Structural Systems
Fujita scale
(F-scale) used to classify tornadoes and their damage F-0: classifies wind speeds below 73 mph that result in light damage F-5 : represents incredible damage and wind speeds from 261 mph to 318 mph
value of gradient height
-900 ft for open country area -1200 ft for suburban areas -1500 ft for urban or metropolitan areas
Stiffener in steel rigid frames
-A plate used to increase the capacity of a member's web and flanges to resist buckling -fits between the flanges, normal to the web, and is welded to the web and flanges
ASD
-Allowable Strength (Previously Stress) Design -uses service load stresses against an Allowable Stress -ASD provides a constant factor of safety for all designs regardless of load types -there is one equation for design strengths that are divided by a safety factor, for ASD and multiplied by a strength reduction factor, for LRFD.
Building codes cover which types of earthquake effects or damages?
-Building codes cover ground shaking or vibration effects -Building codes do not cover landslides, soil liquefaction, and avalanches
complex wind calculations and wind tunnel testing are required for:
-Buildings with heights exceeding 400 ft, for buildings subject to dynamic effects, those sensitive to wind vibrations, and for buildings with height to width ratio of 5 or more. -also often carried out on reduced-scale models of long-span suspension bridges
LRFD
-Load Resistance Factored Design -uses factored load forces against a maximum Strength -LRFD provides a higher factor of safety on the loads that are less well defined (1.6 LL factor compared to a 1.2 DL factor)
Minimum coverage of concrete coverage over rebar according to ACI
-Walls and Slabs: 3/4" -Beams and Columns (interior): 1.5" -Beams and Columns (exterior): 2" -Foundations: 3"
response spectrum
-a graph that shows, for different soil types, the relation between the ratio of building acceleration to ground acceleration and the period (T) in seconds. -peak acceleration of a building or structure increase as the structure's period increases, and it reaches a maximum when the period is 0.5 seconds
Special moment-resisting frame (SMRF)
-a rigid frame that is designed and detailed to ensure ductility and to be able to absorb a large amount of energy in the inelastic phase without failure or unacceptable deformation -could be made of structural steel or reinforced concrete
raft footing
-aka compensated footing -a mat foundation that is placed deep in the soil so that the weight of the excavated soil is about equal to the weight of the building
Ordinary moment-resisting frame (OMRF)
-are permitted only in buildings assigned to seismic design categories A and B -may be built using structural steel or reinforced concrete -Steel ______ are allowed without any limitations in buildings of seismic design categories A, B, and C -In seismic categories D and higher, steel OMRFs are permitted under certain conditions only
Bracing of nonstructural (architectural) elements
-code requires that both structural and nonstructural elements be designed for lateral seismic forces -the seismic factor used in designing the bracing for nonstructural elements is greater than that used for the building structure -bracing for nonstructural elements must be designed for seismic forces coming from any horizontal direction
Stub girder system
-consists of main steel girders framed between columns, above which short lengths of stub girders are welded. -they are typically 5 feet long and spaced about five feet apart -Shear connectors are welded to the tops of the stub girders to provide composite action with the concrete slab. -The concrete slab above the stub girders acts with the main girder to form a sort of Vierendeel truss, in which the main girder is the bottom chord, the slab is the top chord, and the stub girders are the verticals
eccentric braced frames
-developed with the objective of making braced frames more ductile and more flexible -with eccentric chevron bracing, a small gap (called an eccentricity) is provided between the two braces at their top connection point, thereby creating more flexibility in the system -frame in which diagonal members are connected to a beam a short distance from the column joint
Glulam
-fabricated from thin layers of wood glued together -generally consists of 2 to 60 lams -Both dry-use adhesives and watertight glue can be used to fabricate glulam -Standard widths of glulam beams are between 2 in and 14 in -can span more than 100 ft
Wood I-joists are generally spaced:
-like lumber joists, often at 12 in, 16 in, or 24 in o.c. -can be used for spans up to 45 ft.
Strong-motion accelerograph
-measures the acceleration of the ground or building during an earthquake. The ground doesn't move at a uniform velocity or speed during an earthquake. Acceleration is the change of velocity. -normally stop working when subject to strong ground motion. Will be activated by the earthquake, record the earth motion, and then stop working. -IBC requires that in seismic design categories D, E, and F, every building of more than six stories have an aggregate floor area of at least 60,000 SF.
one-way reinforced concrete slab
-most commonly used reinforced concrete slab system -typical short direction spans are 10, 12, 14, and 16 ft.
Bridging of open-web joists
-provided for lateral stability -provides lateral stability mainly to lower chords, upper chords held in place by the decking material -bridging may be horizontal or diagonal -size of bridging depends on the slenderness ratio of the element
pan joist concrete deck
-reinforced concrete slab ribbed in one direction only -ribs usually 18 in. to 30 in. on center -joists generally run in the short direction, and the beams in the long direction -usually used for heavier loads such as storage or industrial buildings
Strain
-the deformation or change in size caused by external forces applied on a member -an elongation under tensile forces and a shortening under compressive forces -Strain = stress/Modulus of elasticity
gradient height
-the height above which ground friction and other obstructions no longer reduce wind speed -depends on the type of area (open, suburban, or urban)
Arch
-the horizontal thrust at each support is directly proportional to the LOAD and SPAN -horizontal thrust at each support is inversely proportional to the rise
The spacing of stirrups in reinforced concrete beams depends on
-the magnitude of shear forces. Stirrups are placed to help resist shear that cannot be resisted by concrete alone -the grades of concrete and steel used. Higher grades better resist shear stress than lower grades
Torsion
-the rotation or twisting in a diaphragm that results when the center of mass is different from the center of rigidity -occurs only in rigid diaphragms
In the floor framing of wood-framed buildings using lumber joists, rows of bridging are generally provided:
1. at the joist midspan 2. on rows not more than 8 ft. apart 3. in the form of cross bridging or solid blocking -the function of bridging is to distribute the strength of the floor and the resistance to lateral loads
The moment of inertia for the cross section of a member is related to the
1. shape of the cross section 2. dimensions of the cross section
Section Modulus (S) for the cross section of a member is related to the
1. the moment of inertia of the cross section 2. the depth of the cross section
in the design of a retaining wall, and according to most codes, the minimum safety factor to consider for sliding is
1.5 2.0 to consider for overturning
In the design of a retaining wall, and according to most codes, the minimum safety factor to consider for sliding is:
1.5 A factor of 2.0 is generally considered for overturning
According to IBC, the dead load resisting moment must be at least _______ the overturning moment caused by wind forces.
1.67 times
According to AISC Specification, the largest size for a fillet weld is
1/16" less than the thickness of the material
Maximum drift should be limited to _______ of a building's height and drift between stories should be limited to ________ times the story height
1/500; 0.0025
Reinforcement bar spacing in reinforced brick masonry walls is generally:
18 in to 36 in. Vertical and horizontal no. 4 and no. 5 rebars are normally placed in this space
IBC requires a minimum dimension of _____ from the top or bottom of joist to holes
2 in.
Glulam members of the stress class 24F have and allowable bending stress of:
2400 psi. The number preceding the letter F in the combination symbol of glulam is always the value of Fb in hundreds of psi.
In a dual system, the moment-resisting frame must be able to resist at least ____% of the base shear.
25
Modulus of elasticity for steel
29,000 ksi
An increase in magnitude of 1 on the Richter scales corresponds to and increase of ____ times in energy released
32
standard height for wind measurements
33 ft
maximum spacing between reinforcing bars
3x the slab thickness or 18 in., whichever is smaller
In residential applications, the minimum thickness for a concrete slab on grade is generally
4 in. 8 in. for industrial buildings and warehouses with heaving loads
Building codes limit the span-to-depth ratio of a plywood diaphragm to
4:1 This prevents excessive horizontal deflection of the diaphragm
International building code requires that seismic design be in accordance with ASCE 7, which requires that, when providing for accidental torsion, the mass at each level of a building is assumed to be displaced in each direction a distance equal to:
5% of the building dimension at that level in the direction perpendicular to the direction of force.
ASTM 992 tensile yield strength
50 ksi
The rotational speed in a tornado may exceed:
500 mph and averages 250 mph
In the open-web steel joists structural system, the deep-long span joists of the DLH-series have depths up to
6 feet
Open-web K-series steel joists can span up to:
60 ft.
Aggregates generally represent _____% of concrete volume
60% to 75%
lowest value of basic wind speed on the U.S wind map provided by the International Building Code is:
85 mph
dual system
A structural system with two components: 1.) a complete frame system supports the gravity dead and live loads 2.) specially detailed moment resisting frame and shear walls or braced frames resist the lateral loads
Seismic design categories
A: buildings in areas where expected ground shaking is minor B: buildings of occupancy categories I, II, and III, in areas where moderately destructive ground shaking is expected. C: buildings of the occupancy category IV, in areas where moderately destructive ground shaking might take place, and to buildings of occupancy I, II, and III, in areas where a somewhat more severe ground shaking is expected. D: buildings and structures in areas expected to experience severe and destructive ground shaking but not close to major active fault lines E: buildings of occupancy I, II, and III in areas located near major active faults F: buildings of occupancy IV in areas located near major active faults
Wind exposure categories
B: refers to urban and suburban wooded areas and terrain with obstructions C: refers to open terrain, such as in a desert area D: refers to flat and unobstructed terrain near large bodies of water D is most severe wind exposure of the three
Importance factor
Considers occupancy type
Modulus of elasticity
E= stress (F)/strain E = (P ÷ A)/ (Deformation ÷ L) Deformation = (P ×L) ÷ (A × E)
What are the different types of seismic waves?
Earthquakes generate three types of seismic waves: P (primary) waves- cause most damage to the building S (secondary) waves surface waves P and S waves penetrate interior of the Earth; known as "body waves"
Modified allowable bending stress (for wood)
F'b=(Fb)(Cm)(Cd)(Ct)(Cf)(Cr) Cm: wet service factor (1 for dry conditions) Cd: load duration factor(for floor live load Cd is 1) Ct: temperature factor (1 for normal temperature) Cf: size factor Cr: repetitive member factor
Allowable bending stress Fb
Fb= (Mmax)/S
What shapes are used for steel piles?
H shapes (8-14 in. deep) and round pipes
For a three hinged arch, the horizontal thrust is directly proportional to ______ & ______ and inversely proportional to the _______.
Horizontal thrust is directly proportional to the load and the span Inversely proportional to the rise
moment of inertia
I=(bd^3)/12
How are lateral loads transferred in a flexible diaphragm vs a rigid diaphragm?
In a flexible diaphragm, lateral loads are transferred to the vertical resisting elements according to tributary area. Ex. if supported by two end walls and one central wall, the central wall will resist 1/2 the total lateral load, while each end wall will resist 1/4 of the total lateral load. In a rigid diaphragm, the lateral loads are transferred to the vertical resisting elements in proportion to the relative stiffness of these elements. Ex. a wall that is twice as rigid as another wall will resist twice the lateral load resisted by the wall of lower rigidity.
According to ACI code req, the minimum thickness for a one way simply supported reinforced concrete slab with a span L is
L/20 L/10 minimum thickness for cantilever slab L/28 minimum thickness for continuous one-way slab
________ forces are directly proportional to the dead load.
Lateral earthquake forces are directly proportional to the dead load; the greater the dead load, the greater the seismic force. Lateral wind forces are unrelated to a building's dead load.
Maximum moment for simply supported beam with uniformly distributed load
M(max)=(wL^2)/8 or M(max)=(wL)/8
Maximum moment for simply supported beam with point load at the center of the span
M(max)=PL/4
direct wind pressure equation
P(wind pressure)=.00256v^2 P: wind pressure in pounds-force per square foot v: wind velocity in mph
Pier Luigi Nervi
Palazzetto dello Sport area Rome Olympic Games Complex
Section Modulus
S=I/c (I): the cross section's moment of inertia (c): the distance from the cross section's neutral axis to its extreme fibers S(min req.)=M(max)/F'b (M): maximum bending moment (F'b): modified or adjusted allowable bending stress -expressed in in^3
Design spectral response at period of 1.0 sec (SD1) equation
SD1= (Cs)/T(R/I) Cs: seismic response coefficient T: actual period of building R: response modification coefficient I: Importance factor
For spans of 300 ft or more, the most appropriate steel structural system is generally:
Steel arches. They are used extensively to support roofs covering large unobstructed floor areas in structures such as hangars, field houses, and exhibition halls. Rigid frames are generally preferred for intermediate spans
Type II Portland Cement
Sulfate resisting cement, used only when the structure is in contact with either soil or water containing sulfate
Period of vibration equation (T)
T=(Cu)(Ta) Cu: coefficient for upper limit on calc. period Ta: approximate period of vibration Ta=(Ct)h^x Ct & x: approximate period parameters h: height
In a steel dome, the ring at the bottom functions in ______________.
Tension. It's often referred to as the tension ring. The dome tends to open up at the bottom and exert outward thrusts continuously around the perimeter.
According to ACI code, the maximum spacing between lateral ties of a reinforced concrete column should be
The least of the following dimensions: 1.) 16 times the reinforcing bar diameter 2.) 48 times the tie diameter 3.) the least dimension of the column
According to IBC, what is the minimum width of a plywood shear wall?
The minimum width of a plywood shear wall is determined by the height of the shear wall
ultimate uniform load
The resulting loads of service loads (real expected loads) muliplied by load factors (safety factors) W(u)=1.2W(dl)+1.6W(ll)
Tendency of steel beam to buckle
The tendency of a beam to buckle increases relative to the ratio lx/ly, where lx is the moment of inertia about the horizontal centroidal axis and ly is the moment of inertia about the vertical centroidal axis.
Yield Point
The unit stress at which a material deforms with no increase in load
Rebar grades
There are three strength of steel used in making rebar today: grades 40 (generally only found in smallest sizes like #3 and #4), 60, and 75 (seldom used) representing steels with yield strengths of 40 ksi, 60 ksi, and 75 ksi, respectively.
The effect of ______________ is not covered in building codes.
Tornadoes. Their effects are difficult to represent and consider so they are no covered in building codes or considered on the wind map. Wind velocity and direction change very quickly in a tornado, and wind researchers have difficulty modeling tornadoes in wind laboratories. The basic rules of design for high winds are generally applied for tornado prone regions. Tornado shelters are most often designed for a wind velocity of 300 mph.
Ultimate tensile capacity of a reinforcing bar equation
Tu=(As)fy As: Area fy: yield strength (ex. grade 40 reinforcing steel has a yield strength of 40 ksi)
Most frequently used types of portland cement
Type I (general purpose)-least expensive, for general applications and Type III (high early strength)- used when high strength is needed at an early age, allows forms to be removed sooner and reused quickly, thus reducing construction time
Base Shear equation
V=(Cs)W Cs: seismic response coefficient W: effective seismic weight of the building
Factor (Kz) in wind design
Velocity pressure exposure coefficient evaluated at height z; takes into account: -building height -exposure -wind gusting
Components of a steel rigid frame are generally:
Wide-flange shapes and built-up sections from a web plate to which flange plates are fillet welded
drift
a building's lateral (side-to-side) displacement due to wind forces
drag strut
a member that collects and transfers lateral forces to a shear wall, braced frame or moment-resisting frame
radius of gyration
a property of the geometry of the section r = (SQRT (I÷A)) is critical in the assessment of the slenderness of steel columns. The lesser the radius of gyration, the more likely a steel column is to buckle (the other factors that affect the slenderness of a steel column are its un-braced length and how strongly its ends are connected.
space frame
a series of trusses of equal depth that intersect each other in a consistent grid pattern and are connected at their points of intersection Loads are supported by trusses in both directions and the entire system works as a unit.
soft story
a story in which the lateral stiffness is less than 70% of that in the story above or less than 80% of the average stiffness of the stories above.
Joist girder
an open web steel truss used as a primary framing member. designed as a simple span member supporting equally spaced concentrated loads from a floor or roof system, generally consisting of open web steel joists.
Throat area of of fillet weld is
assumed to be 0.707 times the weld size
fault
boundary between tectonic plates
The strength design load combinations that must be considered with seismic loads in building design include:
dead load, floor live load, and snow load roof live load, is never combined with seismic loads when strength design load combinations are used.
In wood-framed buildings, the thickness of the plywood subfloor is:
depending on the spacing of the supporting joists. -Traditional floor systems consist of the subfloor, sometimes an underlayment, and a finish floor.
A soil's load-bearing capacity increases with
depth load-bearing capacity is also related to soil type
Base isolators
designed to isolate the structure from the ground in order to reduce the effect of ground motion on the structure. They consist of dampers and steel bearings placed at the base of columns. They reduce the building's acceleration due to ground shaking and absorb the earthquake energy.
Moment-resisting or rigid, frames resist lateral forces by
developing bending.
reinforced bar spacing for reinforced brick masonry walls
from 18 in. to 36 in.
drift
horizontal deflection of a building when a building resists seismic or wind load
tectonic plates
irregular plates that form the earth's crust. These plates are floating and constantly moving, and the movement of these plates is what primary causes earthquakes
What is the balanced steel ratio as applied to reinforced concrete
is the reinforcement ratio that would result in a simultaneous yielding of the steel and crushing of the concrete.
revolving cup anemometer
is used to measure wind speed by measuring the time it takes a 1 mi column of air to pass by. This 1 mi distance is determined by the cup's revolutions.
The moment-resisting frame is an appropriate system to use for resisting wind forces in
low-rise buildings and high rise buildings with 30 stories or less. For taller buildings, other systems should be considered or some additional type of bracing such as X or chevron bracing.
Most flexible lateral load resisting system uses
moment resisting frames this system resists lateral loads by bending and flexure in the members. Joints are designed to be rigid and bending generally occurs in the beam and column components of the frame.
A diaphragm is considered flexible when the maximum lateral deformation is
more than twice the average story drift of the story it's designed for
cantilever wall
most efficient retaining wall for depths between 10 ft and 20 ft.
Moment directions
positive if counterclockwise, negative if clockwise
Adding a shear key to the base of a cantilever retaining wall helps to:
prevent sliding of the wall by increasing the contact surface and therefore the fricitonal forces between the base of the wall and the soil.
epicenter
projection of a hypocenter onto the surface of the earth
basic wind velocity pressure equation
q=0.00256(Kz)(Kzt)(Kd)v^2*I I: importance factor Kz: exposure category Kzt: topographic factor (1 for flat site) Kd: directionality factor (.85 for all building structures)
Prestressed concrete
reinforced concrete in which some initial compression stresses are introduced prior to loading. Prestressing increases a concrete element's capacity to resist the tension stresses generated by loads. Prestressed concrete beams are used for long spans and heavy loads. They can span 100 ft or more.
Thermal stress
s=EαΔT α: coefficient of thermal linear expansion E: modulus of elasticity ΔT: change in temperature (Tf-Ti) If the temperature rises above the normal, the rod will be in compression, and if the temperature drops below the normal, the rod is in tension.
dimension lumber
sawn lumber elements with a nominal width between 2 in and 4 in
timber
sawn lumber elements with a thickness of more than 5 in -minimum size of columns must be 8 in by 8in -requires exterior walls of noncombustible materials, such as masonry or concrete
boards
sawn lumber elements with nominal thickness of less than 2 in.
In fillet welds, the stress is considered to always be_______.
shear stress on the minimum throat area, regardless of the direction of the applied load.
counterfort wall
stronger retaining wall that is generally used for depths of about 25 ft or more.
A shell structure has a curved surface and supports load and supports load by:
tension, compression and shear in its own plane. It is too thin to resist bending stresses, which makes it unable to support any substantial concentrated loads.
P-delta
the increase in bending moments induced by the vertical loads acting on the laterally displaced building frame
hypocenter/focus
the location deep within the earth where the seismic movement of rock begins
The AISC Specification requires a weld length at least equal to:
the perpendicular distance between welds
Equation for total elongation due to the rise in temperature
ΔL=αLΔT α: coefficient of thermal linear expansion L: length in inches ΔT: change in temperature
Maximum Deflection for simply supported beam with uniformly distributed load
Δmax=(5wL^4)/384EI or Δmax=(5WL^3)/384EI P: point load L: length of the beam E: modulus of elasticity I: moment of inertia
Maximum Deflection for simply supported beam with point load at the center of the span
Δmax=(PL^3)/48EI P: point load L: length of the beam E: modulus of elasticity I: moment of inertia
