motion when subject to a lateral load.
Describe the ersulting accelerations and seismic forces of a building with long periods. Short periods?
Long periods = low accelrations and low seismic forces
short periods = high accelerations and high seismic forces.
Torsion is caused because …
…the center of mass and the center of rigidity
cannot geometrically coincide
Reentrant corner is the common characteristic of building forms that assume
the shape of …
… an L, T, H, etc
What is the diffrence between stress and strain?
Stress is the force on a material, strain is what happens to the material under a given stress.
Slip Critical:
when any slippage cannot happen as it would risk the structure (e.g.:
when the joints are subject to fatigue loading, the joints have oversized holes, the
entire load is carried by friction)
Standard round holes are 1/16” larger than the diameter of the bolt
• Slotted holes are used where some adjustment is needed
• The effect of reducing cross sectional area of the members or net area must be
checked.
• Connection’s shear failure is parallel to the load
• Connection’s tension failure is perpendicular to the load
• Spacing of bolts and edge distance from the last bolt to the edge of the member is
critical
Arches are usually top hinged to …
…allow it to remain flexible and avoid developinghigh bending stresses under live loading and loading due to temperature changes and settlement
THe moment of interia depends only on..
the shape and dimentions of the section.
Creep
tendency of a material to move slowly or deform permanently under stress
Fatigue:
progressive damage that occurs when a material is subject to cyclic loading
Laitance:
an accumulation of fine particles on the surface of fresh concrete due to
upward movement of water. Occurs when there’s too much water in the mixture. Concrete appears “chalky”
Abrams Law:
compressive strength of concrete is inversely proportional to ratio of
water to cement
Hydration:
chemical hardening of concrete
What shear/moment digram is this?
Moment diagram for a simple eam with concentrated load at center
What shear/moment digram is this?
moment diagram for a beam fixed at both ends with a concetrated load at center
What shear/moment digram is this?
Shear diagram for a beam (simple or fixed at both ends) with a concetrated load at the center
What shear/moment digram is this?
moment diagram for a beam fixed at both ends with a uniformly distribtued load
Shear stress in a column pad is a function of …
column load, column size, and thickness of the pad
Underpinning:
the process of strengthening and stabilizing the foundation of an existing building
Dowel Type Fasteners:
(nails, screws, bolts) that transmit lateral loads via bearing
stresses between the fastener and members of the connection OR that transfer withdrawal loads parallel to the fasteners axis via friction or bearing to the connected materials
Bearing Type Fasteners:
(shear plates) that transmit lateral loads only by shear forces via bearing on the connected materials
Hangers:
combination of dowel and bearing type fasteners that support one structural member and are connected to another member by a combination of dowel and bearing action
Plate Girder:
assembly of steel plates, or plates and angles, fastened together to form an integral member
Shoring:
supporting a structure in order to prevent collapse so that construction can proceed. (e.g.: support beams and floors of building while a column/wall is removed, shoring in trenches for worker safety in excavation)
Critical net section:
section where the most wood has been removed
Friction Pile:
Driven into softer soil.
• Friction transmits the load between pile and soil. “
• Bearing capacity is limited by whichever is weaker: strength of the pile or soil”
• Pile Foundations:
used when soil is unsuitable for spread footings (e.g.:
expansive soils or clay near surface) by transmitting loads through soil to a more secure bearing farther below
• Located in groups or in alignment under a bearing wall
• Load transferred from wall to pile caps.
• Piles are either driven (timber, steel, precast conc) or drilled (caissons) Belled
Caissons: holes are drilled to firm strata and concrete poured. • They’re basically really, really deep spread footings”
• Mat Foundations:
Very expensive…$$$ method.
• Typically it’s only used when the strata is weak,
• It acts as one continuous foundation.
• Socketed Caissons:
like Belled Caissons, but the hole is drilled deep into the strata. Bearing capacity comes from end baring and frictional forces.
• End Bearing Piles:
2-3x cost of spread footings.
• Driven until tip meets firm resistance from strata
• Occupancy Categories of Building and other Structures (Table 1604.5)
• Category I: buildings/structures that represent a low hazard to human life in the even of failure (eg: agriculture facilities, minor storage)
• Category II: buildings/structures that aren’t in category I, III, or IV
• Category III: buildings/surfaces that represent substantial hazard to human life in
the event of failure (e.g.: schools, jails, anything with occupancy greater than
5,000, healthcare facilities with more than 50 occupants but no surgery/ED)
• Category IV: buildings/structures designated as essential (e.g.: hospitals with
ED/surgeries, fire/police/rescue stations/garages, emergency shelters, defense,
air traffic control)
Slenderness ratio is used to
determine maximum allowable stress for a column
most critical consideration in column design.
the greater the unbraced length the greater the slenderness ratio
The load carrying capacity of a wood columns is determined by several factors:
Modulus of Elasticity, which in turn depends on its species and grade
design compressive stress, which also depends on species and grade
the ratio 1/d, where 1 is the unbraced height and d is the least lateral dimension of the coumn
Cross sectiononal area of the column
A complete penetration groove weld is…
… one whose depth is the same thickness as the member
strength is considered to be the same as that of the connected material
… placed between two butting plates or members and is usually stressed in direct compression or tension
The maximum size of coarse aggregate that may be used depends on …
..the size of concrete members and the spacing of the reinforcing bars
aggergate szie should no exceed 1/5 of the narrowest dimension between sides of forms or 3/4 the clear spacing between reinforcing bars
ususally more water is required for smaller size coarse aggregates
for a given water cement ratio, the amount of cement required increases as the maximum size of the coarse aggregate decreases, Therefore, for economy, the maximum size of coarse aggregate should be as large as possible
The stub girder system:
…consists of main steel girders framed between columns, above which short lengths of stub gurders are welded
Typically 5’ long and spaced 5’ apart
The horizontal thrust at each support of a 3 hinged arch is directly proportional to …
the load
the span
and inversely proportional to the rise
a 3 hinged arch rotates when the temperatur changes
What is the most widely used structural steel for wide glange shapes in buildings?
ASTM A992
It is high strength, low alloy steel.
Teh deflection of a steel beam is related to
The tendency for a column to buckles increase with and increase in –1–
The tendency will also increase with a decrease in –2–
1) increase in length
2) decrease in radius of gyration
How are the diagonal members of the X-bracing of a tall structure normally designed to minimize cost?
One diagonal brace is deisgned to be stressed in tension while the other is not stressed.
Diagonal braces are not designed to work in compression. By designing diagonal members as tension members instead of compression members, their size and therfore their cost is minimized.
Wood I joists are generally used for spans …
…larger than 20 ft up to about 45 ft.
A flat plate is
What is the function of the stirrups in reinforced conrete?
to resist shear
One Way Concrete Joist system (pan joists):
• Prefab metal pan forms are used to create frame to support light/medium loads
with spans of 20’ - 30’ and depths of 1’ - 2’
• Formed with prefab metal pan forms spaced 24” – 36” apart in one direction
Prestressed conrete beams may be used for spans with lengths of
100 ft
Used for long spans and heavy loads
a raft, or compensated footing is
a mat foundation placed deep in the soil so that the weight of the excavated soil is about equal to the weight of the building
How high can each of the following retaining wall go
gravity
cantilever
counterfort
Portland cement: binding agent in concrete made of lime, silica, iron oxide, and
lumina which interacts with water that combines to form paste that binds aggregates together
• Type I: standard cement used for general construction
• Type II: modified cement where heat of hydration needs to be controlled
• Type III: high early strength cement where quick set is required
• Type IV: low heat cement for very slow setting, used to avoid damage caused by
heat
• Type V: sulfate resisting cement, where exposed to water or soil with hight
alkaline content
bridging may be omitted if the joist span is less than…
… 32 feet.
The answer is BRIDGING MAY BE OMITTED IF THE JOIST SPAN IS LESS THAN 32 FEET. The load capacities for open web steel joists are tabulated in the Standard Specifications of the Steel Joist Institute. These tables include spans of up to 144 feet for DLH series joists, and tabulate both the load capacities and the loads which will produce a deflection of 1/360 of the joist span. This deflection load is tabulated because deflection is often critical for joists, especially on long spans. The Specifications also specify the number of rows of bridging that must always be used in steel joist construction.
The ratio Kl/r is a measure of the …
buckling tendency of a steel column;
the larger the value of Kl/r, the greater the tendency of the column to buckle, resulting in a lower column capacity. In this ratio, K is a constant determined by the degree of fixity at the ends of the column (II), and the buckled shape of the column at failure (III), and is found in the table on page xx. An increase in the value of K results in a higher value of Kl/r, a greater tendency of the column to buckle, and consequently a lower column load capacity (IV). The value of K is not related to the yield strength (F y ) of the structural steel (I is incorrect).
The greatest shrinkage in wood members occurs …
…across the grain, and shrinkage parallel to the grain (longitudinal direction of the member) is negligible.
