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.
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!
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)
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
