huangyhg

help - restrained vs unrestrained construction
i have two questions regarding the definition of a "restrained" construction vs "unrestrained" construction, as it relates to fire rating requirements.
1. is the definition located in a building code, such as ibc or fbc? i have searched florida building code and found nothing.
2. i have a 3 story building, tilt panel walls, with precast concrete floor joists, and cast in place slab on the joists. the question has come up about the fire rating requirement for the building. i am trying to determine if this construction type is restrained or not. in this case, it could make a very big difference. the slab is doweled into the wall panels. however, there is no direct connection from the joist to the panel. the joists are simply sitting on a continuous haunch. the joists are of course tied into the slab once it is poured. any thoughts on if this is restrained or not?
thanks
andrew
1. the definition is found in astm e 119 "standard test methods for fire tests of building construction an materials". the pci design handbook also covers this.
2. from pci design handbook, table 9.3.4.1 footnote c: "resistance to potential thermal expansion is considered to be achieved when continuous structural concrete topping is used".
here is an excerpt from the minutes of a meeting at ul. it doesn't give you the answer, but maybe a way to get it.
"2001 fire council page 22 may 8-9, 2001
restrained and unrestrained members
after 30 years, the determination and impact of restraint with respect to fire endurance ratings continues to be a challenging issue. ul’s fire resistance directory contains information on the stiffness of ul’s test frame. this information is provided to enable a structural engineer to determine whether a specific building frame provides an equivalent degree of restraint to that provided in the fire test conditions.
the standard for fire tests of building construction and materials, ul 263; astm e119 standard test methods for fire tests of building constructions and materials;
and nfpa 251 standard methods of tests of fire endurance of building construction and materials include a guide for determining conditions of restraint for floor and roof assemblies and for individual beams. this guide is intended to assist in determining when the restrained or unrestrained rating should be used."
this is the definition from the ul fire resistance directory....
the time ratings assigned to each illustrated design are given in terms of a restrained or an unrestrained condition, or both. a restrained condition in a fire test is considered to be one in which the expansion at the supports of a load-carrying element resulting from the effects of the fire is resisted by forces external to the element. an unrestrained condition is one in which the load-carrying element is free to expand and rotate at its supports.
thanks all for the info. it is still clear as mud to me though. on one hand, i do have a continuous concrete slab, which is doweled into the tilt wall panels. so i think restrained.
on the other hand, the supports for the slab are precast joists, which are free to rotate at the supports. so i think unrestrained.
thanks for the tip on the pci manual. i know i have seen the table somewhere before. i spent about 30 minutes yesterday scouring through my fbc and ibc trying to find the table. the thing that kills me is they reference restrained/unrestrained, but don't give you a definition, or even a link to a source of the definition, that i could find anywhere. very frustrating.
i will have to do some more research i guess, and maybe talk to the building official or fire marshall to get a determination.
the reason this is a problem is the type of construction required is type i. if this is the case, we need a 2 hour floor. the floor joists have a x-sectional area of only 100 sq.in. or so. not enough to get a 2 hour rating according to the fbc. so we could not use these joists. of course, the architect found out about the type i construction requirement only yesterday. the first floor slab is already cast.
thanks for all your help.
andrew
there was an article in some engineering journal a few years ago where the author contended that almost all real-life conditions in buildings with fires are in a restrained condition.
he reasoned that in cases where the fire was on the internal area of the floor plan, the local fire condition was such that the surrounding steel/concrete/whatever framing and the floor diaphragm would constrict lateral expansion of the system such that it would behave as a restrained condition.
the only unrestrained conditions would be where fires occurred on near the perimeter of the floor diaphragm where that restraint would not occur.
according to research published for steel framed construction by robert idling and boris bresler, the important type of restraint is rotaional restraint, not axial or in plane expansion. only moderate amount of rotational restraint is needed for an assembly to perform as restrained. apparently, beams framed with single plate shear tabs provide enough restraint. the authors go on to say: "other factors influence the degree of rotational trstraint in large steel framed floor assemblies. if continuity and/or composite action are part of the floor system, fire tests have shown that the concrete slab plays a significant role in providing rotational restraint and improves fire resistance. if these factors are absent, and the framing connection or the beam or column to which it is connected cannot provide the level of rotaional restraint described above, then such members should be considered unrestrained."
from your description, it sounds like your end conditions don't provide rotational restraint. according to the above authors, a continuous slab provides restraint for a steel beam. whether that reasoning applies to concrete joists is a tough call. it's easy to see how a composite slab would provide rotational restraint.

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