huangyhg

moment frame investigation
i'm doing an investigation of a large transit building. the building consists of primarily rigid frames which on the surface look to be very light. upon reviewing the drawings it appears the frames were designed based on wind load only. i spoke with the original engineer to confirm the loading, upon which he became very elusive when questioned why he did not consider the various load combinations. has anybody encounter rigid frames designed based soley on lateral loads ...i.e no consideration of gravity + lateral????
i've seen where older engineers (from the 50's and 60's) designed entire structures such as grocery stores without any consideration of lateral loads at all other than presuming the structural clay tile set between the perimeter wall columns was adequate.

they could be type 2 connections. see attached.
i have checked some of the connections and it appears they were only designed for wind load. i have spoken with a couple of the older guys in the office and they say they typically designed for wind and considered the gravity load to be taken by the shear connection, the connection was simply designed for the moment due to the wind load. seems strange to me because there obviously has to be some additional moment (from the gravity loading) due to p delta. am i missing something?
this isn't only strange, it is an error.
when you design a mrf building (moment resistant frames) you must consider horizontal loads and vertical loads, because the axial force given by dl will reduce the moment resistance. you can easily see it if you consider a m-n domain, in which you can value the interaction of axial loads with the moment.
so, you can have 3 cases:
- a frame with only axial load => your axial resistance will be full (ndfff">);
- a frame with only flexure => your flexure capacity will be full (mdfff">);
- a frame with axial load and flexure => you have interaction between m-n and you can't get full resistance (nd,reducedfff"> and md,reducedfff">).
how can i value this interaction? it's very simple; you can use:
- a linear interaction in the easiest and most simplified case by taking md and rd on a diagram, then trace a line joining them two.
- a quadratic interaction, that represent better the real behavior of the section.
for the linear interaction you can use:
ms / md + ns/nd <= 1
in which:
ms is the moment given by forces;
md is the flexure resistance;
ns is the axial load;
nd is the axial resistance.
i said it in very simplified way (because you have to take in account shear, buckling, and so forth). sap program operates in this way: with demand/capacity ratios.
so, when you design only with wind loads, you are brushing-off the axial load increment given by dl and ll.
hope it helps.

when you superimpose gravity loads with lateral loads, the knee joint will be subject to a larger design moment than under gravity or lateral load alone.
agree with lexatus, reduction in bending capacity will result when a
cant post why not?
i think calculor1 is correct. it is perfectly acceptable to design the double clip angles for gravity loads only, and the angles at the top and bottom flanges for the wind loads only. the angle at the top flange, if welded properly to the column and the beam flange, will deform sufficiently under gravity loads to allow the beam connection to be designed as a pin for gravity loads. but these same angles will create a moment connection for wind loads. blodgett covers it quite well in his textbook.
daveatkins
dave,
all that is fine for the connections as long as the members can take the combined loads. by the sounds of the op the
keeping the lateral and gravity loads as separate design load cases was certainly seen as a legitimate design method in the past.
i doubt if that is still the case, but it would depend on your design code.
lexatus-
this is not an error. we design buildings with "wind clip" connections every day. i didn't chime in earlier because you said the frames looked light. in actuality a "wind clip" building will have heavier beams and lighter columns, not lighter

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