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advice for horizontal reactions on trusses
3

advice for horizontal reactions on trusses

advice for horizontal reactions on trusses

(OP)
i am bridgedesigner:
happy new year to all people.
i have a question, could some one help me with this: i have a structural analysis program( or maybe i solve the truss mannually) for trusses, but it analyse them as planar, two dimensional, in the simplest manner, the support options are pinned or movil, no more options . i want to support this truss on steel or concrete columns with both connections pinned(truss end to column), and assuming columns fixed on foundation, ¿what is the criteria for use the horizontal reactions the program give to me(no matter load condition or combination), or what i get if i solve it manually?, i have no staadpro, sap2000 or other software like those, that analize industrial building like that as space srtucture and that permit the combination truss-frame element. thank you very much.

RE: advice for horizontal reactions on trusses

Pinned support means that the support will have vertical and horizontal forces. If you wish to have a support that only has a vertical reaction, i.e. roller, then you must release the horizontal force at that support. As the engineer you must elect that support type and you must detail these supports so that additional reactions are not developed.

I am not sure what software you are using, but as I always preach, use of software without understanding the theory, proper input and being able to read the results should not be permitted till and after the engineer have been trained thoroughly in engineering and software use.

If you have a two dimensional truss, I would solve it manually using the section or the joint method. They should give you fairly accurate results. You still must understand and use the right support condition/release which can be obtained from many engineering mechanics or structural analysis text books.

Good luck

RE: advice for horizontal reactions on trusses

(OP)
TO LUFTI:
THANK YOU VERY MUCH FOR YOUR ADVICE. THE PROGRAM I USE IS MAPGC OR PAEM THAT SOLVES TRUSSES AS  TWO DIMENSIONALS, AND THE OPTION FOR SUPPORT(IN THE PROGRAM) IS PINNED OR ROLLER, I KNOW WHAT HAPPEN IF I USE ONE SUPPORT ROLLER. I WANT TO TELL YOY I KNOW I CAN SOLVE THE STRUCTURE MANNUALLY WITH THE SECTION METHOS OR JOINT BY JOINT(IF THE TRUSS IS DETERMINATE, OR USING CASTIGLIANO'METHOD, UNITARY LOAD METHOD OR MATRIX(STIFFNES OR FLEXIBILITY METHODS), BUT THE TRUSS HAS MORE THAN 100 ELEMENTS, THAT WHY I DO NOT SOLVE IT MANNUALLY. MY PROBLEM IS NOT INTERPRETING RESULTS, MY PROBLEM IS UNDERSTAND THE CONNECTION TRUSS END TO COLUMNS(CONCRETE OR STEEL, SUPPOSED FIXED ON FOUNDATION. I READ MCCORMAC(STRUCTURAL ANALYSIS) BOOK ABOUT INDUSTRIAL BUILDINGS CRITERIA, BUT I AM STILL KEEPING WITH THE DOUBT IF I CHOOSE BOTH CONNECTIONS PINNED,¿HOW I SHOULD CONSIDER THE HORIZONTAL REACTIONS THE PROGRAM OR ME MANUALLY  OBTAINS?. I HAVE STRUCTURAL ANALYSIS BOOKS BY LAIBLE, MCCORMAC, YUAN YU HSIEH, GERE AND WEAVER, AND THE ONLY GOOD EXPLANATION I HAVE FOUN IS BY MCCORMAC, BUT THE CRITERIA IS SUPPOSING DEFLEXION POINTS. I AM STARTING TO STUDY FINITE ELEMENTS APROACH, I PURCHASED CHANDRUPATLA BOOK, BUT I AM STARTING AND I AM WAITING FOR BOOK 1 FROM ZIENKIEWICZ. THAN YOU VERY MUCH FOR THE ADVICE, REGARDS, BRIDGE DESIGNER.      

RE: advice for horizontal reactions on trusses

The horizontal truss reaction will be applied as a horizontal load at the top of your column, causing shear and bending in the column.  The vertical truss reaction causes axial force in the column.  Analyze the column for the interaction of the bending moment and the axial force.  The footing must resist the bending moment, the shear force, and the axial force.

DaveAtkins

RE: advice for horizontal reactions on trusses

Dave's response is right on target as far as the design of the column.

good luck

RE: advice for horizontal reactions on trusses

(OP)
THANK YOU VERY MUCH TO LUFTI AND DAVE ATKINS I AGRRE TOTALLY WITH DAVE ATKINS, BUT I SINCERELY COMMENT FROM ME ABOUT THE TOPIC, IS THAT I KNOW MOST PEOPLE IN THE FORUM ARE U.S.A ENGINEERS AND ARCHITECS, MAYBE LOT OF YOY HAVE MASTER DEGREE O EVEN MORE THAN THAT, AND OTHER ARE VERY EXPERIENCED ON STRUCTURES, I WAS WAITING FOR SOME ADVICE ABOUT CRITERIA LIKE MCCORMAS`S INFLEXION POINTS OR ANOTHER APROACH THAT SOME ONE HAD USED OR CONSIDERED, BUT DAVE ATKINS RESPONSE IS ACCORD WITH THE CRITERIA I HAVE USED ON TWO INDUSTRIAL BUILDINGS I HAVE STRUCTURALLY DESIGNED TOTALLY OR IN PART, BOTH HAVE WORKED EXCELLENT, RECENTLY AGAINST NORMAN HURRICANE. REGARDS, ALLWAYS YOURS, BRIDGE DESIGNER.

RE: advice for horizontal reactions on trusses

To bridgedesigner:
In small deformation theory, we ignore horizontal reations in supports, so the calculated result is same for one pined &one rolled with two pined. But when you consider the axial displacement of trusses, you should calculate the horizontal reactions if you can't release any support.

RE: advice for horizontal reactions on trusses

You must be very careful if you tell your software that you have 2 pinned ends.
The analysis will distribute forces in the truss quite differently from the pin-and-roller support case.
This exact design case caused collapse of a large steel school roof truss in Singapore, 2 years ago. The designer analysed a pin-roller support but then detailed a pin-pin, fixed to concrete columns.
The horizontal forces pushed the columns over at the same time as truss chord members buckled, because they experienced compression forces that they had not been designed for.
In the case of small, lightly loaded trusses with horizontal bottom chords, the horizontal reactions can often be (safely) ignored.
For other cases, particularly bridges, the horizontal reactions from a pin-pin design must always be accounted for by appropriate design of the sub-structure.
In your particular case, I would apply the full horizontal force from the analysis at the top of the column and design the column to suit. The column will of course deflect, so this will be a conservative approach. I would also recommend that you analyse your truss both ways, and design for the worst case.

RE: advice for horizontal reactions on trusses

(OP)
TO SVEN:
THANK YOU VERY MUCH FOR YOUR OBSERVATION, THIS IS A VERY HELPFUL ONE TO ME, AND THIS IS THE EXACT APROACH I HAVE USED FOR THE DESIGN OF BOTH INDUSTRIAL BUILDINGS I HAVE MADE PAST YEAR. BUT SOME ENGINEERS CRITICED ABOUT THE STEEL COLUMNS WERE TOO CONSERVATIVE. BUT THE STRUCTURES ARE BUILT AND WITH EXCELLENT PWERFORMANCE AGAINST NORMAN HURRICANE, WHO DESTROYED OR PART DISTROYED OTHER EXISTING STRUCTURES.

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