CHh 17 Structural Elements Flashcards
What are the structural questions a designer needs to know about to make informed decisions?
(1) Is an element structural, and is it feasible to modify it? (2) Can floor penetrations be made? (3) Is building movement expected and how can it be accomodated? (4) Is a structural review necessary for the proposed new floor load? (5) How should new interior construction elements interface with existing structural elements? (6) Will fire protection of structural elements need to be repaired, or should new protection be included in the design?
When will the interior designer most likely encounter common structural systems?
When modifications are proposed or required because of interior design decisions.
What are two of the most common steel structural systems?
Beam-and-girder system and open-web steel joist system.
Beam-and-Girder System (Steel)
Large members span between columns and smaller beams are framed INTO them. The girders span the shorter distance, while the beams span the longer distances. Typical spans are from 25’ to 40’, with beams spaced at 8 ft to 10 ft O.C. The steel framing is usually covered with steel decking, which spans between the beams. A concrete topping is poured over the decking to complete the floor slab. (Common in mid- to high-rise office buildings). THERE IS USUALLY LIMITED SPACE BETWEEN THE BOTTOM OF THE GIRDERS AND THE SUSPENDED CEILING. The possibility of adding new mechanical ductwork or installing large recessed light fixtures should be reviewed before final decisions are made about the reflected ceiling plan.
Open-Web Steel Joist System
Joists span between beams or bearing walls. Web joists can span from 20 ft up to 144 ft. Depths range from 8 in. to 72 in. They are typically spaced 2 ft to 6 ft O.C. Steel decking spans between the joists, and a concrete slab is poured on top of the decking. (Usually in one-story or low rise buildings with wide column spacing.) Because webs are open, mechanical and electrical service ducts, pipes, and conduits can easily be run between the web members. Suspended ceilings and other lightweight interior elements can be easily hung from the bottoms of the joists.
What are the two primary types of concrete structural systems?
Cast-in place and precast concrete.
Cast-in-Place Concrete
Concrete is poured into forms where it hardens before the forms are removed. The majority of cast-in-place concrete systems utiliize only mild steel reinforcing set in the formwork before the concrete is placed. Sometimes it is post-tension where there are long span structures.
Precast Concrete
Formed in a plant and shipped to the job site where they are set in place and rigidly connected to form the structure.
Post-Tensions Concrete
Steel cables within the concrete are tightened after the concrete sets, creating extra compression forces in the beam or slab. If a slab is post tensioned, it should not be penetrated for pipes or conduit.
What are the five basic types of cast-in-place concrete?
(1) Concrete Joist System (2) Flat Plate Construction (3) Flat Slab Construction (4) Waffle Slab construction (5) Beam-and-Girder System
Beam-and-Girder System (Concrete)
One of five types of cast-in-place structural concrete systems that functions in a manor similar to a steel systems in which the slab is supported by intermediate beams, which are carried by larger girders. Typical spans are in the range of 15’ to 30’. The slab is poured INTEGRALLY with the beams.
Concrete Joist System
One of five types of cast-in-place structural concrete systems, and is comprised of concrete members spaced 24” or 36” apart, running in one direction, which frame into larger beams. The slab is poured integrally with the joists. Because the joists are close together, it is more DIFFICULT to drill holes for small pipes and conduit.
Flat Plate Construction
One of five types of cast-in-place structural concrete systems, where the floor slab is designed and reinforced to transfer loads directly to the columns, which generally do not exceed 25’ spacing. Flate plate construction is commonly used in situations where FLOOR-TO-FLOOR HEIGHT MUST BE KEPT TO A MINIMUM. Because of the closely spaced reinforcing required, it is often difficult if not impossible to drill these types of floors for electrical service or small pipes.
Flat Slab Construction
One of five types of cast-in-place structural concrete systems, which is similar to flat plate construction, except that drop panels (increased slab thickness around the columns) are used to increase strength. Sometimes the truncated pyramids or cones are used instead of drop panels.
Waffle Slab Construction (Two-Way Joist System)
One of five types of cast-in-place structural concrete systems, which can provide support for heavier loads at slightly LONGER SPANS than the flat slab system. Waffle slabs are often left exposed, with lighting integrated into the coffers.
What does precast concrete consists of ?
Factory-made pieces with high-strength steel cables stretched in the precasting forms before the concrete is poured. After the concrete attains a certain minimum strength, the cables are released and they transfer compressive stresses to the concrete. The members are then shipped to the construction site and set in place.
What do precast concrete floors include?
Single Tees, Double Tees, and Hollow-Core Slabs.
What elements can be precast concrete?
Floors, Columns and Beams
Why are single- and double-tee members a popular form of precast concrete?
Because they can simultaneously serve as beam and floor decking and are easy and fast to erect.
How are single- and double-tee members installed?
A topping of concrete (about 2”) is placed over the tees to provide a uniform, smooth floor surface. Double-tee construction is commonly found in industrial buildings, one- and two-story commercial buildings, and parking garages.
Masonry in structural walls is generally limited to . . .
Loadbeading Walls
Nearly all masonry structural walls are built of . . .
Concrete Block
Brick use is generally limited to . . .
Veneer over wood stud walls or over concrete block walls.
Concrete Block
Manufactured with cement, water, and various agregates, including gravel, expanded shale or slate, expanded slag or pumice, and limestone cinders. The dimensions of CMU is based on a nominal 4” module, with actual dimensions being 3/8” less to allow for joints. Dimensions are referred to by width, height, then length. One of the most common sizes is 8 x 8 x 16. There are two open cells on either side of an intermediate rib. Cells in teh block may be left open when loading is light, or reinforced and filled with grout if more strength is needed. Interior design elements may be suspended from masonry walls by using appropriate types of fasteners.
When does a structural engineer need to be consulted when building a concrete block or brick wall?
When an opening is required.
Loadbearing walls support . . .
loads from above, including live and dead loads.
Live Loads include . . .
People and Furniture
Dead Loads include . . .
The weight of the structure itself.
A loadbearing wall may be made of . . .
Concrete, Masonry, or Wood Framing
Because of their nature as structural supports, loadbearing walls cannot . . .
be removed and can only be pierced for door and other openings if the top of the opening is framed with an adequately engineered lintel or beam.
Interior designers should be able to . . .
recognize common instances of loadbearing walls, either from architectural drawings or from field observations. When it is not clear if a wall is loadbearing or not, an architect or structural designer should be consulted if any modifications are contemplated.
Where are loadbearing walls located in residential construction?
Most exterior walls are usually loadbearing, with some interior partitions also being loadbearing. The first-story exterior walls of a two-story house are nearly always loadbearing.
How can a moderately sized opening be added to a residential loadbearing wall?
Additional studs are installed with a double header or other type of lintel.
In _________ construction, fewer loadbearing walls are used. Most structures are some form of column and beam system with nonloadbearing infill in this type of construction.
Commercial
The _____ walls of high-rise buildings are nearly always structural and cannot be pierced except for small openings for pipes.
Core
When must a structural engineer be consulted?
When the interior designer is unsure whether a wall is loadbearing or not, and when an opening needs to be created in the wall.
What is a building load?
A force acting on a building element.
What are the three major load types?
Gravity, Lateral and Dynamic
What do GRAVITY loads include?
Dead & Live Loads
Dead Loads
The vertical loads due to the weight of the building and any permanent equipment. These include things such as columns, beams, exterior and interior walls, floors, and mechanical equipment. Building designs are initially designed to support all the dead loads. Even though the interior designer will plan interior partitions and other construction elements that change the dead loads, the original design of the building considers these normal dead loads on an average square-foot basis; in initial design, an allowance is made for interior partitioning. THE ONLY TIME A DESIGNER NEEDS TO CONSULT A STRUCTURAL ENGINEER RELATIVE TO DEAD LOADING IS IF PLANS CALL FOR THE INSTALLATION OF UNUSUALLY HEAVY PARTITIONING (such as masonry walls) OR HEAVY EQUIPMENT.
Live Loads
Include the weight of people, furniture and other moveable equipment. Buildings are originally designed to accomodate a particular amount of uniform live load, which is established by building codes for different occupancies.
Concentrated Loads
Codes require that floors be designed to support concentrated loads if the specified load on an otherwise unloaded floor would produce stresses greater than those caused by the uniform load. The concentrated load is assumed to be located on any space 2-1/2 sq. ft.
What should the designer do if a space is being designed for a use other than its original purpose?
If the floor loading will be increased, the designer should consult with a structural engineer to determine if the floor is capable of carrying the additional load and, if not, to have additional structural reinforcement engineered.
Where are structural bays located in some high-rise buildings?
They’re located near the center of the building, and are designed for heavier loading, so file rooms and book stacks should be located in these areas in early planning stages.
Lateral Loads
Include wind loads and earthquake loads. These are provided for in the original design of the building. They may need to be taken into account if interior construction elements will be attached to the structure of the building or are required by code to resist earthquake loads.
In the least severe earthquake zones . . .
No special detailing is required by the interior designer.
